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Information about “PASCAL TUTOR 1 OF 2 (ALSO 580)”
PASCAL TUTOR is a text-based tutorial on programming with the Pascal
language. The assumption is that you know nothing about Pascal in
particular and programming in general.
It begins with the most elementary aspects of programming and takes you
through each step from dynamic allocation to the use of linked lists.
The program was specifically written for use with Turbo Pascal versions
3.0, 4.0, and 5.0. Version 5.5 is also supported through two additional
chapters covering object oriented programming. The Pascal source code
used for examples throughout the tutorial is included.
CHAP01.TXT
Chapter 1
WHAT IS A COMPUTER PROGRAM?
THIS CHAPTER IS FOR NEW PROGRAMMERS
____________________________________________________________
If you are a complete novice to computers you will find the
information in this chapter useful. If however, you have had
some experience with programming, you can completely ignore
this chapter. It will deal with a few fundamentals of
computers in general and will introduce nothing that is
specific to Pascal.
WHAT IS A COMPUTER PROGRAM?
____________________________________________________________
A computer is nothing but a very dumb machine that has the
ability to perform mathematical operations very rapidly and
very accurately, but it can do nothing without the aid of a
program written by a human being. Moreover, if the human
being writes a program that turns good data into garbage, the
computer will very obediently, and very rapidly, turn the good
data into garbage. It is possible to write a computer program
with one small error in it that will do that very thing, and
in some cases appear to be generating good data. It is up to
the human programmer to design a program to achieve the
desired results.
A computer program is simply a "recipe" which the computer
will use on the input data to derive the desired output data.
It is similar to the recipe for baking a cake. The input data
is comparable to the ingredients, including the heat supplied
by the oven. The program is comparable to the recipe
instructions to mix, stir, wait, heat, cool, and all other
possible operations on the ingredients. The output of the
computer program can be compared to the final cake sitting on
the counter ready to be cut and served. A computer program
is therefore composed of two parts, the data upon which the
program operates, and the program that operates on the data.
The data and program are inseparable as implied by the last
sentence.
WHAT ARE CONSTANTS?
____________________________________________________________
Nearly any computer program requires some numbers that never
change throughout the program. They can be defined once and
used as often as needed during the operation of the program.
To return to the recipe analogy, once you have defined how big
a tablespoon is, you can use the same tablespoon without
regard to what you are measuring with it. When writing a
Page 1-1
What is a Computer Program?
computer program, you can define the value of PI = 3.141592,
and continue to use it wherever it makes sense knowing that
it is available, and correct.
WHAT ARE VARIABLES?
____________________________________________________________
In addition to constants, nearly every computer program uses
some numbers that change in value throughout the program.
They can be defined as variables, then changed to any values
that make sense to the proper operation of the program. An
example would be the number of eggs in the above recipe. If
a single layer of cake required 2 eggs, then a triple layer
cake would require 6 eggs. The number of eggs would therefore
be a variable.
HOW DO WE DEFINE CONSTANTS OR VARIABLES?
____________________________________________________________
All constants and variables have a name and a value. In the
last example, the name of the variable was "eggs", and the
value was either 2 or 6 depending on when we looked at the
stored data. In a computer program the constants and
variables are given names in much the same manner, after which
they can store any value within the defined range. Any
computer programming language has a means by which constants
or variables can be first named, then assigned a value. The
means for doing this in Pascal will be given throughout the
remainder of this tutorial.
WHAT IS SO GOOD ABOUT PASCAL?
____________________________________________________________
Some computer languages allow the programmer to define
constants and variables in a very haphazard manner and then
combine data in an even more haphazard manner. For example,
if you added the number of eggs, in the above recipe, to the
number of cups of flour, you would arrive at a valid
mathematical addition, but a totally meaningless number. Some
programming languages would allow you to do just such an
addition and obediently print out the meaningless answer.
Since Pascal requires you to set up your constants and
variables in a very precise manner, the possibility of such
a meaningless answer is minimized. A well written Pascal
program has many cross checks to minimize the possibility of
a completely scrambled and meaningless output.
Notice however, in the last statement, that a "well written"
Pascal program was under discussion. It is still up to the
programmer to define the data structure in such a way that the
program can help prevent garbage generation. In the end, the
Page 1-2
What is a Computer Program?
program will be no better than the analysis that went into the
program design.
If you are a novice programmer, do not be intimidated by any
of the above statements. Pascal is a well designed, useful
tool that has been used successfully by many computer novices
and professionals. With these few warnings, you are ready to
begin.
Page 1-3
CHAP02.TXT
Chapter 2
GETTING STARTED IN PASCAL
YOUR FIRST PASCAL PROGRAM
____________________________________________________________
Lets get right into a program that really =================
does nothing, but is an example of the TRIVIAL.PAS
most trivial Pascal program. Load Turbo =================
Pascal, select TRIVIAL.PAS as a Work file,
and select Edit. This assumes that you
have been successful in learning how to use the TURBO Pascal
system. If you are using TURBO Pascal 4.0 through 5.5, you
will need to load TRIVIAL.PAS from the File menu.
You should now have the most trivial Pascal program possible
on your display, and we can take a look at each part to define
what it does.
The first line is required in the standard Pascal definition
and is the program name which can be any name you like, as
long as it follows the rules for an identifier given in the
next paragraph. It can have no blanks, otherwise it would be
considered as two words and it would confuse the compiler.
The first word program is the first of the reserved words
mentioned earlier and it is the indicator to the Pascal
compiler that this is the name of the program. Notice that
the line ends with a semicolon. Pascal uses the semicolon as
a statement separator and although all statements do not
actually end in a semicolon, most do, and use of the semicolon
will clear up later in your mind.
TURBO Pascal does not require the program statement, but to
remain compatible with standard Pascal, it will simply ignore
the entire statement. It is recommended that you include a
program name both to aid your thinking in standard Pascal, and
to add a little more indication of the purpose of each
program.
WHAT IS AN IDENTIFIER?
____________________________________________________________
All identifiers, including the program name, procedure and
function names, type definitions, and constant and variable
names, will start with an alphabetical character and be
composed of any combination of alphabetic and numeric
characters with no embedded blanks. Upper or lower case
alphabetic characters are not significant and may be mixed at
will. (If you find this definition confusing at this point,
don't worry about it, it will be clear later but it must be
defined early). The standard definition of Pascal requires
that any implementation (i.e. any compiler written by some
Page 2-1
Getting Started in Pascal
company) must use at least 8 characters of the identifier as
significant and may ignore the remaining characters if more
than 8 are used. Most implementations use far more than 8.
TURBO Pascal uses at least 63 characters in an identifier as
being significant.
Standard Pascal does not allow the use of underlines in an
identifier but most implementations of Pascal allow its use
after the first character. All versions of TURBO Pascal
compilers permit the use of the underline in an identifier,
so it will be freely used throughout this tutorial. The
underline is used in the program name Puppy_Dog which should
be on your display at this time.
Returning to the example program, the next line is a blank
line which is ignored by all Pascal compilers. More will be
said about the blank line at the end of this chapter.
NOW FOR THE PROGRAM
____________________________________________________________
Lines 3 and 4 comprise the actual Pascal program, which in
this case does absolutely nothing. It is an illustration of
the minimum Pascal program. The two words begin and end are
the next two reserved words we will consider. Any logical
grouping of Pascal code can be isolated by bracketing it with
the two reserved words begin and end. You will use this
construct repeatedly as you write Pascal code so it is well
to learn it thoroughly. Code to be executed by conditional
jumps will be bracketed by begin and end, as will code within
a loop, and code contained within a subroutine (although they
are called procedures in Pascal), and in many other ways. In
the present program, the begin and end are used to bracket the
main program and every Pascal program will have the main
program bracketed in this manner. Because there is nothing
to do in this program, there are no statements between the
begin and end reserved words.
Finally, although it could be very easily overlooked, there
is one more very important part of the program, the period
following the reserved word end. The period is the signal to
the compiler that it has reached the end of the executable
statements and is therefore finished compiling. Every Pascal
program will have one, and only one period in it and that one
period will be at the end of the program. I must qualify that
statement in this regard, a period can be used in comments,
and in text to be output. In fact there are some data formats
that require using a period as part of their structure. The
statement is true however, that there is only one period in
the executable part of a Pascal program. Think of a Pascal
program as one long sentence with one period at the end.
Ignore lines 9 through 13 for a few minutes and we will
describe them fully later.
Page 2-2
Getting Started in Pascal
That should pretty well describe our first program. Now it
is time to compile and run it after you exit the editor. The
means of doing this is given for each compiler as an aid to
get you started.
TURBO Pascal 3.0 - <ctrl>k, r
TURBO Pascal 4.0 - <alt>r
TURBO Pascal 5.x - <alt>r (then <alt>F5 to view the
results)
Since this program doesn't do anything, it is not very
interesting, so let's get one that does something.
A PROGRAM THAT DOES SOMETHING
____________________________________________________________
Load the Pascal program WRITESM.PAS and =================
view it on your monitor. The filename is WRITESM.PAS
sort of cryptic for "Write Some" and it =================
will display a little output on the
monitor. The program name is Kitty_Cat
which says nothing about the program itself but can be any
identifier we choose. We still have the begin and end to
define the main program area followed by the period. However,
now we have two additional statements between the begin and
end. Writeln is a special word and it is probably not
surprising that it means to write a line of data somewhere.
Without a modifier, which will be fully explained in due time,
it will write to the default device which, in the case of our
IBM compatible, is the video display. The data within the
parentheses is the data to be output to the display and
although there are many kinds of data we may wish to display,
we will restrict ourselves to the simplest for the time being.
Any information between apostrophes will simply be output as
text information.
The special word Writeln is not a reserved word but is defined
by the system to do a very special job for you, namely to
output a line of data to the monitor. It is, in fact, a
procedure supplied for you by the writers of TURBO Pascal as
a programming aid for you. You can, if you so desire, use
this name for some other purpose in your program, but doing
so will not allow you to use the standard output procedure.
It will then be up to you to somehow get your data out of the
program.
Note carefully that some words are reserved and cannot be
redefined and used for some other purpose, and some are
special since they can be redefined. You will probably not
want to redefine any of the special words for a long time so
simply use them as tools.
Page 2-3
Getting Started in Pascal
Notice the semicolon at the end of line 4. This is the
statement separator referred to earlier and tells Pascal that
this line is complete as it stands, nothing more is coming
that could be considered part of this statement. The next
statement, in line 5, is another statement that will be
executed sequentially following the statement in line 4. This
program will output the two lines of text and stop. Now it
is time to go try it. Compile and run the program in the same
manner as you did for the first example program.
You should see the two lines of text output to the video
display every time you run this program. When you grow bored
of running WRITESM.PAS let's go on to another example.
ANOTHER PROGRAM WITH MORE OUTPUT
____________________________________________________________
Examine the example program named =================
WRITEMR.PAS. This new program has three WRITEMR.PAS
lines of output but the first two are =================
different because another special word is
introduced to us, namely Write. Write is
a procedure which causes the text to be output in exactly the
same manner as Writeln, but Write does not cause a carriage
return to be output. Writeln causes its output to take place
then returns the "carriage" to the first character of the next
line. The end result is that all three of the lines of text
will be output on the same line of the monitor when the
program is run. Notice that there is a blank at the end of
each of the first two lines so that the formatting will look
nice. Compile and execute the new program.
Now might be a good time for you to return to editing
WRITEMR.PAS and add a few more output commands to see if they
do what you think they should do. When you tire of that, we
will go on to the next file and learn about comments within
a Pascal program.
ADDING COMMENTS IN THE PROGRAM
____________________________________________________________
The file named PASCOMS.PAS is similar to =================
the others except that comments have been PASCOMS.PAS
added to illustrate their use. Pascal =================
defines comments as anything between (*
and *) or anything between { and }.
Originally only the wiggly brackets were defined, but since
many keyboards didn't have them available, the parenthesis
star combination was defined as an extension and is universal
by now, so you can use either. Most of the comments are self
Page 2-4
Getting Started in Pascal
explanatory except for the one within the code. Since
comments can go from line to line, the two lines that would
print "send money" are not Pascal code but are commented out.
Try compiling and running this program, then edit the comments
out so that "send money" is printed also.
A fine point should be mentioned here. Even though some
compilers allow comments to start with (* and end with }, or
to start with { and end with *), it is very poor programming
practice and should be discouraged. The ANSI Pascal standard
allows such usage but TURBO Pascal does not allow this funny
use of comment delimiters.
TURBO Pascal does not allow you to nest comments using the
same delimiters but it does allow you to nest one type within
the other. This could be used as a debugging aid. If you
generally use the (* and *) for comments, you could use the
{ and } in TURBO Pascal to comment out an entire section of
code during debugging even if it had a few comments in it.
This is a trick you should remember when you reach the point
of writing programs of significant size.
When you have successfully modified and run the program with
comments, we will go on to explain good formatting practice
and how Pascal actually searches through your source file
(Pascal program) for its executable statements.
It should be mentioned that the program named PASCOMS.PAS does
not indicate good commenting style. The program is meant to
illustrate where and how comments can be used and looks very
choppy and unorganized. Further examples will illustrate good
use of comments to you as you progress through this tutorial.
THE RESULT OF EXECUTION SECTION
____________________________________________________________
You should now be able to discern the purpose for lines 20
through 26 of this program. Each of the example programs in
this tutorial lists the result of execution in a similar
comments section at the end of the program. This makes it
possible to study this tutorial anywhere once you print out
the example programs as described in the READ.ME file on the
distribution disk. With this text, and a hard copy of the
example programs containing the result of execution, you do
not need access to a computer to study. Of course you would
need access to a computer to write, compile, and execute the
programming exercises, which you are heartily encouraged to
do.
Page 2-5
Getting Started in Pascal
GOOD FORMATTING PRACTICE
____________________________________________________________
Examine GOODFORM.PAS to see an example of ================
good formatting style. It is important to GOODFORM.PAS
note that Pascal doesn't give a hoot where ================
you put carriage returns or how many
blanks you put in when a blank is called
for as a delimiter. Pascal only uses the combination of
reserved words and end-of- statement semicolons to determine
the logical structure of the program. Since we have really
only covered two executable statements, I have used them to
build a nice looking program that can be easily understood at
a glance. Compile and run this program to see that it really
does what you think it should do.
VERY POOR FORMATTING PRACTICE
____________________________________________________________
Examine UGLYFORM.PAS now to see an example ================
of terrible formatting style. It is not UGLYFORM.PAS
really apparent at a glance but the ================
program you are looking at is exactly the
same program as the last one. Pascal
doesn't care which one you ask it to run because to Pascal,
they are identical. To you they are considerably different,
and the second one would be a mess to try to modify or
maintain sometime in the future.
UGLYFORM.PAS should be a good indication to you that Pascal
doesn't care about programming style or form. Pascal only
cares about the structure, including reserved words and
delimiters such as blanks and semicolons. Carriage returns
are completely ignored as are extra blanks. You can put extra
blanks nearly anywhere except within reserved words or
variable names. You should pay some attention to programming
style but don't get too worried about it yet. It would be
good for you to simply use the style illustrated throughout
this tutorial until you gain experience with Pascal. As time
goes by you will develop a style of statement indentation,
adding blank lines for clarity, and a method of adding clear
comments to Pascal source code. Programs are available to
read your source code, and put it in a "pretty" format, but
that is not important now.
Not only is the form of the program important, the names used
for variables can be very helpful or hindering as we will see
in the next chapter. Feel free to move things around and
modify the format of any of the programs we have covered so
far and when you are ready, we will start on variables in the
next chapter. Be sure you compile and execute UGLYFORM.PAS.
Page 2-6
Getting Started in Pascal
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program that displays your name on the video
monitor.
2. Modify your program to display your name and address on
one line, then modify it by changing the Write's to
Writeln's so that the name and address are on different
lines.
Page 2-7
CHAP03.TXT
Chapter 3
SIMPLE DATA TYPES
WHAT IS A DATA TYPE?
____________________________________________________________
A type in Pascal, and in several other popular programming
languages, defines a variable in such a way that it defines
a range of values which the variable is capable of storing,
and it also defines a set of operations that are permissible
to be performed on variables of that type. TURBO Pascal has
5 basic data types which are predefined and can be used
anywhere in a program provided you use them properly. This
chapter is devoted to illustrating the use of these five data
types by defining the allowable range of values that can be
assigned to them, and by illustrating the operations that can
be done to variables of these types. The five types and a
very brief description follows;
integer Whole numbers from -32768 to 32767
byte The integers from 0 to 255
real Floating point numbers from 1E-38 to 1E+38
boolean Can only have the value TRUE or FALSE
char Any character in the ASCII character set
Please note that the byte type of data is not a part of the
standard Pascal definition but is included as an extension to
the TURBO Pascal compiler.
TURBO Pascal versions 4.0 and 5.x have three additional types
available of the integer class, which are not available with
version 3.0. They are defined as follows;
shortint The integers from -128 to 127
word The integers from 0 to 65535
longint The integers from -2147483648 to 2147483647
In addition to the above data types TURBO Pascal version 4.0
has the following data types available but in order to use
them, you must have an 80X87 math coprocessor installed in
your system;
single Real type with 7 significant digits
double Real type with 15 significant digits
extended Real type with 19 significant digits
comp The integers from about -10E18 to 10E18
TURBO Pascal version 5.x has these four types available but
because they have a software emulator for the floating point
operations, an 80X87 math coprocessor is not required to use
these with these versions. Of course, your resulting program
Page 3-1
Simple Data Types
will run much faster if you have the coprocessor available for
use by the program.
A complete definition of the available types for each compiler
can be found on pages 41 and 42 of the TURBO Pascal version
3.0 reference manual, and on pages 39 through 44 of the
reference manual for version 4.0. They are defined on pages
41 to 46 of the TURBO Pascal 5.x User's guide. It would be
good to read these pages now for a good definition prior to
learning how to define and use them in a program. Note that
all of these will be used in example programs in this chapter.
OUR FIRST VARIABLES
____________________________________________________________
The integers are by far the easiest to ================
understand so we will start with a simple INTVAR.PAS
program that uses some integers in a very ================
simple way. Load INTVAR.PAS into your
TURBO system and let's take a look at it.
Immediately following the program statement is another
reserved word, var. This reserved word is used to define a
variable before it can be used anywhere in the program. There
is an unbroken rule of Pascal that states "Nothing can be used
until it is defined." The compiler will complain by
indicating a compilation error if you try to use a variable
without properly defining it. It seems a bit bothersome to
have to define every variable prior to its use, but this rule
will catch many spelling errors of variables before they cause
trouble. Some other languages will simply define a new
variable with the new name and go merrily on its way producing
some well formatted garbage for you.
Notice that there is only one var, but it is used to define
three different variables, Count, X, and Y. Once a var is
recognized, the compiler will continue to recognize variable
definitions line after line until it finds another reserved
word. It would be permissible to put a var on the second line
also but it is not necessary. It would also be permissible
to put all three variables on one line but your particular
programming style will dictate where you put the three
variables. Following the colon on each line is the word
integer which is a standard identifier, and is therefore
different from a reserved word. A standard identifier is
predefined like a reserved word, but you can redefine it,
thereby losing its original purpose and meaning. For now and
for a long time, don't do that. Page 38 contains a list of
standard identifiers in TURBO Pascal 3.0. There is no
corresponding list in the reference manual for TURBO Pascal
4.0 or for TURBO Pascal 5.x.
Page 3-2
Simple Data Types
OUR FIRST ARITHMETIC
____________________________________________________________
Now that we have three variables defined as integer type
variables, we are free to use them in a program in any way we
desire as long as we use them properly. If we tried to assign
a real value to X, the compiler will generate an error, and
prevent a garbage output. Observe the start of the main body
of the program. There are three statements assigning values
to X, Y, and Count. A fine point of mathematics would state
that Count is only equal to the value of X+Y until one of them
was modified, therefore the equal sign used in so many other
languages is not used here. The sign := is used, and can be
read as "is replaced by the value of," when reading a listing.
Another quicker way is to use the word "gets". Thus X := X
+ 1 would be read, "X gets the value of X plus 1". We will
see later that the simple equal sign is reserved for use in
a different manner.
The first three statements give X the value of 12, Y the value
of 13, and Count the value of 12 + 13 or 25. If we have a
requirement to get those values out of the computer, we need
another extension to the Writeln statement. The first part
of the data within the parentheses should be very familiar to
you now, but the second part is new.
Multiple outputs can be handled within one Writeln if the
fields are separated by a comma. To output a variable, simply
write the variable's name in the output field. The number
following the variable in each case is the number of output
columns to be used by the output data. This number is
optional and can be omitted allowing the system to use as many
columns as it needs. For purposes of illustration, they have
all been assigned different numbers of columns. At this
point, you can compile and run INTVAR.PAS and examine its
output.
To illustrate the various ways to output =================
data, load INTVAR2.PAS and observe that INTVAR2.PAS
even though the output is identical, it is =================
output in a completely different manner.
Observe especially that a Writeln all by
itself simply moves the cursor to the beginning of a new line
on the video monitor. Compile and run this program and
observe its output after you are certain that the two programs
are actually identical.
NOW LET'S USE LOTS OF VARIABLES
____________________________________________________________
Load ALLVAR.PAS to observe a short program using all 5 of the
basic data types. The variables are simply assigned values
Page 3-3
Simple Data Types
and the values are printed. A complete ================
and detailed description of the options ALLVAR.PAS
available in the Write statement is given ================
in the TURBO reference manual version 3.0
on pages 111 through 113, and on pages 500
through 502 for version 4.0. Pages 52 and 53 of the User's
Guide has the corresponding information for TURBO Pascal
version 5.x. It would be to your advantage to read this
section at this time since very little explanation will be
given about Write statements from this point on. We will
discuss the method by which we can write to disk files or
other output devices in a later chapter of this tutorial.
Back to the basic types. Pascal does lots of cross checking
for obvious errors. It is illegal to assign the value of any
variable with a value that is of the wrong type or outside the
allowable range of that variable. There are routines to
convert from one system to another when that is necessary.
Suppose, for example, that you wished to use the value of an
integer in a calculation of real numbers. That is possible
by first converting the integer into a real number of the same
value and using the new real type variable in the desired
calculations. The new real type variable must of course be
defined in a var statement as a real type variable before it
can be used. Details of how to do several conversions of this
kind will be given in the example program named CONVERT.PAS
later in this chapter.
Since we have some variables defined, it ================
would be nice to use the properties of REALMATH.PAS
computers for which they are famous, ================
namely some arithmetic. Two programs are
available for your observation to
illustrate the various kinds of math available, REALMATH.PAS
using real variables, and INTMATH.PAS using integer variables.
You can edit, compile, and run these on your own with no
comment from me except the comments
embedded into the source files. Chapter 6 =================
on pages 51 to 54 of your version 3.0 INTMATH.PAS
TURBO reference manual completely defines =================
the simple mathematics available. The
corresponding list for version 4.0 is
found in chapter 3 on pages 46 through 49, and pages 48
through 51 of the TURBO Pascal User's Guide gives the list for
version 5.x of the compiler.
A byte type variable is used just like an integer variable but
with a much smaller allowable range. Only one byte of
computer memory is used for each variable defined as a byte
type variable, but 2 are used for each integer type variable.
Page 3-4
Simple Data Types
BOOLEAN VARIABLES
____________________________________________________________
Let's take a look at a boolean variable, which is only allowed
to take on two different values, TRUE or FALSE. This variable
is used for loop controls, end of file indicators or any other
TRUE or FALSE conditions in the program. Variables can be
compared to determine a boolean value. A complete list of the
relational operators available with Pascal is given in the
following list.
= equal to
<> not equal to
> greater than
< less than
>= greater than or equal to
<= less than or equal to
These operators can be used to compare any ================
of the simple types of data including BOOLMATH.PAS
integer, char, byte, and real type ================
variables or constants, and they can be
used to compare boolean variables. An
illustration is the best way to learn about the boolean
variable so load BOOLMATH.PAS and observe it.
In BOOLMATH.PAS we define a few boolean variables and two
integer type variables for use in the program and begin by
assigning values to the two integer variables. The expression
Junk = Who in line 14 is actually a boolean operation that is
not true since the value of Junk is not equal to the value of
Who. The result is therefore FALSE and that value is assigned
to the boolean variable A. The boolean variable B is assigned
the value of TRUE because the expression Junk = (Who - 1) is
true. The boolean variables C and D are likewise assigned
some values in a manner that should not need any comment.
After assigning a value to the variable with the big name, the
values are all printed out.
WHERE DO WE USE THE BOOLEAN VARIABLES?
____________________________________________________________
We will find many uses for the boolean type variable when we
study the loops and conditional statements soon, but until
then we can only learn what they are. Often, in a conditional
statement, you will want to do something if either of two
things are true, in which case you will use the reserved word
and with two boolean expressions. If either of the two are
true, the result will be true. Line 29 is an example of this.
If the boolean variables B, C, and D, are all true, then the
result will be true and A will be assigned the value of TRUE.
If any one of them is false, the result will be false and A
will be assigned the value of FALSE.
Page 3-5
Simple Data Types
In Line 31, where the or operator is illustrated, if any of
the three boolean variables is true, the result will be true,
and if all three are false, the result will be false. Another
boolean operator is the not which is illustrated in line 30.
Examine line 33 which says the result is true only if the
variable Junk is one less than Who, or if Junk is equal to
Who.
Compile and run this program, then add some additional
printout to see if the boolean variables change the way you
think they should in the last few statements.
SHORT CIRCUIT OR COMPLETE EVALUATION?
____________________________________________________________
Suppose you have several boolean expressions "and"ed together,
and when evaluation starts, the first expression results in
a FALSE. Since the first expression is FALSE, it is
impossible for the following expressions to ever allow the
final result to be TRUE because the first FALSE will force the
answer to be FALSE. It seems like a waste of execution time
to continue evaluating terms if the final result is already
known, but that is exactly what standard Pascal will do
because of the language definition. This is known as complete
evaluation of a boolean expression. If the system is smart
enough to realize that the final result is known, it could
stop evaluation as soon as the final result is known. This
is known as short circuit evaluation of a boolean expression,
and could also be applied if a term of an "or"ed boolean
expression resulted in a TRUE, since the result would always
be TRUE.
TURBO Pascal version 3.0 always does complete evaluation of
boolean expressions but TURBO Pascal versions 4.0 and 5.x
allows you to choose between complete evaluation or short
circuit evaluation. The default for both compilers is the
short circuit form but it can be changed through the Options
menu when you are using the integrated environment, or through
use of a compiler directive.
LET'S LOOK AT THE CHAR TYPE VARIABLE
____________________________________________________________
A char type variable is a very useful ================
variable, but usually not when used alone. CHARDEMO.PAS
It is very powerful when used in an array ================
or some other user defined data structure
which is beyond the scope of this chapter.
A very simple program, CHARDEMO.PAS is included to give you
an idea of how a char type variable can be used. Study then
Page 3-6
Simple Data Types
compile and run CHARDEMO.PAS for a very brief idea of what the
char type variable is used for.
Examine the sample program CONVERT.PAS for =================
several examples of converting data from CONVERT.PAS
one simple variable to another. The =================
program is self explanatory.
THIS IS FOR TURBO PASCAL 4.0 OR 5.X USERS
____________________________________________________________
If you are using TURBO Pascal version 3.0, you are finished
with this chapter because the data types illustrated in the
last two programs are not available with that compiler.
If you are using TURBO Pascal 4.0 or 5.x, =================
display the program NEWINT4.PAS for an NEWINT4.PAS
example of using the extended integer =================
types available with that compiler. Four
variables are defined and values assigned
to each, then the results are displayed. When you compile and
run the program, you will see that the variable Big_int can
indeed handle a rather large number.
It must be pointed out that the calculation in lines 13 and
21 result in a different answer even though they appear to be
calculating the same thing. An explanation is in order. The
quantity named MaxInt used in lines 10 and 13 is a constant
built into the system that represents the largest value that
an integer type variable can store. On the first page of this
chapter we defined that as 32767 and when running the program
you will find that Index displays that value as it should.
The constant MaxInt has a type that is of a universal_integer
type as do all of the numeric constants in line 13. The
result then is calculated to the number of significant digits
dictated by the left hand side of the assignment statement
which is of type longint resulting in a very large number.
When we get to line 21, however, the variable Index is of type
integer so the calculations are done as though the constants
were of type integer also which causes some of the more
significant digits to be truncated. The truncated result is
converted to type longint and assigned to the variable Big_int
and the truncated value is displayed by line 22.
After that discussion it should be apparent to you that it is
important what types you use for your variables. It must be
emphasized that it would not be wise to use all large type
variables because they use more storage space and slow down
calculations. Experience will dictate the proper data types
to use for each application.
Page 3-7
Simple Data Types
NOW FOR THE NEW REAL TYPES
____________________________________________________________
If you are using TURBO Pascal 4.0 or 5.x, ================
display the program NEWREAL4.PAS for an NEWREAL4.PAS
example using the new "real" types ================
available with the newer versions of TURBO
Pascal. Note that you must have an 80X87
math coprocessor installed to compile and run this program if
you are using TURBO Pascal version 4.0. There is a note given
in the file to aid you in selecting it for use.
If you are using TURBO Pascal version 5.x, you can use the
80X87 math coprocessor, once again getting help from the note
embedded in the file. If you do not have a math coprocessor,
TURBO Pascal version 5.x has an emulator mode which can be
used as instructed on page 42 of the User's Guide. Keep in
mind that, even though the emulator will allow you to use
these newer data types, the resulting program will execute
much slower due to the extra calculations required.
This program should be self explanatory so nothing will be
said except that when you run it you can observe the relative
accuracy of each of the variable types. Once again, you
should keep in mind that use of the larger "real" types costs
you extra storage space and reduced run-time speed, but gives
you more accuracy.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program containing several variable definitions
and do some math on them, printing out the results.
Page 3-8
CHAP04.TXT
Chapter 4
LOOPS & CONTROL STRUCTURES
Every program we have examined to this point has been a simple
one pass through with no statements being repeated. As in all
other languages, Pascal has extensive capabilities to do
looping and conditional branching. We will look at these now.
THE FOR LOOP
____________________________________________________________
We will start with what may be the easiest ================
structure to understand, the for loop. LOOPDEMO.PAS
This is used to repeat a single Pascal ================
statement any number of times we desire.
Load LOOPDEMO.PAS and we will discuss the
loops presented there.
The example illustrated in lines 13 and 14, is the simplest
loop and does nothing more than execute a Writeln 7 times.
We have three new reserved words, for, to, and do which are
used as shown. Any simple variable of type integer, byte, or
char can be used for the loop index but due to the requirement
that everything must be defined prior to use in Pascal, the
loop index must be defined in a var statement. Following the
reserved word do is any single Pascal statement that will be
repeated the specified number of times. Note that the loop
is an incrementing loop but substitution of downto for to will
make it a decrementing loop as is illustrated in the last
example in this program. It should be pointed out that the
loop control variable can only be incremented or decremented
by 1 each time through the loop in Pascal.
A COMPOUND PASCAL STATEMENT
____________________________________________________________
The example given in lines 18 through 22 contains our first
compound Pascal statement. It was mentioned in Chapter 1 that
the begin end pair of reserved words could be used to mark the
limits of a compound statement. In this case, the single
statement starting with the begin at the end of line 18 and
extending through and including the end statement in line 22
is the single Pascal statement that will be executed 10 times.
A second variable Total has been introduced to simply add
another operation to the loop. Any valid Pascal operation can
be performed within the begin end pair, including another for
loop, resulting in nested loops to whatever depth you desire.
The third example shows how the char type variable could be
used in a for loop. Pascal requires that the loop index, the
Page 4-1
Loops and Control Structures
starting point, and the ending point all be of the same type
or it will generate an error message during compilation. In
addition, they must be variables of type integer, byte, or
char. The starting point and ending point can be constants
or expressions of arbitrary complexity.
The fourth example is a decrementing loop as mentioned
earlier. It uses the reserved word downto, and should be self
explanatory.
THE IF STATEMENT
____________________________________________________________
Pascal has two conditional branching ================
capabilities, the if and the case IFDEMO.PAS
statements. We will look at the simplest ================
of the two now, the if statement. Load
IFDEMO.PAS for an onscreen look at the if
then pair of reserved words first illustrated in lines 11 and
12. Any condition that can be reduced to a boolean answer is
put between the if then pair of words. If the resulting
expression resolves to TRUE, then the single Pascal statement
following the reserved word then is executed, and if it
resolves to FALSE, then the single statement is skipped over.
Of course, you can probably guess that the single statement
can be replaced with a compound statement bracketed with a
begin end pair and you are correct. Study example 1 and you
will see that the line will always be printed in this
particular fragment because Three is equal to One + Two. It
is very difficult to come up with a good example without
combining some of the other control structures but we will do
so in the next example program.
The second example in lines 14 through 19, is similar to the
first but has the single statement replaced with a compound
statement and should be simple for you to understand.
The third example in lines 21 through 24, contains a new
reserved word, else. When the if condition is FALSE, the
single statement following the word then is skipped and if a
semicolon is encountered, the if clause is totally complete.
If instead of a semicolon, the reserved word else is
encountered, then the single Pascal statement following else
is executed. One and only one of the two statements will be
executed every time this if statement is encountered in the
program. Examination of the third example should clear this
up in your mind.
Notice that the Pascal compiler is looking for either a
semicolon to end the if, or the reserved word else to continue
the logic. It is therefore not legal to use a semicolon
immediately preceding the reserved word else. You will get
a compiler error if you include the semicolon.
Page 4-2
Loops and Control Structures
THE IF-THEN-ELSE BLOCK
____________________________________________________________
Put on your thinking cap because the next principle is
difficult to grasp at first but will suddenly clear up and be
one of the most useful facts of Pascal programming. Since the
entire if then else block of code is itself a single Pascal
statement by definition, it can be used anywhere that an
executable statement is legal without begin end separators.
This is shown in the fourth example of the IFDEMO.PAS Pascal
example program. Lines 27 through 30 comprise a single Pascal
statement, and lines 32 through 35 comprise another. The if
statement begun in line 26 therefore has a single statement
in each of its branches, and it is a single Pascal statement
itself beginning in line 26 and ending in line 35.
The if then else construct is one of the most used, most
useful, and therefore most important aspects of Pascal. For
this reason you should become very familiar with it.
Try changing some of the conditions in the example program to
see if you can get it to print when you expect it to for your
own practice. When you are ready, we will go on to a program
with loops and conditional statements combined and working
together.
LOOPS AND IFS TOGETHER
____________________________________________________________
Load LOOPIF.PAS and study it for a few ================
minutes. It contains most of what you LOOPIF.PAS
have studied so far and should be ================
understandable to you at this point. It
contains a loop (lines 7 through 17) with
two if statements within it (lines 8 & 9 and lines 10 through
16), and another loop (lines 11 through 15) within one of the
if statements.
You should make careful note of the formatting used here. The
begin is at the end of the line which starts the control and
the end is lined up under the control word such that it is
very clear which control word it is associated with. You will
develop your own clear method of formatting your code in time
but until then it is suggested that you follow this example
which is written in a manner which is acceptable within the
Pascal programming community.
An easily made error should be pointed out at this time. If
an extraneous semicolon were put at the end of the if
statement in line 8, the code following the statement would
always be executed because the null statement (the nothing
statement between the then and the semicolon) would be the
Page 4-3
Loops and Control Structures
conditional statement. The compiler would not generate an
error and you would get no warning. Add a semicolon at the
end of line 8 to see the error. Of course, you will need to
compile and execute the program to see line 9 print for all
10 values of Count.
FINALLY, A MEANINGFUL PROGRAM
____________________________________________________________
Load TEMPCONV.PAS and study its structure. ================
Notice the header block that defines the TEMPCONV.PAS
program and gives a very brief explanation ================
of what the program does. This program
should pose no problem to you in
understanding what it does since it is so clearly documented.
Run it and you will have a list of Centigrade to Fahrenheit
temperature conversions with a few added notes.
Load, examine, and run DUMBCONV.PAS for a ================
good example of poor variable naming. The DUMBCONV.PAS
structure of the program is identical to ================
the last program and when you run it, you
will see that it is identical in output,
but compared to the last program, it is difficult to
understand what it does by studying the listing. We studied
UGLYFORM.PAS in chapter 2 of this tutorial and it illustrated
really stupid formatting that nobody would ever use. The poor
choice of variable names and lack of comments in the present
program is nearly as unreadable, but many programmers are
content to write code similar to this example. You should be
conscious of good formatting style and naming conventions from
the start and your programs will be clear, easy to understand,
and will run efficiently. This program, like the last should
be easily understood by you, so we will go on to our next
Pascal control structure.
THE REPEAT UNTIL LOOP
____________________________________________________________
The next two Pascal constructs are very similar because they
are both indefinite loops (indefinite because they are not
executed a fixed number of times). One of the loops is
evaluated at the top and the other at the bottom. It will
probably be easier to start with the repeat until construct
which is the loop that is evaluated at the bottom.
Retrieve the file REPEATLP.PAS to see an ================
example of a repeat loop. Two more REPEATLP.PAS
reserved words are defined here, namely ================
repeat and until. This rather simple
construct simply repeats all statements
between the two reserved words until the boolean expression
Page 4-4
Loops and Control Structures
following the until is found to be TRUE. This is the only
expression in Pascal that operates on a range of statements
rather than a single statement and begin end delimiters are
not required.
A word of caution is in order here. Since the loop is
executed until some condition becomes TRUE, it is possible
that the condition will never be TRUE and the loop will never
terminate. It is up to you, the programmer, to insure that
the loop will eventually terminate. Compile and execute
REPEATLP.PAS to observe the output.
THE WHILE LOOP
____________________________________________________________
The file WHILELP.PAS contains an example =================
of another new construct, the while loop. WHILELP.PAS
This uses the while do reserved words and =================
will execute one Pascal statement (or one
compound statement bounded with begin and
end) continuously until the boolean expression between the two
words becomes FALSE.
This loop is also indeterminate and could, like the repeat
until loop, never terminate. You should therefore exercise
care in using it.
There are two basic differences in the last two loops. The
repeat until loop is evaluated at the bottom of the loop and
must therefore always go through the loop at least one time.
The while loop is evaluated at the top and may not go through
even once. This gives you flexibility when choosing the loop
to do the job at hand. Compile, run, and examine the output
from the example program WHILELP.PAS.
THE CASE STATEMENT
____________________________________________________________
The final control structure introduces one more reserved word,
case. The case construct actually should be included with the
if statement since it is a conditional execution statement,
but we saved it for last because it is rather unusual and will
probably be used less than the others we have discussed in
this chapter.
The case statement is used to select one ================
of many possible simple Pascal statements CASEDEMO.PAS
to execute based on the value of a simple ================
variable. Load the file CASEDEMO.PAS and
observe the program for an example of a
case statement. The variable between the case and of reserved
words in line 9 is the variable used to make the selection and
Page 4-5
Loops and Control Structures
is called the case selector. Following that, the various
selections are listed as a possible value or range, followed
by a colon, a single Pascal statement, and a semicolon for
each selector. Following the list of selections, an else can
be added to cover the possibility that none of the selections
were executed. Finally, an end statement is used to terminate
the case construct. Note that this is one of the few places
in Pascal that an end is used without a corresponding begin.
The example file uses Count for the case selector, prints the
numbers one through five in text form, and declares that
numbers outside this range are not in the allowable list. The
program should be self explanatory beyond that point. Be sure
to compile and run this example program.
Load and display the sample program =================
BIGCASE.PAS for another example of a case BIGCASE.PAS
statement with a few more added features. =================
This program uses the identical structure
as the previous program but in line 11 a
range is used as the selector so that if the value of Count
is 7, 8, or 9 this selection will be made. In line 12, three
different listed values will cause selection of this part of
the code. Of greater importance are the compound statements
used in some of the selections. If the variable Count has the
value of 2, 4, or 6, a compound statement will be executed and
if the value is 3, a for loop is executed. If the value is
1, an if statement is executed which will cause a compound
statement to be executed. In this case the if statement will
always be executed because TRUE will always be true, but any
Boolean expression could be used in the expression. Be sure
to compile and run this program, then study the output until
you understand the result thoroughly.
This brings us to the end of chapter 4 and you now have enough
information to write essentially any program desired in
Pascal. You would find however, that you would have a few
difficulties if you attempted to try to write a very big
program without the topics coming up in the next few chapters.
The additional topics will greatly add to the flexibility of
Pascal and will greatly ease programming with it.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program that lists the numbers from 1 to 12 and
writes a special message beside the number representing
your month of birth.
2. Write a program that lists all of the numbers from 1 to
12 except for the numbers 2 and 9.
Page 4-6
CHAP05.TXT
Chapter 5
PROCEDURES AND FUNCTIONS
A PASCAL PROGRAM OUTLINE
____________________________________________________________
In order to properly define procedures and functions we need
to lay some groundwork in the form of a few definitions.
These are important concepts, so pay close attention. Every
Pascal program is composed of three fundamental parts which
can be defined as follows.
Program Heading - This is the easiest part since it is only
one line, at least it has been in all of our programs up
to this point. It is simply the program line, and it
never needs to be any more involved or complicated than
it has been up to this point in TURBO Pascal. You may
remember that we said it is not even necessary in TURBO
Pascal.
Declaration Part - This is the part of the Pascal source code
in which all constants, variables, and user defined
auxiliary operations are defined. In some of the
programs we have examined, there have been one or more
var declarations. These are the only components of the
declaration part we have used to this point. There are
actually five components in the declaration part, and
procedures and functions which are the topics of this
chapter, are the fifth part. We will cover the other
components in the next chapter.
Statement Part - This is the last part of any Pascal program,
and it is what we have been calling the main program.
It is one compound statement bracketed with the reserved
words begin and end. Note that neither of these two
words are optional since both are required to have a
Pascal program.
It is very important that you grasp the above definitions
because we will be referring to them constantly during this
chapter and throughout the remainder of this tutorial. With
that introduction, let's go on to our first Pascal program
with a procedure in it, in fact it will have three procedures.
THE FIRST PROCEDURES
____________________________________________________________
Load PROCED1.PAS as your first example =================
program with a procedure and display it on PROCED1.PAS
your monitor. You will notice that it =================
doesn't look like anything you have seen
up to this point because it has procedures
Page 5-1
Procedures and Functions
in it. Lets go back to our definitions from above. The first
line is the Program Heading which should pose no difficulty.
The Declaration Part begins with the var statement in line 4
and continues down through and including all three procedures
ending in line 19. Lines 21 through 26 constitute the
Statement Part. It may seem strange that what appears to be
executable Pascal statements, and indeed they are executable
statements, are contained in the Declaration Part rather than
the Statement Part. This is because of the Pascal definition
and it will make sense when we have completed our study of
procedures and functions.
Continuing to examine PROCED1.PAS, we will make note of the
program itself, which is the Statement Part. The program, due
to the nature of Pascal and the carefully chosen procedure
names, clearly tells us what it will do. It will write a
header, eight messages, and an ending. The only problem we
are faced with is, how will it write these messages? This is
where the Declaration Part is called upon to define these
operations in detail. The Declaration Part contains the three
procedures which will completely define what is to be done by
the procedure calls in the main program.
DEFINITIONS GO IN THE DEFINITION PART
____________________________________________________________
It should be clear to you that the definitions of the
procedures should be in the Definition Part of the program
because that is exactly what they do. In the case of a var,
a variable is defined for later use by the main program, and
in the case of a procedure, the procedure itself is defined
for later use by the main program.
Let's arbitrarily pick one of the procedures, the first, and
examine it in detail. The first executable statement we come
to in the main program is line 22 and says simply,
Write_A_Header, followed by the usual statement separator, the
semicolon. This is a simple procedure call. When the
compiler finds this statement, it goes looking for a
predefined procedure of that name which it can execute. If
it finds one in the Declaration Part of the program, it will
execute that procedure. If it doesn't find a user defined
procedure, it will search the Pascal library for a system
defined procedure and execute it. The Write and Writeln
statements are system procedures, and you have already been
using them quite a bit, so procedures are not completely new
to you. If the compiler doesn't find the procedure defined
in either place, it will generate an error message. Depending
on which version of TURBO Pascal you are using, the system may
search several libraries to find the procedures called by the
program. Much more will be said about this later in this
tutorial.
Page 5-2
Procedures and Functions
HOW TO DEFINE & CALL A PROCEDURE
____________________________________________________________
To call a procedure, we simply need to state its name. To
define a simple procedure, we use the reserved word procedure
followed by its calling name, with a semicolon as a
terminator. Following the Procedure Heading, there is the
Declaration Part of the procedure followed by a body which is
nothing more than a compound statement bracketed by the
reserved words begin and end. This is identical to the
Statement Part of the main program except that the procedure
ends with a semicolon instead of a period. Any valid Pascal
statements can be put between the begin and end, and in fact,
there is no difference in what can be put in a procedure and
what can be put in the main program.
The program we are examining would be no different if we would
eliminate the first procedure completely and move the Writeln
contained in it down to the Statement Part in place of
Write_A_Header. If that is not clear, go back and reread the
last two paragraphs until it is.
Lines 23 and 24 will cause the procedure named Write_A_Message
to be called 8 times, each time writing a line of output to
the monitor. Suffice it to say at this time, that the value
of the variable Count, as defined here, is available globally,
meaning anywhere in the entire Pascal program. We will define
the scope of variables shortly. Finally, the last procedure
call is made, causing the ending message to be displayed, and
the program execution is complete.
THE UNBROKEN RULE OF PASCAL
____________________________________________________________
Having examined your first Pascal procedures, there is a fine
point that is obvious but could be easily overlooked. We
mentioned the unbroken rule of Pascal in an earlier chapter
and it must be followed here too. "Nothing can be used in
Pascal until it has been defined". The procedures must all
be defined ahead of any calls to them, once again emphasizing
the fact that they are part of the Declaration Part of the
program, not the Statement Part.
Compile and run PROCED1.PAS to verify that it does what you
expect it to do.
MORE PROCEDURE CALLS
____________________________________________________________
Assuming you have run PROCED1.PAS successfully and understand
its output, let's go on to PROCED2.PAS and examine it. In
this program we will see how to call a procedure and take
Page 5-3
Procedures and Functions
along some data for use within the =================
procedure. To begin with, notice that PROCED2.PAS
there are three procedure calls in the =================
Statement Part of the program and each has
an additional term not contained in the
calls in the last program, namely the variable name Index
within brackets. This is Pascal's way of taking a variable
parameter to the procedure when it is called.
You will notice that the variable Index is defined as an
integer type variable in the very top of the Declaration Part.
Since we are taking an integer type variable along when we
visit the procedure Print_Data_Out, it had better be expecting
an integer variable as input or we will have a type mismatch.
In fact, observing the procedure heading itself in line 7,
indicates that it is indeed expecting an integer variable but
it prefers to call the variable Puppy inside of the procedure.
Calling it something different poses no problem as long as the
main program doesn't try to call its variable Puppy, and the
procedure doesn't try to use the name Index. Both are
actually referring to the same piece of data but they simply
wish to refer to it by different names.
FORMAL AND ACTUAL PARAMETERS
____________________________________________________________
The parameters listed within the parentheses of the procedure
header are called the formal parameters, whereas the
parameters listed within the parentheses of the procedure call
are referred to as the actual parameters. Observe that the
next procedure is called with Index as the actual parameter
and the procedure prefers to use the name Cat as the formal
parameter name. In both cases, the procedures simply print
out the parameter passed to it, and each then try to modify
the value passed to it before passing it back. We will see
that one will be successful and the other will not.
We are in a loop in which Count is incremented from 1 to 3 and
Pascal does not allow us to modify the loop variable so we
make a copy of the value in line 21 and call it Index. We can
then modify Index in the main program if we desire.
CALL BY VALUE
____________________________________________________________
In line 7, the procedure heading does not contain the reserved
word var in front of the passed parameter and therefore the
parameter passing is only one way because of the way Pascal
is defined. Without the reserved word var in front of the
variable Puppy, the system makes a copy of Index, and passes
the copy to the procedure which can do anything with it, using
its new name, Puppy, but when control returns to the main
Page 5-4
Procedures and Functions
program, the original value of Index is still there. The copy
of Index named Puppy is modified in the procedure, but the
original variable named Index remains unchanged. You can
think of the passed parameter without the var as one way
parameter passing. This is a "call by value" because only the
value of the actual variable is passed to the procedure.
CALL BY REFERENCE
____________________________________________________________
In line 13, the second procedure has the reserved word var in
front of its desired name for the variable, namely Cat, so it
can not only receive the variable, it can modify it, and
return the modified value to the main program. A copy is not
made, but the original variable named Index is actually passed
to this procedure and the procedure can modify it, therefore
communicating with the main program. The formal parameter
name, Cat in the procedure, is actually another name for the
actual variable named Index in the main program. A passed
parameter with a var in front of it is therefore a two way
situation. This is a "call by reference" since the reference
to the original variable is passed to the procedure.
SOME NEW TERMINOLOGY
____________________________________________________________
To restate some of the new terminology in the last few
paragraphs, the parameter name in the calling program is
referred to as the actual parameter, and the parameter name
in the procedure is referred to as the formal parameter. In
the last example then, the actual parameter is named Index and
the formal parameter in the procedure is named Cat. It should
be pointed out that it is called a formal parameter whether
it is a "call by reference" or a "call by value". This
terminology is used in many other programming languages, not
only in Pascal.
When you run this program, you will find that the first
procedure is unable to return the value of 12 back to the main
program, but the second procedure does in fact succeed in
returning its value of 35 to the main program. Spend as much
time as you like studying this program until you fully
understand it. It should be noted that as many parameters as
desired can be passed to and from a procedure by simply making
a list separated by commas in the calls, and separated by
semicolons in the procedure. This will be illustrated in the
next example program.
Compile and run PROCED2.PAS and study the output. You should
be able to comprehend all of the output. If it is not clear,
reread the last few paragraphs.
Page 5-5
Procedures and Functions
To add to your knowledge of Pascal, =================
examine the program PROCED3.PAS for an PROCED3.PAS
example of a procedure call with more than =================
one variable in the call. Normally, you
would group the three input values
together to make the program more readable, but for purposes
of illustration, they are separated. Observe that the
variable Fruit is a two way variable because it is the 3rd
variable in the actual parameter list and corresponds to the
3rd formal parameter in the procedure header. Compile and run
PROCED3.PAS to see that it does what you expect it to do based
on the above explanation.
"CALL BY REFERENCE" OR "CALL BY VALUE"?
____________________________________________________________
It may seem to you that it would be a good idea to simply put
the word var in front of every formal parameter in every
procedure header to gain maximum flexibility, but using all
"call by references" could actually limit your flexibility.
There are two reasons to use "call by value" variables when
you can. The first is simply to shield some data from being
corrupted by the procedure. This is becoming a very important
topic in Software Engineering known as "information hiding"
and is the primary basis behind Object Oriented Programming
which will be discussed in chapters 15 and 16 of this
tutorial.
Secondly is the ability to use a constant in the procedure
call. Modify line 17 of PROCED3.PAS as follows;
Add_The_Fruit(12,Orange,Fruit,Pear);
and compile and run the program. Since Value1 is a "call by
value", the constant 12 can be used and the program will
compile and run. However, if you change line 17 to;
Add_The_Fruit(Apple,Orange,32,Pear);
you will find that it will not compile because Total is a
"call by reference" and the system must be able to return a
value for the formal parameter Total. It cannot do this
because 32 is a constant, not a variable. Even if no value
would be returned, the call by reference must have a variable
for the actual parameter. As a programming exercise, make
Value1 a call by reference by adding the word var in front of
it in line 6, and you will find that the compiler will not
allow you to replace the variable Apple with the constant 12
as was suggested earlier in this section.
The prior discussion should indicate to you that both "call
by value" and "call by reference" have a useful place in
Page 5-6
Procedures and Functions
Pascal programming and it is up to you to decide which you
should use.
When you are satisfied with the present illustration and you
have compiled and executed PROCED3.PAS several times to study
the results of the suggested changes, we will go on to study
the scope of variables using PROCED4.PAS.
A MULTIPLY DEFINED VARIABLE
____________________________________________________________
If you will examine PROCED4.PAS, you will =================
notice that the variable Count is defined PROCED4.PAS
twice, once in the main program var block =================
and once in the var block contained within
the procedure named Print_Some_Data. This
is perfectly legal and is within the Pascal definition.
The variable Index is defined only in the main program var
block and is valid anywhere within the entire Pascal program,
including the procedures and is said to be a global variable.
The variable Count is also defined in the main program var
block and is valid anywhere within the entire Pascal program,
except within the procedure where another variable is defined
with the same name Count. The two variables with the same
name are in fact, two completely different variables, one
being available only outside of the procedure and the other
being available only within the procedure. The variable
More_Stuff is defined within the procedure, so it is invisible
to the main program, since it is defined at a lower level than
that of the main program. It is only available for use within
the procedure in which it is defined.
Any variable is available at any point in the program
following its definition but only at the level of definition
or below. This means that any procedure in the Declaration
Part of a program can use any variable defined in the
Declaration Part of the program provided that the definition
occurs prior to the procedure. Any variable defined within
a procedure cannot be used by the main program since the
definition is at a lower level than the main program.
Be sure to compile and run PROCED4.PAS before continuing on
to the next example program.
PROCEDURES CALLING OTHER PROCEDURES
____________________________________________________________
Load and examine PROCED5.PAS to see an example of procedures
that call other procedures. Keep in mind that, "Nothing can
be used in Pascal until it has been previously defined", and
the order of procedures will be clear in this example. Note
Page 5-7
Procedures and Functions
that procedure Three calls procedure Two
which in turn calls procedure One. =================
PROCED5.PAS
Compile and run PROCED5.PAS and study the =================
output until you understand why it outputs
each line in the order that it does.
Now that you have a good working knowledge of procedures, we
need to make another important point. Remember that any
Pascal program is made up of three parts, the Program Heading,
the Declaration Part, and the Statement Part. The Declaration
Part is composed of five unique components, four of which we
will discuss in detail in the next chapter, and the last
component, which is composed of some number of procedures and
functions. We will cover functions in the next example, so
for now simply accept the fact that it is like a procedure.
A procedure is also composed of three parts, a Procedure
Heading, a Declaration Part, and a Statement Part. A
procedure, by definition, is therefore nothing more or less
than another complete Pascal program embedded within the main
program, and any number of procedures can be located in the
Declaration Part of the main program. These procedures are
all in a line, one right after another.
Since a procedure is defined like the main program, it would
seem to be possible to embed another procedure within the
Declaration Part of any procedure. This is perfectly valid
and is often done, but remember that the embedded procedure
can only be called by the procedure in which it is embedded,
not by the main program. This is a form of information hiding
which is becoming popular in modern software engineering.
The previous paragraph is probably a bit difficult to grasp.
Don't worry about it too much now, as you become proficient
as a Pascal programmer, you will very clearly see how embedded
procedures are used.
NOW LET'S LOOK AT A FUNCTION
____________________________________________________________
Now to keep a promise, let's examine the ================
program named FUNCTION.PAS to see what a FUNCTION.PAS
function is and how to use it. In this ================
very simple program, we have a function
that simply multiplies the sum of two
variables by 4 and returns the result. The major difference
between a function and a procedure is that the function
returns a single value and is called from within a
mathematical expression, a Writeln command, or anywhere that
it is valid to use a variable, since it is really a variable
itself. Observing the Function Heading of the function, in
line 6, you will notice that a function begins with the
reserved word function. Further observation reveals the two
Page 5-8
Procedures and Functions
input variables inside the parenthesis pair being defined as
integer variables, and following the parenthesis is a colon
and another integer. The last integer is used to define the
type of the variable being returned to the main program.
Any call to this function is actually replaced by an integer
value upon completion of the call. Therefore in line 14, the
function is evaluated and the value returned is used in place
of the function call. The result of the function is therefore
assigned to the variable named Feet.
Note that a function always returns a value and it may return
additional values if some of its formal parameters are defined
as "call by reference". Be sure to compile and run this
program.
NOW FOR THE MYSTERY OF RECURSION
____________________________________________________________
One of the great mysteries of Pascal and ================
several other popular programming RECURSON.PAS
languages, is the recursion of procedure ================
calls. Simply defined, recursion is the
ability of a procedure to call itself.
Examine the Pascal example file RECURSON.PAS for an example
of recursion. The main program is very simple, it sets the
variable named Count to the value 7 and calls the procedure
Print_And_Decrement. The procedure prefers to refer to the
variable by the name Index but that poses no problem for us
because we understand that the name of the formal parameter
can be any legal identifier. The procedure writes a line to
the video display with the value of Index written within the
line, and decrements the variable.
The if statement introduces the interesting part of this
program. If the variable is greater than zero, and it is now
6, then the procedure Print_And_Decrement is called once
again. This might seem to create a problem except for the
fact that this is perfectly legal in Pascal. Upon entering
the procedure the second time, the value of Index is printed
as 6, and it is once again decremented. Since it is now 5,
the same procedure will be called again, and it will continue
until the value of Index is reduced to zero when each
procedure call will be completed one at a time and control
will return to the main program.
ABOUT RECURSIVE PROCEDURES
____________________________________________________________
This is really a stupid way to implement this particular
program, but it is the simplest recursive program that can be
written and therefore the easiest to understand. You will
Page 5-9
Procedures and Functions
have occasional use for recursive procedures, so don't be
afraid to try them. Remember that the recursive procedure
must have some variable converging to something, or you will
have an infinite loop. Compile and run this program and
observe the value decrementing as the recursion takes place.
THE FORWARD REFERENCE
____________________________________________________________
Occasionally you will have a need to refer =================
to a procedure before you can define it. FORWARD.PAS
In that case you will need a forward =================
reference. The program FORWARD.PAS has an
example of a forward reference in it. In
this program, each one of the procedures calls the other, a
form of recursion. This program, like the last, is a very
stupid way to count from 7 to 0, but it is the simplest
program possible with the forward reference.
The first procedure, Write_A_Line, has its header defined in
exactly the same manner as any other procedure but instead of
the normal procedure body, only the reserved word forward is
given. This tells the compiler that the procedure will be
defined later. The next procedure is defined as usual, then
the body of Write_A_Line is given with only the reserved word
procedure and the procedure name. The variable reference has
been defined earlier. In this way, each of the procedure
names are defined before they are called.
It would be possible, by using the forward reference in great
numbers, to move the main program ahead of all procedure
definitions and have the program structured like some other
languages. This style of programming would be perfectly legal
as far as the compiler is concerned, but the resulting program
would be very nonstandard and confusing. You would do well
to stick with conventional Pascal formatting techniques and
use the forward reference sparingly. Be sure you compile and
run this program.
THE PROCEDURE TYPE IN TURBO PASCAL 5.X
____________________________________________________________
Examine the program named PROCTYPE.PAS ================
which can only be compiled and run if you PROCTYPE.PAS
are using TURBO Pascal version 5.x since ================
this is a new extension to the Pascal
language by Borland. In this program, the
procedure Do_Math is defined as a procedure type in line 12,
and three regular procedures are defined each of which have
the same structure of formal parameters. In the program,
since Do_Math is a procedure type that is compatible with each
of the defined procedures, it can be assigned one of the other
Page 5-10
Procedures and Functions
procedures, and a call to Do_Math is identical to a call to
that procedure to which it is currently assigned. The program
should be self explanatory with those few comments so you will
be left to study the details on your own.
Note the comments in lines 4 and 5 of the program. When using
a procedure type or a function type, which is the topic of the
next example program, TURBO Pascal requires that you use the
compiler directive F+, which forces the system to use far
calls for all procedure calls.
Examine the program named FUNCTYPE.PAS for ================
an example of a program using some of the FUNCTYPE.PAS
same techniques as the last program but ================
instead uses a function type for the
subprogram variable. This program should
be simple for you to study on your own concerning the details
of operation. The only rule concerning the procedure and
function types which must be stated, is that a subprogram type
variable can only be assigned subprogram names if the list of
actual parameters are identical for the type and the
subprogram. This includes the type of the return value for
a function. Since this is a new extension to the TURBO Pascal
languages, it has not been used much, so don't worry about it
too much. You should know that it can be done, because
someday you will find a piece of Pascal code with this
construct used. Of course, you will someday find a good use
for it yourself.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program to write your name, address, and phone
number with each Writeln in a different procedure.
2. Add a statement to the procedure in RECURSON.PAS to
display the value of Index after the call to itself so
you can see the value increasing as the recurring calls
are returned to the next higher level.
3. Rewrite TEMPCONV.PAS putting the centigrade to fahrenheit
formulas in a function call.
Page 5-11
CHAP06.TXT
Chapter 6
ARRAYS, TYPES, CONSTANTS, AND LABELS
ARRAYS
____________________________________________________________
At the beginning of this tutorial we said that a computer
program is composed of data and executable statements to do
something with that data. Having covered nearly all of the
programming statements, we must now go back and fill in some
gaps in our data definition and look at the array in
particular.
One of the most useful Pascal data ================
structures is the array, which is, in the ARRAYS.PAS
simplest terms, a group of 2 or more ================
identical terms, all having the same type.
Let's go directly to an example to see
what an array looks like. Display the Pascal program
ARRAYS.PAS and notice line 5 starting with the word
Automobiles. The variable Automobiles is defined as an
integer variable but in addition, it is defined to have twelve
different integer variables, namely Automobile[1],
Automobile[2], Automobile[3], .. Automobile[12].
The square braces are used in Pascal to denote a subscript for
an array variable. The array definition given in line 5 is
the standard definition for an array, namely a variable name,
followed by a colon and the reserved word array, with the
range of the array given in square brackets followed by
another reserved word of and finally the type of variable for
each element of the array.
USING THE ARRAY
____________________________________________________________
In using the elements of the array in a program, each of the
elements of the array are required to be used in exactly the
same manner as any simple variable having the same type. Each
time one of the variables is used, it must have the subscript
since the subscript is now part of the variable name. The
subscript moreover, must be of the type used in the definition
and it must be within the range defined or it will be
construed as an error.
Now consider the program itself. As Index is varied from 1
to 12, the range of the subscripts of the variable Automobile,
the 12 variables are set to the series of values 11 to 22.
Any integer values could be used, this was only a convenient
way to set the values to some well defined numbers. With the
values stored, a header is now printed and the list of values
contained in the array is printed. Note carefully that,
Page 6-1
Arrays, Types, Constants, and Labels
although the subscripts are limited to 1 through 12, the
values stored in each of the 12 variables are limited only by
the range of integers, namely -32768 to 32767. Review this
material and this program as long as needed to fully
understand it, as it is very important.
Keep in mind that the array is actually composed of 12
different integer type variables that can be used in any way
that it is legal to use any other integer type variable.
Compile and run this program.
DOUBLY INDEXED ARRAYS
____________________________________________________________
After understanding the above example =================
program, load the program ARRAYS2.PAS to ARRAYS2.PAS
see the next level of complexity of =================
arrays. You will see that Checkerboard is
defined as an array from 1 to 8, but
instead of it being a simple data type, it is itself another
array from 1 to 8 of type integer. The variable Checkerboard
is actually composed of 8 elements, each of which is 8
elements, leading to a total of 64 elements, each of which is
a simple integer variable. This is called a doubly
subscripted array and it can be envisioned in exactly the same
manner as a real checker board, an 8 by 8 matrix. Another way
to achieve the same end is to define the double array as in
the next line of the program where Value is defined as a total
of 64 elements.
To use either of the two variables in a program, we must add
two subscripts to the variable name to tell the program which
element of the 64 we desire to use. Examining the program
will reveal two loops, one nested within the other, and both
ranging in value from 1 to 8. The two loop indices can
therefore be used as subscripts of the defined array
variables. The variable Checkerboard is subscripted by both
of the loop indices and each of the 64 variables is assigned
a value as a function of the indices. The assigned value has
no real meaning other than to illustrate to you how it is
done. Since the value of Checkerboard is now available, it
is used to define some values to be used for the variable
Value in line 12 of the program.
After defining all of those variables, and you should
understand that we have defined a total of 128 variables in
the double loop, 64 of Checkerboard and 64 of Value, they can
be printed out. The next section of the program does just
that, by using another doubly nested loop, with a Write
statement in the center. Each time we go through the center
of the loop we tell it to print out one of the 64 variables
in the Checkerboard matrix with the indices Index and Count
Page 6-2
Arrays, Types, Constants, and Labels
defining which of the variables to write each time. Careful
study of the loop should reveal its exact operation.
After printing out the matrix defined by the variable
Checkerboard we still have the matrix defined by the variable
Value intact (In fact, we still have all of Checkerboard
available because we haven't changed any of it). Before
printing out the matrix defined by Value, let's change a few
of the elements just to see how it is done. The code in lines
24 to 26 simply change three of the variables to illustrate
that you can operate on all of the matrix in loops, or on any
part of the matrix in simple assignment statements. Notice
especially line 26, in which Value[3,6] (which was just set
to the value of 3), is used as a subscript. This is perfectly
legal since it is defined as a simple integer variable and is
within the range of 1 to 8, which is the requirement for a
subscript of the variable Value. The last part of the program
simply prints out the 64 values of the variable Value in the
same manner as above. Notice that when you run the program,
the three values are in fact changed as expected.
ARRAYS ARE FLEXIBLE
____________________________________________________________
A few more words about arrays before we go on. The arrays in
the last program were both defined to be square, namely 8 by
8, but that choice was purely arbitrary. The subscripts were
chosen to go from 1 to 8 but they could have been chosen to
go from 101 to 108 or any other range needed to clearly define
the problem at hand. And, as you may have guessed, you are
not limited to a doubly subscripted matrix but you can define
a variable with as many subscripts as you need to achieve your
desired end. There is a practical limit to the number of
subscripts because you can very quickly use up all of your
available memory with one large subscripted variable.
THE TYPE DEFINITION
____________________________________________________________
Now that you understand arrays, lets look =================
at a more convenient way to define them by TYPES.PAS
examining the Pascal file TYPES.PAS. You =================
will notice a new section at the beginning
of the listing which begins with the word
type. The word type is another reserved word which is used
at the beginning of a section to define "user-defined types".
Beginning with the simple predefined types we studied earlier,
we can build up as many new types as we need and they can be
as complex as we desire. The six names (from Array_Def to
Boat) in the type section are not variables, but are defined
to be types and can be used in the same manner as we use
integer, byte, real, etc.
Page 6-3
Arrays, Types, Constants, and Labels
PASCAL CHECKS TYPES VERY CAREFULLY
____________________________________________________________
This is a very difficult concept, but a very important one.
The Pascal compiler is very picky about the types you use for
variables in the program, doing lots of checking to insure
that you do not use the wrong type anywhere in the program.
Because it is picky, you could do very little without the
ability to define new types when needed, and that is the
reason Pascal gives you the ability to define new types to
solve a particular problem.
Some of these types are used in the var declaration part of
the program. Notice that since Airplane is an array of
Dog_Food and Dog_Food is in turn an array of boolean, then
Airplane defines a doubly subscripted array, each element
being a boolean variable. This does not define any variables,
only a user defined type, which can be used in a var to define
a matrix of boolean variables. This is in fact done in the
definition of Puppies, which is an array composed of 72 (6
times 12) boolean variables. In the same manner, Stuff is
composed of an array of 14 variables, each being an integer
variable. The elements of the array are, Stuff[12],
Stuff[13], .. Stuff[25]. Notice also that Stuff2 is also
defined in exactly the same manner and is also composed of 14
variables.
Careful inspection will reveal that Kitties is a variable
which has the same definition as Puppies. It would probably
be poor programming practice to define them in different
manners unless they were in fact totally disassociated. In
this example program, it serves to illustrate some of the ways
user-defined types can be defined. Be sure to compile and run
this program.
IS THE CONCEPT OF "TYPES" IMPORTANT?
____________________________________________________________
If you spend the time to carefully select the types for the
variables used in the program, the Pascal compiler will do
some debugging for you since it is picky about the use of
variables with different types. Any aid you can use to help
find and remove errors from your program is useful and you
should learn to take advantage of type checking. The type
checking in Pascal is relatively weak compared to some other
languages such as Modula-2 or Ada, but still very useful.
In a tiny program like this example, the value of the type
declaration part cannot be appreciated, but in a large program
with many variables, the type declaration can be used to great
advantage. This will be illustrated later.
Page 6-4
Arrays, Types, Constants, and Labels
THE CONSTANT DECLARATION
____________________________________________________________
Examining the Pascal example program ================
CONSTANT.PAS will give us an example of a CONSTANT.PAS
constant definition. The reserved word ================
const is the beginning of the section that
is used to define constants that can be
used anyplace in the program as long as they are consistent
with the required data typing limitations. In this example,
Max_Size is defined as a constant with the value of 12. This
is not a variable and cannot be changed in the program, but
is still a very valuable number. For the moment ignore the
next two constant definitions. As we inspect the type
declarations, we see two user-defined types, both of which are
arrays of size 1 to 12 since Max_Size is defined as 12. Then
when we get to the var declaration part, we find five
different variables, all defined as arrays from 1 to 12 (some
are type integer and some are type char). When we come to the
program we find that it is one big loop which we go through
12 times because the loop is executed Max_Size times.
In the above definition, there seems to be no advantage to
using the constant, and there is none, until you find that for
some reason you wish to increase the range of all arrays from
12 to 18. In order to do so, you only need to redefine the
value of the constant, recompile, and the whole job is done.
Without the constant definition, you would have had to change
all type declarations and the upper limit of the loop in the
program. Of course that would not be too bad in the small
example program, but could be a real mess in a 2000 line
program, especially if you missed changing one of the 12's to
an 18. That would be a good example of data in and garbage
out. This program should give you a good idea of what the
constant can be used for, and as you develop good programming
techniques, you will use the constant declaration to your
advantage.
THE TURBO PASCAL TYPED CONSTANT
____________________________________________________________
We skipped over the second and third constant declarations for
a very good reason. They are not constant declarations.
TURBO Pascal has defined, as an extension, the "typed
constant". Using the syntax shown, Index_Start is defined as
an integer type variable and is initialized to the value of
49. This is a true variable and can be used as such in the
program. The same effect can be achieved by simply defining
Index_Start as an integer type variable in the var declaration
part and setting it to the value of 49 in the program itself.
Since it does not really fit the definition of a constant,
it's use is discouraged until you gain experience as a Pascal
Page 6-5
Arrays, Types, Constants, and Labels
programmer. Until then it will probably only be confusing to
you. In like manner, Check_It_Out is a boolean type variable
initialized to the value TRUE. It is not a constant.
The typed constants defined in the last paragraph have one
additional characteristic, they are initialized only once,
when the program is loaded. Even when used in a procedure or
function, they are only initialized when the program is
loaded, not upon each call to the procedure or function.
Don't worry too much about this at this point, when you gain
experience with Pascal, you will be able to use this
information very effectively.
THE LABEL DECLARATION
____________________________________________________________
Finally, the example program LABELS.PAS ================
will illustrate the use of labels. In the LABELS.PAS
Pascal definition, a label is a number ================
from 0 to 9999 that is used to define a
point in the program to which you wish to
jump. All labels must be defined in the label definition part
of the program before they can be used. Then a new reserved
word goto is used to jump to that point in the program. The
best way to see how the goto is used with labels is to examine
the program before you.
TURBO Pascal has an extension for labels. Any valid
identifier, such as used for variables, can be used as a label
in addition to the values from 0 to 9999. These are
illustrated in the example program.
When you compile and run this program, the output will look
a little better than the program does.
THE PACKED ARRAY
____________________________________________________________
When Pascal was first defined in 1971, many of the computers
in use at that time used very large words, 60 bits being a
typical word size. Memory was very expensive, so large
memories were not too common. A Pascal program that used
arrays was inefficient because only one variable was stored
in each word. Most of the bits in each word were totally
wasted, so the packed array was defined in which several
variables were stored in each word. This saved storage space
but took extra time to unpack each word to use the data. The
programmer was given a choice of using a fast scheme that
wasted memory, the array, or a slower scheme that used memory
more efficiently, the packed array.
Page 6-6
Arrays, Types, Constants, and Labels
The modern microcomputer has the best of both schemes, a short
word, usually 16 bits, and a large memory. The packed array
is therefore not even implemented in many compilers and will
be ignored during compilation. The packed array is
specifically ignored by all versions of TURBO Pascal.
ONE MORE TURBO PASCAL EXTENSION
____________________________________________________________
Standard Pascal, as defined by Nicklaus Wirth, requires that
the various fields in the definition part of the program come
in a specific order and each must appear only once. The
specific order is, label, const, type, var, and finally the
procedures and functions. Of course, if any are not needed,
they are simply omitted. This is a rather rigid requirement
but it was required by the pure Pascal definition probably to
teach good programming techniques to beginning students.
All versions of TURBO Pascal are not nearly as rigid as the
standard Pascal requirement. You are permitted to use the
fields in any order and as often as you wish provided that you
define everything before you use it, which is the unbroken
rule of Pascal. It sometimes makes sense to define a few
variables immediately after their types are defined to keep
them near their type definitions, then define a few more types
with the variables that are associated with them also. TURBO
Pascal gives you this extra flexibility that can be used to
your advantage.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program to store the integers 201 to 212 in an
array then display them on the monitor.
2. Write a program to store a 10 by 10 array containing the
products of the indices, therefore a multiplication
table. Display the matrix on the video monitor.
3. Modify the program in 2 above to include a constant so
that by simply changing the constant, the size of the
matrix and the range of the table will be changed.
Page 6-7
CHAP07.TXT
Chapter 7
STRINGS & STRING PROCEDURES
PASCAL STRINGS
____________________________________________________________
According to the Pascal definition, a ================
string is simply an array of 2 of more STRARRAY.PAS
elements of type char, and is contained in ================
an array defined in a var declaration as a
fixed length. Look at the example program
STRARRAY.PAS. Notice that the strings are defined in the type
declaration even though they could have been defined in the
var part of the declaration. This is to begin getting you
used to seeing the type declaration. The strings defined here
are nothing more than arrays with char type variables.
A STRING IS AN ARRAY OF CHAR
____________________________________________________________
The interesting part of this file is the executable program.
Notice that when the variable First_Name is assigned a value,
the value assigned to it must contain exactly 10 characters
or the compiler will generate an error. Try editing out a
blank and you will get an invalid type error. Pascal is neat
in allowing you to write out the values in the string array
without specifically writing each character in a loop as can
be seen in the Writeln statement. To combine the data, called
concatenation, requires the use of the rather extensive
looping and subscripting seen in the last part of the program.
It would be even messier if we were to consider variable
length fields which is nearly always the case in a real
program.
Two things should be observed in this program. First, notice
the fact that the string operations are truly array operations
and will follow all of the characteristics discussed in the
last chapter. Secondly, it is very obvious that Pascal is
rather weak when it comes to its handling of text type data.
Pascal will handle text data, even though it may be difficult
to do so using the standard description of Pascal as
illustrated in this program. We will see next that TURBO
Pascal really shines here. Compile and run STRARRAY.PAS and
observe the output.
THE TURBO PASCAL STRING TYPE
____________________________________________________________
Look at the example program STRINGS.PAS. You will see a much
more concise program that actually does more. TURBO Pascal
has, as an extension to standard Pascal, the string type of
Page 7-1
Strings and String Procedures
variable. It is used as shown, and the =================
number in the square brackets in the var STRINGS.PAS
declaration is the maximum length of the =================
string. In actual use in the program, the
variable can be used as any length from
zero characters up to the maximum given in the declaration.
The variable First_Name, for example, actually has 11
locations of storage for its data. The current length is
stored in First_Name[0] and the data is stored in
First_Name[1] through First_Name[10]. All data are stored as
byte variables, including the size, so the length is therefore
limited to a maximum of 255 characters.
STRINGS HAVE VARIABLE LENGTHS
____________________________________________________________
Now look at the program itself. Even though the variable
First_Name is defined as 10 characters long, it is perfectly
legal to assign it a 4 character constant, with First_Name[0]
automatically set to 4 by the system and the last six
characters undefined and unneeded. When the program is run
the three variables are printed out all squeezed together
indicating that the variables are indeed shorter than their
full size as defined in the var declaration.
Using the string type is even easier when you desire to
combine several fields into one as can be seen in the
assignment to Full_Name. Notice that there are even two
blanks, in the form of constant fields, inserted between the
component parts of the full name. When it is written out, the
full name is formatted neatly and is easy to read. Compile
and run STRINGS.PAS and observe the output.
WHAT'S IN A STRING TYPE VARIABLE?
____________________________________________________________
The next example program named ================
WHATSTRG.PAS, is intended to show you WHATSTRG.PAS
exactly what is in a string variable. ================
This program is identical to the last
program except for some added statements
at the end. Notice the assignment to Total. The function
Length is available in TURBO Pascal to return the current
length of any string type variable. It returns a byte type
variable with the value contained in the [0] position of the
variable. We print out the number of characters in the string
at this point, and then print out each character on a line by
itself to illustrate that the TURBO Pascal string type
variable is simply an array variable.
Page 7-2
Strings and String Procedures
The TURBO Pascal reference manual has a full description of
several more procedures and functions for use with strings
which are available in TURBO Pascal only. Refer to your TURBO
Pascal version 3.0 reference manual in chapter 9, beginning
on page 67, or if you are using TURBO Pascal version 4.0, you
will find the string functions throughout chapter 27. The
TURBO Pascal Reference Guide for version 5.X has a list of
string procedures and functions on page 120. The use of these
should be clear after you grasp the material covered here.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program in which you store your first, middle,
and last names as variables, then display them one to a
line. Concatenate the names with blanks between them and
display your full name as a single variable.
Page 7-3
CHAP08.TXT
Chapter 8
SCALARS, SUBRANGES, AND SETS
PASCAL SCALARS
____________________________________________________________
A scalar, also called an enumerated type, =================
is a list of values which a variable of ENTYPES.PAS
that type may assume. Look at the Pascal =================
program ENTYPES.PAS for an example of some
scalars. The first type declaration
defines Days as being a type which can take on any one of
seven values. Since, within the var declaration, Day is
assigned the type of Days, then Day is a variable which can
assume any one of seven different values. Moreover Day can
be assigned the value Mon, or Tue, etc., which is considerably
clearer than using 0 to represent Monday, 1 for Tuesday, etc.
This makes the program easier to follow and understand.
Internally, Pascal does not actually assign the value Mon to
the variable Day, but it uses an integer representation for
each of the names. This is important to understand because
you need to realize that you cannot print out Mon, Tue, etc.,
but can only use them for indexing control statements.
The second line of the type definition defines Time_Of_Day as
another scalar which can have any of four different values,
namely those listed. The variable Time can only be assigned
one of four values since it is defined as the type
Time_Of_Day. It should be clear that even though it can be
assigned Morning, it cannot be assigned Morning_time or any
other variant spelling of Morning, since it is simply another
identifier which must have an exact spelling to be understood
by the compiler. Several real variables are defined to allow
us to demonstrate the use of the scalar variables. After
writing a header in lines 16 through 20, the real variables
are initialized to some values that are probably not real life
values, but will serve to illustrate the scalar variable.
A BIG SCALAR VARIABLE LOOP
____________________________________________________________
The remainder of the program is one large loop being
controlled by the variable Day as it goes through all of its
values, one at a time. Note that the loop could have gone
from Tue to Sat or whatever portion of the range desired, it
does not have to go through all of the values of Day. Using
Day as the case variable, the name of one of the days of the
week is written out each time we go through the loop. Another
loop controlled by Time is executed four times, once for each
value of Time. The two case statements within the inner loop
are used to calculate the total pay rate for each time period
Page 8-1
Scalars, Subranges, and Sets
and each day. The data is formatted carefully to make a nice
looking table of pay rates as a function of Time and Day.
Take careful notice of the fact that the scalar variables
never entered into the calculations, and they were not printed
out. They were only used to control the flow of logic. It
was much neater than trying to remember that Mon is
represented by a 0, Tue is represented by a 1, etc. In fact,
those numbers are used for the internal representation of the
scalars but we can relax and let Pascal worry about the
internal representation of our scalars. Compile and run this
program and observe the output.
LET'S LOOK AT SOME SUBRANGES
____________________________________________________________
Examine the program SUBRANGE.PAS for an ================
example of subranges and some additional SUBRANGE.PAS
instruction on scalar variables. It may ================
be expedient to define some variables that
only cover a part of the full range as
defined in a scalar type. Notice that Days is declared a
scalar type as in the last program, and Work is declared a
type with an even more restricted range. In the var
declaration, Day is once again defined as the days of the week
and can be assigned any of the days by the program. The
variable Workday, however, is assigned the type Work, and can
only be assigned the days Mon through Fri. If an attempt is
made to assign Workday the value Sat, a run-time error will
be generated. A carefully written program will never attempt
that, and it would be an indication that something is wrong
with either the program or the data. This is one of the
advantages of Pascal over older languages and is a reason for
the relatively strong type checking built into the language.
Further examination will reveal that Index is assigned the
range of integers from 1 through 12. During execution of the
program, if an attempt is made to assign Index any value
outside of that range, a run time error will be generated.
Suppose the variable Index was intended to refer to your
employees, and you have only 12. If an attempt was made to
refer to employee number 27, or employee number -8, there is
clearly an error somewhere in the data and you would want to
stop running the payroll to fix the problem. Pascal would
have saved you a lot of grief.
SOME STATEMENTS WITH ERRORS IN THEM.
____________________________________________________________
In order to have a program that would compile without errors,
and yet show some errors, the section of the program in lines
Page 8-2
Scalars, Subranges, and Sets
16 through 27 is not really a part of the program since it is
within a comment area. This is a trick to remember when you
are debugging a program, a troublesome part can be commented
out until you are ready to include it with the rest of the
code. The errors are self explanatory and it would pay for
you to spend enough time to understand each of the errors.
There are seven assignment statements as examples of subrange
variable use in lines 29 through 35. Notice that the variable
Day can always be assigned the value of either Workday or
Weekend, but the reverse is not true because Day can assume
values that would be illegal to assign to the others.
THREE VERY USEFUL FUNCTIONS
____________________________________________________________
Lines 37 through 42 of the example program demonstrate the use
of three very important functions when using scalars. The
first is the Succ function that returns the value of the
successor to the scalar used as an argument, the next value.
If the argument is the last value, a run time error is
generated. The next function is the Pred function that
returns the predecessor to the argument of the function.
Finally the Ord function which returns the ordinal value of
the scalar.
All scalars have an internal representation starting at 0 and
increasing by one until the end is reached. In our example
program, Ord(Day) is 5 if Day has been assigned Sat, but
Ord(Weekend) is 0 if Weekend has been assigned Sat. As you
gain experience in programming with scalars and subranges, you
will realize the value of these three new functions.
A few more thoughts about subranges are in order before we go
on to another topic. A subrange is always defined by two
predefined constants, and is always defined in an ascending
order. A variable defined as a subrange type is actually a
variable defined with a restricted range. Good programming
practice would dictate that subranges should be used as often
as possible in order to prevent garbage data. There are
actually very few variables ever used that cannot be
restricted by some amount. The limits may give a hint at what
the program is doing and can help in understanding the program
operation. Subrange types can only be constructed using the
simple types, integer, char, byte, or scalar.
Compile and run this program even though it has no output.
Add some output statements to see what values some of the
variables assume.
Page 8-3
Scalars, Subranges, and Sets
SETS
____________________________________________________________
Now for a new topic, sets. Examining the ================
example Pascal program SETS.PAS will SETS.PAS
reveal some sets. A scalar variable is ================
defined first, in this case the scalar
type named Goodies. A set is then defined
with the reserved words set of followed by a predefined scalar
type. Several variables are defined as sets of Treat, after
which they can individually be assigned portions of the entire
set.
Consider the variable Ice_Cream_Cone which has been defined
as a set of type Treat. This variable is composed of as many
elements of Goodies as we care to assign to it. In the
program, we define it as being composed of Ice_Cream, and
Cone. The set Ice_Cream_Cone is therefore composed of two
elements, and it has no numerical or alphabetic value as most
other variables have.
In lines 21 through 26, you will see 4 more delicious deserts
defined as sets of their components. Notice that the banana
split is first defined as a range of terms, then another term
is added to the group illustrating how you can add to a set.
All five are combined in the set named Mixed, then Mixed is
subtracted from the entire set of values to form the set of
ingredients that are not used in any of the deserts. Each
ingredient is then checked to see if it is in the set of
unused ingredients, and printed out if it is. Note that in
is another reserved word in Pascal. Running the program will
reveal a list of unused elements.
In this example, better programming practice would have
dictated defining a new variable, possibly called Remaining
for the ingredients unused in line 32. It was desirable to
illustrate that Mixed could be assigned a value based on
subtracting itself from the entire set, so the poor variable
name was used.
When you compile and run this program you will see that this
example results in some nonsense results but hopefully it led
your thinking toward the fact that sets can be used for
inventory control, possibly a parts allocation scheme, or some
other useful system.
SEARCHING WITH SETS
____________________________________________________________
The Pascal program FINDCHRS.PAS is more useful than the last
one. In it we start with a short sentence and search it for
all lower case alphabetic letters and write a list of those
used. Since we are using a portion of the complete range of
Page 8-4
Scalars, Subranges, and Sets
char, we do not need to define a scalar ================
before defining the set, we can define the FINDCHRS.PAS
set using the range 'a'..'z'. The set ================
Data_Set is assigned the value of no
elements in the first statement of the
program, and the print string, named Print_Group, is set to
blank in the next. The variable Storage is assigned the
sentence to search, and the search loop is begun. Each time
through the loop, one of the characters is checked. It is
either declared as a non-lower-case character, as a repeat of
one already found, or as a new character to be added to the
list.
You are left to decipher the details of the program, which
should be no problem since there is nothing new here. Run the
program and observe how the list grows with new letters as the
sentence is scanned.
PROGRAMMING EXERCISE
____________________________________________________________
1. Modify FINDCHRS.PAS to search for upper-case letters.
Page 8-5
CHAP09.TXT
Chapter 9
RECORDS
A VERY SIMPLE RECORD
____________________________________________________________
We come to the grandaddy of all data ================
structures in Pascal, the record. A SMALLREC.PAS
record is composed of a number of ================
variables any of which can be of any
predefined data type, including other
records. Rather than spend time trying to define a record in
detail, lets go right to the first example program,
SMALLREC.PAS. This is a program using nonsense data that will
illustrate the use of a record.
There is only one entry in the type declaration part of the
program, the record identified by the name Description. The
record is composed of three fields, the Year, Model, and
Engine variables. Notice that the three fields are each of
a different type, indicating that the record can be of mixed
types. You have a complete example of the way a record is
defined before you. It is composed of the identifier
Description, the = sign, the reserved word record, the list
of elements, and followed by the reserved word end. This is
one of the places in Pascal where an end is used without a
corresponding begin. Notice that this only defines a type,
it does not define any variables. That is done in the var
declaration where the variable Truck is defined as a record
of type Description and Cars is defined to have 10 complete
records of the type Description. The variable Truck has three
components, Year, Model, and Engine, and any or all of these
components can be used to store data pertaining to Truck.
When assigning data to the variable Truck, for example, there
are actually three parts to the variable, so we use three
assignment statements, one for each of the sub-fields. In
order to assign values to the various sub-fields, the variable
name is followed by the sub-field name with a separating
period. The "var.sub_field" combination is a variable name.
Keep in mind that Truck is a complete record containing three
variables, and to assign or use one of the variables, you must
designate which sub-field you are interested in. Examine
lines 16 through 18 of the program where the three fields are
assigned meaningless data for illustration. The Year field
is assigned an integer number, the Model field is assigned the
name Pickup, and the Engine variable is assigned the value
Diesel.
A loop is then used to assign data to all 10 records of Car.
In order to further illustrate that there are actually 30
Page 9-1
Records
variables in use here, a few are changed at random in lines
26 through 30, being very careful to maintain the required
types as defined in the type declaration part of the program.
Finally, all ten composite variables, consisting of 30 actual
variables in a logical grouping are printed out using the same
"var.sub-field" notation described above.
If the preceding description of a record is not clear in your
mind, review it very carefully. It's a very important concept
in Pascal, and you won't have a hope of a chance of
understanding the next example until this one is clear. Be
sure to compile and run SMALLREC.PAS so you can study the
output.
A SUPER RECORD
____________________________________________________________
Examine the Pascal example file BIGREC.PAS ================
for a very interesting record. First we BIGREC.PAS
have a constant defined. Ignore it for ================
the moment, we will come back to it later.
Within the type declaration we have three
records defined, and upon close examination, you will notice
that the first two records are included as part of the
definition of the third record. The record identified as
Person, actually contains 9 variable definitions, three within
the Full_Name record, three of its own, and three within the
Date record. This is a type declaration and does not actually
define any variables, that is done in the var part of the
program.
The var part of the program defines some variables beginning
with the array of Friend containing 50 (because of the
constant definition in the const part) records of the user
defined type, Person. Since the type Person defines 9 fields,
we have now defined 9 times 50 = 450 separate and distinct
variables, each with its own defined type. Remember that
Pascal is picky about assigning data by the correct type.
Each of the 450 separate variables has its own type associated
with it, and the compiler will generate an error if you try
to assign any of those variables the wrong type of data.
Since Person is a type definition, it can be used to define
more than one variable, and in fact it is used again to define
three more records, Self, Mother, and Father. These three
records are each composed of 9 variables, so we have 27 more
variables which we can manipulate within the program. Finally
we have the variable Index defined as a simple byte type
variable.
Page 9-2
Records
HOW TO MANIPULATE ALL OF THAT DATA
____________________________________________________________
In the program we begin by assigning data to all of the fields
of Self in lines 31 through 43. Examining the first three
statements of the main program, we see the construction we
learned in the last example program being used, namely the
period between descriptor fields. The main record is named
Self, and we are interested in the first part of it,
specifically the Name part of the Person record. Since the
Name part of the Person record is itself composed of three
parts, we must designate which component of it we are
interested in. Self.Name.First_Name is the complete
description of the first name of Self and is used in the
assignment statement in line 31 where it is assigned the name
of "Charley". The next two fields are handled in the same way
and are self explanatory.
WHAT IS THE WITH STATEMENT?
____________________________________________________________
Continuing on to the fourth field, the City, there are only
two levels required because City is not another record
definition. The fourth field is therefore completely defined
by Self.City. Notice the with Self do statement. This is a
shorthand notation used with record definitions to simplify
coding. From the begin in line 34 to the matching end in line
43, any variables within the Self record are used as though
they had a Self. in front of them. It greatly simplifies
coding to be able to omit the leading identifier within the
with section of code. You will see that City, State, and
Zipcode are easily assigned values without further reference
to the Self variable. When we get to the Day part of the
birthday, we are back to three levels and the complete
definition is Self.Birthday.Day but once again, the Self. part
is taken care of automatically because we are still within the
with Self do area.
To illustrate the with statement further, another is
introduced in line 39, with Birthday do, and an area is
defined by the begin end pair which extends from line 39
through line 42. Within this area both leading identifiers
are handled automatically to simplify coding, and Month is
equivalent to writing Self.Birthday.Month if both with
statements were removed.
HOW FAR DOWN CAN YOU NEST THE WITH STATEMENT?
____________________________________________________________
You may be wondering how many levels of nesting are allowed
in record definitions. There doesn't appear to be a limit
according to the Pascal definition, but we do get a hint at
Page 9-3
Records
how far it is possible to go. In TURBO Pascal, you are
allowed to have with statements nested to nine levels, and it
would be worthless to nest with statements deeper than the
level of records. Any program requiring more levels than nine
is probably far beyond the scope of your programming ability,
and mine, for a long time.
After assigning a value to Year, the entire record of Self is
defined, all nine variables. It should be pointed out that
even though Self is composed of nine separate variables, it
is proper to call Self a variable itself because it is a
record variable.
SUPER-ASSIGNMENT STATEMENTS
____________________________________________________________
The statement in line 45, "Mother := Self;" is very
interesting. Since both of these are records, both are the
same type of record, and both therefore contain 9 variables,
Pascal is smart enough to recognize that, and assign all nine
values contained in Self to the corresponding variables of
Mother. So after one statement, the record variable Mother
is completely defined. The statement in line 46 assigns the
same values to the nine respective variables of Father, and
the next two lines assign all 50 Friend variables the same
data. By this point in the program, we have therefore
generated 450 + 27 = 477 separate pieces of data so far in
this program. We could print it all out, but since it is
nonsense data, it would only waste time and paper. Lines 49
through 52 write out three sample pieces of the data for your
inspection.
WHAT GOOD IS ALL OF THIS
____________________________________________________________
It should be obvious to you that what this program does, even
though the data is nonsense, appears to be the beginning of
a database management system, which indeed it is. Instead of
assigning nonsense data, a list could be read in and stored
for manipulation. It is a crude beginning, and has a long way
to go to be useful, but you should see a seed for a useful
program.
Now to go back to the const in line 4 as promised. The number
of friends was defined as 50 and used for the size of the
array and in the assignment loop in line 47. You can now edit
this number and see how big this database can become on your
computer. If you are using TURBO Pascal, you will be limited
to slightly more than 1000 because of the 64K limitation of
an executable program, and the fact that all of this data is
stored within that 64K boundary. It should be noted that
TURBO Pascal 4.0 or 5.x allows a program larger than 64K but
Page 9-4
Records
still places a limitation of 64K on each compilation unit.
See how big you can make the number of friends before you get
the memory overflow message. Keep the number in mind because
when we get to the chapter on Pointers and Dynamic Allocation,
you will see a marked increase in allowable size, especially
if you have a large amount of RAM installed in your computer.
A VARIANT RECORD
____________________________________________________________
If any part of this chapter is still unclear, it would be good
for you to go back and review it at this time. The next
example will really tax your mind to completely understand it,
and this will be true especially if the prior material is not
clear.
Examine the Pascal program VARREC.PAS for ================
an example of a program with a variant VARREC.PAS
record definition. In this example, we ================
first define a scalar type, namely
Kind_Of_Vehicle for use within the record.
Then we have a record defining Vehicle, intended to define
several different vehicles, each with different kinds of data.
It would be possible to define all variables for all types of
vehicles, but it would be a waste of storage space to define
the number of tires for a boat, or the number of propeller
blades used on a car or truck. The variant record lets us
define the data precisely for each vehicle without wasting
data storage space.
WHAT IS A TAG-FIELD?
____________________________________________________________
In the record definition we have the usual record header
followed by three variables defined in the same manner as the
records in the last two example programs. Then we come to the
case statement. Following this statement, the record is
different for each of the four types defined in the associated
scalar definition. The variable What_Kind is called the
tag-field and must be defined as a scalar type prior to the
record definition. The tag-field is used to select the
variant, when the program uses one of the variables of this
record type. The tag-field is followed by a colon and its
type definition, then the reserved word of. A list of the
variants is then given, with each of the variants having the
variables for its particular case defined. The list of
variables for one variant is called the field list.
A few rules are in order at this point. The variants do not
have to have the same number of variables in each field list,
and in fact, one or more of the variants may have no variables
at all in its variant part. If a variant has no variables,
Page 9-5
Records
it must still be defined with a pair of empty parentheses
followed by a semi-colon. All variables in the entire variant
part must have unique names. The three variables, Wheels,
Tires, and Tyres, all mean the same thing to the user, but
they must be different for the compiler. You may use the same
identifiers again in other records and for simple variables
anywhere else in the program. The Pascal compiler can tell
which variable you mean by its context. Using the same
variable name should be discouraged as bad programming
practice because it may confuse you or another person trying
to understand your program at a later date.
The final rule is that the variant part of the record must be
the last part of it, and in fact, the last part of any or all
variants can itself have a variant part to it. That is
getting pretty advanced for our level of use of Pascal at this
time however.
USING THE VARIANT RECORD
____________________________________________________________
We properly define four variables with the record type Vehicle
in line 22 and go on to examine the program itself.
We begin by defining one of our variables of type Vehicle,
namely the variable named Ford. The seven lines assigning
values to Ford are similar to the prior examples with the
exception of line 28. In that line the tag-field which
selects the particular variant used is set equal to the value
Truck, which is a scalar definition, not a variable. This
means that the variables named Motor, Tires, and Payload are
available for use with the record Ford, but the variables
named Wheels, Engine, Tyres, etc. are not available in the
record named Ford.
Next, we will define the record Sunfish as a Boat, and define
all of its variables in lines 33 through 41. All of Sunfish's
variables are defined but in a rather random order to
illustrate that they need not be defined in a particular
order. You should remember the with statement from the last
example program.
To go even further in randomly assigning the variables to a
record, we redefine Ford as having an Engine which it can only
have if it is a car. This is one of the fine points of the
Pascal record. If you assign any of the variant variables,
the record is changed to that variant, but it is the
programmers responsibility to assign the correct tag-field to
the record, not Pascal's. Good programming practice would be
to assign the tag-field before assigning any of the variant
variables. The remainder of the Ford variables are assigned
to complete that record, the non-variant part remaining from
the last assignment.
Page 9-6
Records
The variable Mac is now set equal to the variable Sunfish in
line 48. All variables within the record are copied to Mac
including the tag-field, making Mac a Boat.
NOW TO SEE WHAT WE HAVE IN THE RECORDS
____________________________________________________________
We have assigned Ford to be a car, and two boats exist, namely
Sunfish and Mac. Since Schwinn was never defined, it has no
data in it, and is at this point useless. The Ford tag-field
has been defined as a car, so it should be true in the if
statement, and the message in line 51 should print. The
Sunfish is not a bicycle, so it will not print. The Mac has
been defined as a boat in the single assignment statement, so
it will print a message with an indication that all of the
data in the record was transferred to its variables.
Even though we can make assignment statements with records,
they cannot be used in any mathematical operations such as
addition, or multiplication. They are simply used for data
storage. It is true however, that the individual elements in
a record can be used in any mathematical statements legal for
their respective types.
One other point should be mentioned. The tag-field can be
completely eliminated resulting in a "free union" variant
record. This is possible because Pascal, as you may remember
from above, will automatically assign the variant required
when you assign data to one of the variables within a variant.
This is the reason that all variables within any of the
variants must have unique names. The free union record should
be avoided in your early programming efforts because you
cannot test a record to see what variant it has been assigned
to it. It is definitely an advanced technique in Pascal.
Be sure you compile and run VARREC.PAS and study the output
until you understand it completely.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a simple program with a record to store the names
of five of your friends and display the names.
Page 9-7
CHAP10.TXT
Chapter 10
STANDARD INPUT/OUTPUT
WE'VE USED THIS ALREADY
____________________________________________________________
During the course of this tutorial we have been using the
Write and Writeln procedures to display data, and it is now
time to discuss them fully. Actually there is little to be
said about them that has not already been said, but in order
to get all of the data in one place, they will be redefined
here.
As mentioned earlier, Write and Writeln ================
are not actually reserved words but are WRITELNX.PAS
procedure calls. They are therefore ================
merely identifiers that could be changed,
but there should never be a reason to do
so. Let's get on to our first example program WRITELNX.PAS
which has lots of output.
MANY OUTPUT STATEMENTS
____________________________________________________________
Pascal has two output statements with only slight differences
in the way they work. The Writeln statement outputs all of
the data specified within it, then returns the cursor to the
beginning of the next line. The Write statement outputs all
of the data specified within it, then leaves the cursor at the
next character where additional data can be output. The Write
statement can therefore be used to output a line in bits and
pieces if desired for programming convenience. The first
example program for this chapter, WRITELNX.PAS, has many
output statements for your observation. All outputs are
repeated so you can observe where the present field ends and
the next starts.
Observe the two integer output statements in lines 13 and 14.
The first simply directs the system to output Index twice, and
it outputs the value with no separating blanks. The second
statement says to output Index twice also, but it instructs
the system to put each output in a field 15 characters wide
with the data right justified in the field. This makes the
output look much better. This illustrates that you have
complete control over the appearance of your output data.
The real output statements in lines 19 and 20 are similar to
the integer except that when the data is put into a field 15
characters wide, it is still displayed in scientific format.
Adding a second field descriptor as illustrated in lines 21
Page 10-1
Standard Input/Output
through 23, tells the system how many digits you want
displayed after the decimal point.
The boolean, char, and string examples should be self
explanatory. Notice that when the string is output, even
though the string has been defined as a maximum of 10
characters, it has been assigned a string of only 8
characters, so only 8 characters are output. Compile and run
this program and observe the results.
If you are using TURBO Pascal version 4.0 or 5.0, the added
data types described in chapter 3 of this tutorial are output
in the same manner as those illustrated in the program
WRITELNX.PAS.
NOW FOR SOME INPUT FROM THE KEYBOARD
____________________________________________________________
The example file READINT.PAS will =================
illustrate reading some integer data from READINT.PAS
the keyboard. A message is output in line =================
8 with an interesting fact that should be
pointed out. Anyplace where Pascal uses a
string variable or constant, it uses the apostrophe for a
delimiter. Therefore, anyplace where an apostrophe is used
in a string, it will end the string. Two apostrophes in a row
will be construed as a single apostrophe within the string and
will not terminate the string. The term 'Read' within the
string will therefore be displayed as shown earlier in this
sentence.
The variable Index is used to loop five times through a
sequence of statements with one Read statement in it. The
three integer values are read in and stored in their
respective variables with the one statement. If less than
three are entered at the keyboard, only as many as are read
in will be defined, the rest will be unchanged. Following
completion of the first loop, there is a second loop in lines
19 through 25 that will be executed 5 times with only one
minor change, the Read statement is replaced by the Readln
statement. At this point it would be best run this program
trying several variations with input data.
When you run READINT.PAS, it will request three integers.
Reply with three small integers of your choice with as many
blank spaces between each as you desire, followed by a
carriage return. The system will echo your three numbers back
out, and request three more. Respond with only one number
this time, different from each of the first three, and a
carriage return. You will get your new number followed by
your previous second and third number indicating that you did
not re-enter the last two integer variables. Enter three more
Page 10-2
Standard Input/Output
numbers, this time including a negative number and observe the
echo once again.
Continue entering numbers until the system outputs the message
indicating that it will now be using the Readln for reading
data. At this point enter the same numbers that you did in
the previous section and notice the difference, which is only
very slight. Each time you hit the enter key to cause the
computer to process the data you have just given it, it will
echo the carriage return to the display, and the "Thank you"
message will be on a new line. When entering data from the
keyboard, the only difference in Read and Readln is whether
or not the carriage return is echoed to the display following
the data read operation.
It should not be a surprise to you that after you enter the
data, the data is stored within the program and can be used
anywhere that integer data is legal for use. Thus, you could
read in an integer, and use the integer to control the number
of times through a loop, as a case selector, etc.
TIME TO CRASH THE COMPUTER
____________________________________________________________
Crashing the computer will not hurt a thing. Rerun the above
program and instead of entering integer data, enter some real
data with decimal points, or even some character data. The
computer should display some kind of message indicating that
you have caused an I/O error (Input/Output), and TURBO Pascal
will abort operation (that simply means to stop the program
and return control to the operating system). No harm has been
done, simply start it again to enter more numbers or errors.
READING REAL NUMBERS
____________________________________________________________
The example program READREAL.PAS will ================
illustrate how to read real numbers into READREAL.PAS
the computer. It will read an integer and ================
three real numbers each time through the
loop. It is perfectly fine to give the
system a number without a decimal point for a real number.
The computer will simply read it as a decimal number with
zeros after the decimal point and consider it as a real number
internally. As you found out in the last example program,
however, it is not permissible to include a decimal point in
the data if the computer is looking for an integer variable.
Include some character data for a real number and crash the
system in this program too.
Page 10-3
Standard Input/Output
READING CHARACTER DATA
____________________________________________________________
The next example program, READCHAR.PAS, ================
will read in one character each time READCHAR.PAS
through the loop and display it for you. ================
Try entering more than one character and
you will see that the extra characters
will simply be ignored. It is not possible to crash this
program because any character you enter will be valid.
The next example, READARRY.PAS, will read ================
in a string of characters and display them READARRY.PAS
for you if you are using TURBO Pascal 3.0. ================
Neither TURBO Pascal 4.0 nor 5.x permits
reading into an array but does allow
reading into the individual elements of the array one element
at a time. This program does not work with TURBO Pascal 4.0
or 5.x so you should go directly to the next program,
READSTRG.PAS, if you are using either of the two newer
versions.
Continuing our discussion of READARRY.PAS, up to 10 characters
will be read, and if less than 10 are read, the rest will be
blank filled. Try entering 10 characters, then 4, to see that
the residual 6 characters are blanked out before storing and
printing. Since the array is fixed at ten characters, ten
characters are always printed out, including trailing blanks.
Finally, READSTRG.PAS will also read up to ================
10 characters, but since a string is a READSTRG.PAS
dynamic length variable, it will only ================
print out the characters you input each
time, up to the maximum of 10 as defined
in the var declaration. It will display trailing blanks if
you type them in because blanks are valid characters.
BULLET PROOF PROGRAMMING
____________________________________________________________
It can be frustrating to be running a program and have it
declare an I/O error and terminate operation simply because
you have entered an incorrect character. The integer and real
data inputs defined earlier in this chapter are fine for quick
little programs to do specific calculations, but if you are
writing a large applications program it is better to use
another technique. Since the character and string inputs
cannot abort operation of the program, it is best to use them
to input the variable data and check the data internally under
your own program control. An error message can then be given
to the operator and another opportunity granted to input the
correct data. All well written large application programs use
this technique.
Page 10-4
Standard Input/Output
HOW DO I PRINT SOMETHING ON THE PRINTER
____________________________________________________________
With all of the Pascal knowledge you now ================
have, it is the simplest thing in the PRINTOUT.PAS
world to get data to the printer. The ================
example file PRINTOUT.PAS will show you
graphically how to do it. Every Write or
Writeln statement is required to have a device identifier
prior to the first output field. If there is none, it is
automatically defaulted to the standard output device, the
display monitor. The example program has a few outputs to the
monitor in lines 9 and 10 with the device identifier included,
namely Output. This is only done to show you the general form
of the Write statements, but if you desire, you can add the
standard device identifier to every monitor output.
There are many statements in this program with the device
identifier Lst, which is the standard name for the list device
or the printer. It should be obvious to you that the first
field is the device selector which is used to direct the
output to the desired device.
Compile and run this program with your printer turned on for
some printer output. If you are using TURBO Pascal 3.0, you
will have to comment out line 4 since it will not be
understood by your compiler. It is required with version 4.0
or 5.x to tell the system where to find the definition of the
output device named Lst.
Just to supply you with a bit more information, every Read and
Readln statement is also required to have a device identifier
prior to the first input field. As you may suspect, it is
also defaulted to Input if none is specified, and the standard
input device is the keyboard.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program containing a loop to read in a character
string up to 60 characters long, then print the string
on your printer. When you run the program, you will have
the simplest word processing program in the world. Be
sure to include a test to end the loop, such as when
"END" is typed in.
Page 10-5
CHAP11.TXT
Chapter 11
FILE INPUT/OUTPUT
FILES HANDLE SERIAL DATA
____________________________________________________________
One of the most common operations when using a computer is to
either read from, or write to a file. You are already
somewhat experienced in file handling from the last chapter,
because in computer terminology, the keyboard, terminal, and
printer are all classified as files. A file is any serial
input or output device that the computer has access to. Since
it is serial, only one piece of information is available to
the computer at any instant of time. This is in contrast to
an array, for example, in which all elements of the array are
stored internally and are all available at any time.
A SHORT HISTORY LESSON
____________________________________________________________
Several years ago computers were all large cumbersome machines
with large peripheral devices such as magnetic tape drives,
punch card readers, paper tape readers or punches, etc. It
was a simple task to assign the paper tape reader a symbol and
use that symbol whenever it was necessary to read a paper
tape. There was never more than one file on the paper tape
being read, so it was simply read sequentially, and hopefully
the data was the desired data. With the advent of floppy
disks, and hard disks, it became practical to put several
files of data on one disk, none of which necessarily had
anything to do with any of the other files on that disk. This
led to the problem of reading the proper file from the disk,
not just reading the disk.
Pascal was originally released in 1971, before the
introduction of the compact floppy disk. The original release
of Pascal had no provision for selecting a certain file from
among the many included on the disk. Each compiler writer had
to overcome this deficiency and he did so by defining an
extension to the standard Pascal system. Unfortunately, all
of the extensions were not the same, and there are now several
ways to accomplish this operation. There are primarily two
ways, one using the Assign statement, and the other using the
Open statement. They are similar to each other and they
accomplish the same end result.
BACK TO THE PRESENT TIME
____________________________________________________________
All of the above was described to let you know that we will
have a problem in this chapter, namely, how do we cover all
Page 11-1
File Input/Output
of the possible implementations of Pascal available? The
answer is, we can't. Most of what is covered in this chapter
will apply to all compilers, and all that is covered will
apply to the TURBO Pascal compilers, versions 3.0 through 5.x.
This tutorial is especially written for the TURBO Pascal
compilers, but you should be warned you will find differences
in Pascal implementations if you ever find a need to use a
different Pascal compiler someday. You may, for example, need
to use a mini-computer or a mainframe computer someday to
complete a programming assignment. When that happens, you
will find that the area of input/output control will probably
be the biggest difference in the implementations of Pascal.
READING AND DISPLAYING A FILE
____________________________________________________________
Examine the file READFILE.PAS for an ================
example of a program that can read a text READFILE.PAS
file from the disk. In fact it will read ================
itself from the disk and display it on the
video monitor. The first statement in the
program is the Assign statement. This is TURBO Pascal's way
of selecting which file on the disk will be either read from
or written to. In this case we will read from the disk. The
first argument in the Assign statement is the device specifier
similar to Lst used in the last chapter for the printer. We
have chosen to use the name Turkey for the device identifier,
but could have used any valid identifier. This identifier
must be defined in a var declaration as a TEXT type variable.
The next argument is the filename desired. The filename can
be defined as a string constant, as it is here, or as a string
variable.
The TEXT type is a predefined type and is used to define a
file identifier. It is predefined as a "file of char", so it
can only be used for a text file. We will see later that
there is another type of file, a binary file.
Now that we have a file identified, it is necessary to prepare
it for reading by executing a reset statement in line 9. The
Reset statement positions the read pointer at the beginning
of the file, ready to read the first piece of information in
the file. Once we have done that, data is read from the file
in the same manner as it was when reading from the keyboard.
In this program, the input is controlled by the while loop
which is executed until we exhaust the data in the file.
WHAT ARE THE "EOF" AND "EOLN" FUNCTIONS?
____________________________________________________________
The Eof function is new and must be defined. When we read
data from the file, we move closer and closer to the end,
Page 11-2
File Input/Output
until finally we reach the end and there is no more data to
read. This is called "end of file" and is abbreviated Eof.
Pascal has this function available as a part of the standard
library which returns FALSE until we reach the last line of
the file. When there is no more data to read left in the
file, the function Eof returns TRUE. To use the function, we
merely give it our file identifier as an argument. It should
be clear to you that we will loop in this program until we
read all of the data available in the input file.
The Eoln function is not used in this program but is a very
useful function. If the input pointer is anywhere in the text
file except at the end of a line, the Eoln returns FALSE, but
at the end of a line, it returns a value of TRUE. This
function can therefore be used to find the end of a line of
text for variable length text input.
To actually read the data, we use the Readln procedure, giving
it our identifier Turkey and the name of the variable we want
the data read into. In this case, we read up to 80 characters
into the string and if more are available, ignore them. You
should remember when we did this in the last chapter from the
keyboard input. We are using the same technique here except
we are reading from a file this time. Since we would like to
do something with the data, we output the line to the default
device, the video monitor. It should be clear to you by now
that the program will read the entire file and display it on
the monitor.
Finally, we Close the file Turkey. It is not really necessary
to close the file because the system will close it for you
automatically at program termination, but it is a good habit
to get into. It must be carefully pointed out here, that you
did not do anything to the input file, you only read it and
left it intact. You could Reset it and reread it again in
this same program. Compile and run this program to see if it
does what you expect it to do.
A PROGRAM TO READ ANY FILE
____________________________________________________________
Examine the next program READDISP.PAS for ================
an improved file reading program. This is READDISP.PAS
very similar except that it asks you for ================
the name of the file that you wish to
display, and enters the name into a 12
character string named Name_Of_File_To_Input. This is then
used in the Assign statement to select the file to be read,
and the file is reset as before. Lines 15 through 18 display
a header, and from that point on, the program is identical to
the last one with a few small additions. In order to
demonstrate the use of a function within the Writeln
specification, the program calls for the length of the input
Page 11-3
File Input/Output
string in line 23 and displays it before each line. The lines
are counted as they are read and displayed, and the line count
is displayed at the end of the listing.
You should be able to see clearly how each of these operations
is accomplished. Compile and run this program, entering any
filename we have used so far (be sure to include the .PAS
extension). After a successful run, enter a nonexistent
filename and see the I/O error.
HOW TO COPY A FILE (SORT OF)
____________________________________________________________
Examine the file READSTOR.PAS for an ================
example of reading from a file and writing READSTOR.PAS
to another one. In this program we ================
request an operator input for the filename
to read, after which we Assign the name to
the file and Reset it. When we reset the file however, we go
to a bit of extra trouble to assure that the file actually
exists.
Suppose we input a filename, and the file did not exist
because the file was actually missing, or because we entered
the filename wrong. Without the extra effort, the TURBO
Pascal runtime system would indicate a run-time error, and
terminate the program returning us to the operating system.
In order to make a program easier to use, it would be nice to
tell the operator that the file didn't exist and give him the
opportunity to try again with another file name. The method
given in lines 16 through 20 of this program will allow you
to do just that.
USING A COMPILER DIRECTIVE
____________________________________________________________
First you must disable the built in TURBO Pascal I/O checking
by inserting the compiler directive in line 16. This tells
the system to ignore any I/O errors from this point on and if
the file doesn't exist, the system will not abort when you
attempt to reset it in line 17. Another compiler directive
is given in line 18 to enable I/O checking again for the
remainder of the program.
WE DO OUR OWN FILE CHECKING
____________________________________________________________
If the file didn't exist and could not therefore be reset, we
have a problem because the program thinks the file is
available for use but it actually isn't. Fortunately, TURBO
Pascal has a built in variable, named IOResult, that informs
Page 11-4
File Input/Output
us of the result of each I/O operation. Following any I/O
operation, if this variable contains the value of zero, the
I/O operation was correct, and if it contains any other value,
the operation had some sort of error. In our case, we simply
compare it to zero to generate a boolean value, then based on
the boolean value we either give an error message and stop,
or perform the desired file operations.
It would be good programming practice to check all file
openings in this manner to allow the operator to recover from
a simple oversight or spelling error.
If the file was opened properly, then in line 21 through 24
we request a different filename to write to, which is assigned
to a different identifier. Note that the output file is not
checked for a valid opening as it should be. The statement
in line 24 is new to us, the Rewrite statement. This name
apparently comes from the words REset for WRITEing because
that is exactly what it does. It clears the entire file of
any prior data and prepares to write into the very beginning
of the file. Each time you write into it, the file grows by
the amount of the new data written.
Once the identifier has been defined, and the Rewrite has been
executed, writing to the file is identical to writing to the
display with the addition of the identifier being specified
prior to the first output field. With that in mind, you
should have no trouble comprehending the operation of the
program. This program is very similar to the last, except
that it numbers the lines as the file is copied. After
running the program, look in your default directory for the
new filename which you input when it asked for the output
filename. Examine that file to see if it is truly a copy of
the input file with line numbers added.
One word of caution. If you used an existing filename for the
output file, the file was overwritten, and the original
destroyed. In that case, it was good that you followed
instructions at the beginning of this tutorial and made a
working copy of the distribution disk. You did do that,
didn't you?
Compile and run this program two different ways, once with a
valid input filename that should run properly, and the second
time with an input filename that doesn't exist to prove to
yourself that the test actually does work correctly.
HOW TO READ INTEGER DATA FROM A FILE
____________________________________________________________
It is well and good to be able to read text from a file, but
now we would like to read other forms of data from a file.
First we will look at an example program to read data from a
Page 11-5
File Input/Output
text file, then later we will see an example program that
reads from a binary file.
Examine the program READINTS.PAS for an ================
example of reading data from a text file. READINTS.PAS
A text file is an ASCII file that can be ================
read by a text editor, printed, displayed,
or in some cases, compiled and executed.
It is simply a file made up of a long string of char type
data, and usually includes linefeeds, carriage returns, and
blanks for neat formatting. Nearly every file on the Tutorial
disk you received with this package is a text file. One
notable exception is the file named LIST.EXE, which is an
executable program file.
The example program has nothing new, you have seen everything
in it before. We have an assignment, followed by a reset of
our file, followed by four read and write loops. Each of the
loops has a subtle difference to illustrate the Read and
Readln statements. Notice that the same file is used for
reading four times with a Reset prior to each, illustrating
the nondestructive read mentioned a few paragraphs ago.
The file we will be using is named INTDATA.TXT and is on your
disk. You could display it at this time using the program
READDISP.PAS we covered recently. Notice that it is simply
composed of the integer values from 101 to 148 arranged four
to a line with a couple of spaces between each for separation
and a neat appearance. The important thing to remember is
that there are four data points per line.
READ AND READLN ARE SLIGHTLY DIFFERENT
____________________________________________________________
As variables are read in with either procedure, the input file
is scanned for the variables using blanks as delimiters. If
there are not enough data points on one line to satisfy the
arguments in the input list, the next line is searched also,
and the next, etc. Finally when all of the arguments in the
input list are satisfied, the Read is complete, but the Readln
is not. If it is a Read procedure, the input pointer is left
at that point in the file, but if it is a Readln procedure,
the input pointer is advanced to the beginning of the next
line. The next paragraph should clear that up for you.
The input data file INTDATA.TXT has four data points per line
but the first loop in the program READINTS.PAS requests only
three each time through the loop. The first time through, it
reads the values 101, 102, and 103, and displays those values,
leaving the input pointer just prior to the 104, because it
is a Read procedure. The next time through, it reads the
value 104, advances to the next line and reads the values 105,
Page 11-6
File Input/Output
and 106, leaving the pointer just prior to the 107. This
continues until the 5 passes through the loop are completed.
The loop in lines 19 through 22 contains a Readln procedure
and also reads the values 101, 102, and 103, but when the
input parameter list is satisfied, it moves the pointer to the
beginning of the next line, leaving it just before the 105.
The values are printed out and the next time we come to the
Readln, we read the 105, 106, and 107, and the pointer is
moved to the beginning of the next line. It would be good to
run the program now to see the difference in output data for
the two loops. Remember that the only difference is that the
first loop uses the Read procedure, and the second uses the
Readln procedure.
When you come back to the program again, observe the last two
loops, which operate much like the first two except that there
are now five requested integer variables, and the input file
still only has four per line. This is no problem. Both input
procedures will simply read the first four in the first line,
advance to the second line for its required fifth input, and
each will do its own operation next. The Read procedure will
leave the input pointer just before the second data point of
the second line, and the Readln will advance the input pointer
to the beginning of the third line. Run this program and
observe the four output fields to see an illustration of these
principles.
NOW TO READ SOME REAL VARIABLES FROM A FILE
____________________________________________________________
Examine the file named REALDATA.TXT supplied on your Pascal
Tutorial disk. You will see 8 lines of what appears to be
scrambled data, but it is good data that Pascal can read.
Notice especially line 4 which has some data missing, and line
6 which has some extra data.
Examine the program file READDATA.PAS ================
which will be used to illustrate the READDATA.PAS
method of reading real type data. ================
Everything should be familiar to you,
since there is nothing new here. The
Readln statement is requesting one integer variable, and three
real variables, which is what most of the input file
contained. When we come to the fourth line, there are not
enough data points available, so the first two data points of
the next line are read to complete the fourth pass through the
loop. Since the file pointer is advanced to the beginning of
the next line, we are automatically synchronized with the data
again. When we come to the sixth line, the last two data
points are simply ignored. Run the program to see if the
results are as you would predict.
Page 11-7
File Input/Output
If a Read were substituted for the Readln in line 14 of the
program, the file pointer would not be advanced to the
beginning of line 6 after the fourth pass through the loop.
The next attempt to read would result in trying to read the
value 0.0006 as an integer, and a run time error would result.
Modify the program, substituting a Read for the Readln in line
14, and see if this is not true.
It should be pointed out that TURBO Pascal 4.0 and 5.x both
require a digit both before and after the decimal point in all
data that is to be read in as real type data or it will be
flagged as a run-time error and the program will be halted.
The digits can be zero as they are in several places in the
example file but they must be there. If you are using TURBO
Pascal 3.0, the leading and trailing digits are not required.
That is all there is to reading and writing text files. If
you learn the necessities, you will not be stumbling around
in the area of input/output which is very intimidating to many
people. Remember to Assign, then Reset before reading,
Rewrite before writing, and Close before quitting. It is of
the utmost importance to close a file you have been writing
to before quitting to write the last few buffers to the file,
but it is not as important to close read files unless you are
using a lot of them, as there is an implementation dependent
limit of how many files can be open at once. It is possible
to read from a file, close it, reopen it, and write to it in
one program. You can reuse a file as often as you desire in
a program, but you cannot read from and write into a file at
the same time.
NOW FOR BINARY INPUT AND OUTPUT
____________________________________________________________
Examine the file BINOUT.PAS for an example ================
of writing data to a file in binary form. BINOUT.PAS
First there is a record defined in the ================
type declaration part composed of three
different variable types. In the var
part, Output_File is defined as a "file of Dat_Rec", the
record defined earlier. The variable Dog_Food is then defined
as an array of the record, and a simple variable is defined.
Any file assigned a type of TEXT, which is a "file of char",
is a text file. A text file can be read and modified with a
text editor, printed out, displayed on the monitor, etc. If
a file is defined with any other definition, it will be a
binary file and will be in an internal format as defined by
the Pascal compiler and may not be readable by any compiler
other than the one used to write it. Attempting to display
such a file will result in very strange looking gibberish on
the monitor.
Page 11-8
File Input/Output
When we get to the program, the output file is assigned a name
in line 15, and a Rewrite is performed on it to reset the
input pointer to the beginning of the file, empty the file,
and prepare for writing data into it. The loop in lines 18
through 22 simply assigns nonsense data to all of the
variables in the 20 records so we have something to work with.
We write a message to the display that we are ready to start
outputting data, then we output the data one record at a time
with the standard Write statement. A few cautions are in
order here. The output file can be defined to store any
simple variable type, integer, byte, real, or a record, but
the types cannot be mixed. The record itself however, can be
any combination of data including other records if desired,
but any file can only have one type of record written to it.
A Writeln statement is illegal when writing to a binary file
because a binary file is not line oriented. A Write statement
is limited to one output field per statement. This is not a
serious limitation since it is a simple matter to put one
Write statement in the program for each variable you wish to
write out to the file. It is important to Close the file when
you are finished writing to it.
WHY USE A BINARY FILE
____________________________________________________________
A binary file written by a Pascal program cannot be read by
a word processor, a text editor or any other application
program such as a database or spreadsheet, and it may not even
be readable by a Pascal program compiled by a different
companies compiler because the actual data structure is
implementation dependent. It can't even be read by a Pascal
program using the same compiler unless the data structure is
identical to the one used to write the file. With all these
rules, it seems like a silly way to output data, but there are
advantages to using a binary output.
A binary file uses less file space than a corresponding text
file because the data is stored in a packed mode. Since all
significant digits of real data are stored, it is more precise
unless you are careful to output all significant data to the
corresponding TEXT file. Finally, since the binary data does
not require formatting into ASCII characters, it will be
considerably faster than outputting it in TEXT format. When
you run this example program, it will create the file
KIBBLES.BIT, and put 20 records in it. Return to DOS and look
for this file and verify its existence. If you try to TYPE
it, using the DOS TYPE command, you will have a real mess on
your monitor because it does not contain char type data, but
that might be a good exercise.
Page 11-9
File Input/Output
READING A BINARY FILE
____________________________________________________________
BININ.PAS is another example program that =================
will read in the file we just created. BININ.PAS
Notice that the variables are named =================
differently, but the types are all
identical to those used to write the file
and they are in the same order. An additional line is found
in the program, the if statement. We must check for the "end
of file" marker to stop reading when we find it or Pascal will
list an error and terminate operation. Three pieces of
information are written out to verify that we actually did
read the data file in.
Once again, a few rules are in order. A Readln is illegal
since there are no lines in a binary file, and only one
variable or record can be read in with each Read statement.
FILE POINTERS, GET, AND PUT STATEMENTS
____________________________________________________________
File pointers and the Get and Put procedures are a part of
standard Pascal, but since they are redundant and therefore
not needed, they are not a part of TURBO Pascal. The standard
Read and Write procedures are more flexible, more efficient,
and easier to use. The use of Get and Put will not be
illustrated or defined here. If you ever have any need for
them, they should be covered in detail in your Pascal
reference manual for the particular implementation you are
using.
Pointers will be covered in detail in the next chapter of this
tutorial.
PROGRAMMING EXERCISES
____________________________________________________________
1. Modify READFILE.PAS so that after reading and displaying
the file, the file is reset, then read and displayed
again. This was suggested in the text.
2. Write a program to read the data from any text file, and
display it on the monitor with line numbers and the
number of characters in each line. Finally display the
number of lines found in the file, and the total number
of characters in the entire file. Compare this number
with the filesize given by the DOS command DIR.
Page 11-10
CHAP12.TXT
Chapter 12
POINTERS AND DYNAMIC ALLOCATION
THIS IS ADVANCED MATERIAL
____________________________________________________________
For certain types of programs, pointers and dynamic allocation
can be a tremendous advantage, but many programs do not need
such a high degree of data structure. For that reason, it
would probably be to your advantage to lightly skim over these
topics and come back to them later when you have a substantial
base of Pascal programming experience. It would be good to
at least skim over this material rather than completely
neglecting it, so you will have an idea of how pointers and
dynamic allocation work and that they are available for your
use when needed.
A complete understanding of this material will require deep
concentration as it is complex and not at all intuitive.
Nevertheless, if you pay close attention, you will have a good
grasp of pointers and dynamic allocation in a short time.
WHAT ARE POINTERS, AND WHAT GOOD ARE THEY?
____________________________________________________________
Examine the program named POINT.PAS for =================
your first example of a program using POINT.PAS
pointers. In the var declaration you will =================
see two variables named Where and Who that
have the symbol ^ in front of their types.
This defines them, not as variables, but as pointers to
integer type variables and since they are pointers, they store
an address. Figure 12-1 is a graphical representation of the
data space prior to beginning execution of the program. A box
represents a variable, and a box with a dot in it represents
a pointer. In line 12 of the program, the variable Index is
assigned the value of 17 for purposes of illustration. The
pointer named Where is then assigned the address of the
variable Index which means that it does not contain the value
of 17, it contains the address of the storage location where
the variable Index is stored. In like manner, we assign the
address of Index to the pointer named Who. It should be
obvious to you that Addr is a TURBO Pascal function that
returns the address of its argument.
HOW DO WE USE THE POINTERS?
____________________________________________________________
It should be clear to you that we now have a single variable
named Index with two pointers pointing at it as depicted in
figure 12-2. If the pointers are useful, we should be able
Page 12-1
Pointers and Dynamic Allocation
to do something with them now, so we simply print out the same
variable three different ways in line 15. When we write
"Where^", we are telling the system that we are not interested
in the pointer itself but instead we are interested in the
data to which the pointer points. This is referred to as
dereferencing the pointer. Careful study of the output fields
in line 15 will reveal that we first display the value of
Index, then the value to which the pointer Where points, and
finally the value to which the pointer Who points. Since both
pointers point to the variable Index, we are essentially
displaying the value of Index three times. You will confirm
this when you compile and run this program.
In line 17, we tell the system to assign the value of 23 to
the variable to which the pointer Where points as an
illustration and figure 12-3 pictures the data space at this
time. If you understood the discussion in the previous
paragraph, you will understand that we are actually assigning
the variable named Index the value of 23 because that is where
the pointer named Where is pointing. In line 18, we once
again display the value of the variable Index 3 times just as
we did in line 15. It would be to your advantage to compile
and run this program to see that the value of 17 is output
three times, then the value of 23 is output three times.
In a program as simple as this, the value of pointers is not
at all clear but a simple program is required in order to make
the technique clear. Display the program named POINT.PAS on
your monitor again because we are not yet finished with it.
A FEW MORE POINTERS
____________________________________________________________
In line 4, we define a new type named Int_Point which is a
pointer type to an integer variable. We use this new type in
line 9 to define three more pointers and in line 20, we assign
one of them the address of the variable named Index. Since
the pointers are of identical types, in line 21 we can assign
Pt2 the value of Pt1, which is actually the address of the
variable named Index. Likewise, the pointer Pt3 is assigned
the value of Pt2, and we have all three pointers pointing to
the variable named Index. If you are using TURBO Pascal
version 4.0 or 5.x, you are allowed to assign pointers like
this only if they have the same type, which these three do.
However, since the pointers named Where and Who are declared
individually, they are not of the same type according to the
rules of Pascal and if line 14 were changed to read "Who :=
Where;", a compilation error would occur with TURBO Pascal
version 4.0 or 5.x. This error would not occur with TURBO
Pascal 3.0 since it is a little less stringent in its type
checking. The variables are only assignment compatible if
they are declared with the same type name.
Page 12-2
Pointers and Dynamic Allocation
Finally, we assign the only variable in this program which is
named Index the value of 151 in line 23 and display the value
151 three times as we did above. Compile and run this program
again to see that it does indeed display the value 151 three
times.
THIS IS FOR TURBO PASCAL VERSION 4.0 OR 5.X
____________________________________________________________
If you are using TURBO Pascal version 4.0 ================
or 5.x, you should display the program POINT4.PAS
named POINT4.PAS on your monitor for an ================
example of another new extension to the
Pascal programming language by Borland.
This program is identical to the last except in lines 13, 14
and 20, where the symbol @ is used to denote the address of
the variable Index rather than the function Addr. This is
only available with TURBO Pascal version 4.0 or 5.x as a
convenience to you. In ANSI standard Pascal the @ symbol is
used as a synonym for the ^ symbol but Borland chose to use
it for a completely different purpose. If you are using TURBO
Pascal 3.0, you will not be able to compile and run this
program, but nothing is lost because it is identical to the
previous one.
OUR FIRST LOOK AT DYNAMIC ALLOCATION
____________________________________________________________
If you examine the file named ================
POINTERS.PAS, you will see a very trivial POINTERS.PAS
example of pointers and how they are used ================
with dynamically allocated variables. In
the var declaration, you will see that the
two variables have a ^ in front of their respective types once
again, defining two pointers. They will be used to point to
dynamically allocated variables that have not yet been
defined.
The pointer My_Name is a pointer to a 20 character string.
The pointer actually points to an address somewhere within the
computer memory, but we don't know where yet. Actually, there
is nothing for it to point at because we have not defined a
variable. After we assign it something to point to, we can
use the pointer to access the data stored at that address.
Your computer has some amount of memory installed in it. If
it is an IBM-PC or compatible, it can have up to 640K of RAM
which is addressable by various programs. The operating
system requires about 60K of the total, and the TURBO Pascal
run time system requires about 4K to 8K depending on which
version you are using, and what functions you have called.
The TURBO Pascal program can use up to 64K. Adding those
Page 12-3
Pointers and Dynamic Allocation
three numbers together results in about 128K or 132K. Any
memory you have installed in excess of that is available for
the stack and the heap. The stack is a standard area defined
and controlled by DOS that can grow and shrink as needed.
Many books are available to define the stack and its use if
you are interested in more information on it.
WHAT IS THE HEAP?
____________________________________________________________
The heap is a Pascal defined entity that utilizes otherwise
unused memory to store data. It begins immediately following
the program and grows as necessary upward toward the stack
which is growing downward. As long as they never meet, there
is no problem. If they meet, a run-time error is generated.
The heap is therefore outside of the 64K limitation of TURBO
Pascal and many other Pascal compilers.
TURBO Pascal version 4.0 or 5.x does not limit us to 64K, but
there are other reasons for using the heap in addition to the
64K limitation. These should be evident as we learn how the
heap works.
If you did not understand the last few paragraphs, don't
worry. Simply remember that dynamically allocated variables
are stored on the heap and do not count in the 64K limitation
placed upon you by some compilers.
Back to our example program, POINTERS.PAS. When we actually
begin executing the program, we still have not defined the
variables we wish to use to store data in. The first
executable statement in line 10 generates a variable for us
with no name and stores it on the heap. Since it has no name,
we cannot do anything with it, except for the fact that we do
have a pointer My_Name that is pointing to it. By using the
pointer, we can store up to 20 characters in it, because that
is its type, and later go back and retrieve it.
WHAT IS DYNAMIC ALLOCATION?
____________________________________________________________
The variable we have just described is a dynamically allocated
variable because it was not defined in a var declaration, but
with a New procedure. The New procedure creates a variable
of the type defined by the pointer, puts it on the heap, and
finally assigns the address of the variable to the pointer
itself. Thus My_Name contains the address of the variable
generated. The variable itself is referenced by using the
pointer to it followed by a ^, just like in the last program,
and is read, "the variable to which the pointer points".
Page 12-4
Pointers and Dynamic Allocation
The statement in line 11 assigns a place on the heap to an
integer type variable and puts its address in My_Age.
Following the New statements we have two assignment statements
in which the two variables pointed at are assigned values
compatible with their respective types, and they are both
written out to the video display in much the same manner as
we did in the program named POINT.PAS. This is illustrated
in figure 12-5.
Following execution of lines 13 and 14, the data space is
configured as illustrated in figure 12-6.
GETTING RID OF DYNAMICALLY ALLOCATED DATA
____________________________________________________________
The two statements in lines 19 and 20 are illustrations of the
way the dynamically allocated variables are removed from use.
When they are no longer needed, they are disposed of with the
Dispose procedure which frees up their space on the heap so
it can be reused.
In such a simple program, pointers cannot be appreciated, but
it is necessary for a simple illustration. In a large, very
active program, it is possible to define many variables,
dispose of some of them, define more, and dispose of more,
etc. Each time some variables are disposed of, their space
is then made available for additional variables defined with
the New procedure.
The heap can be made up of any assortment of variables, they
do not have to all be the same. One point must be kept in
mind. Anytime a variable is defined, it will have a pointer
pointing to it. The pointer is the only means by which the
variable can be accessed. If the pointer to the variable is
lost or changed, the data itself is lost for all practical
purposes. Compile and run this program and examine the
output.
DYNAMICALLY STORING RECORDS;
____________________________________________________________
The next example program, DYNREC.PAS, is a ================
repeat of one we studied in an earlier DYNREC.PAS
chapter. For your own edification, review ================
the example program BIGREC.PAS before
going ahead in this chapter. Assuming
that you are back in DYNREC.PAS, you will notice that this
program looks very similar to the earlier one, and in fact
they do exactly the same thing. The only difference in the
type declaration is the addition of a pointer Person_Id, and
in the var declaration, the first four variables are defined
as pointers here, and were defined as record variables in the
last program.
Page 12-5
Pointers and Dynamic Allocation
A point should be made here. Pointers are not generally used
in very small programs. This program is a good bit larger
than the last and should be a clue to you as to why such a
trivial program was used to introduce pointers in this
tutorial. A very small, concise program can illustrate a
topic much better that an large complex program, but we must
go on to more useful constructs of any new topic.
WE JUST BROKE THE GREAT RULE OF PASCAL
____________________________________________________________
Notice in the type declaration that we used the identifier
Person in line 18 before we defined it in line 19, which is
illegal to do in Pascal. Foreseeing the need to define a
pointer prior to the record, the designers of Pascal allow us
to break the rule in this one place. The pointer could have
been defined after the record in this particular case, but it
was more convenient to put it before, and in the next example
program, it will be required to put it before the record. We
will get there soon.
Since Friend is really 50 pointers, we have now defined 53
different pointers to records, but so far have defined no
variables other than Temp and Index. We immediately use the
New procedure to dynamically allocate a record with Self
pointing to it, and use the pointer so defined to fill the
dynamically allocated record. Compare this to the program
named BIGREC and you will see that it is identical except for
the addition of the New and adding the ^ to each use of the
pointer to designate the data pointed to.
THIS IS A TRICK, BE CAREFUL
____________________________________________________________
Now go down to line 48 where Mother is allocated a record and
is then pointing to the record. It seems an easy thing to do
then to simply assign all of the values of self to all the
values of mother as shown in the next statement, but it
doesn't work. All the statement does, is make the pointer
Mother point to the same place where Self is pointing because
we did a pointer assignment. The data that was allocated to
the pointer Mother is now somewhere on the heap, but we don't
know where, and we cannot find it, use it, or deallocate it.
This is an example of losing data on the heap. The proper way
is given in the next two statements where all fields of Father
are defined by all fields of Mother which is pointing at the
original Self record. Note that since Mother and Self are
both pointing at the same record, changing the data with
either pointer results in the data appearing to be changed in
both because there is, in fact, only one field where the data
is stored.
Page 12-6
Pointers and Dynamic Allocation
In order to Write from or Read into a dynamically assigned
record it is necessary to use a temporary record since
dynamically assigned records are not allowed to be used in I/O
statements. This is illustrated in lines 57 through 63 of the
program where some data is written to the monitor.
Finally, the dynamically allocated variables are disposed of
prior to ending the program. For a simple program such as
this, it is not necessary to dispose of them because all
dynamic variables are disposed of automatically when the
program is terminated and we return to DOS or the TURBO Pascal
integrated environment. Notice that if the "Dispose(Mother);"
statement was included in the program, the data could not be
found due to the lost pointer, and the program would be
unpredictable, probably leading to a system crash.
It would be a meaningful exercise for you to diagram the data
space for this program, at a few selected points in its
execution, in a manner similar to that done in figure 12-1 to
figure 12-5 of this chapter.
SO WHAT GOOD IS THIS ANYWAY?
____________________________________________________________
Remember when you were initially studying BIGREC.PAS? I
suggested that you see how big you could make the constant
Number_Of_Friends before you ran out of memory. At that time
we found that it could be made slightly greater than 1000
before we got the memory overflow message at compilation. Try
the same thing with DYNREC.PAS to see how many records it can
handle, remembering that the records are created dynamically,
so you will have to run the program to actually run out of
memory. The final result will depend on how much memory you
have installed, and how many memory resident programs you are
using such as "Sidekick". If you have a full memory of 640K,
I would suggest you start somewhere above 8000 records of
Friend.
Now you should have a good idea of why Dynamic Allocation can
be used to greatly increase the usefulness of your programs.
There is, however, one more important topic we must cover on
dynamic allocation. That is the linked list.
WHAT IS A LINKED LIST?
____________________________________________________________
Understanding and using a linked list is ================
by far the most baffling topic you will LINKLIST.PAS
confront in Pascal. Many people simply ================
throw up their hands and never try to use
a linked list. I will try to help you
understand it by use of an example and lots of explanation.
Page 12-7
Pointers and Dynamic Allocation
Examine the program named LINKLIST.PAS for an example of a
linked list. I tried to keep it short so you could see the
entire operation and yet do something meaningful.
To begin with, notice that there are two types defined in
lines 4 and 6, a pointer to the record and the record itself.
The record, however, has one thing about it that is new to us,
the last entry, Next is a pointer to another record of this
type. This record then, has the ability to point to itself,
which would be trivial and meaningless, or to another record
of the same type which would be extremely useful in some
cases. In fact, this is the way a linked list is used. I
must point out, that the pointer to another record, in this
case called Next, does not have to be last in the list, it can
be anywhere it is convenient for you.
A couple of pages ago, we discussed the fact that we had to
break the great rule of Pascal and use an identifier before
it was defined. This is the reason the exception to the rule
was allowed. Since the pointer points to the record, and the
record contains a reference to the pointer, one has to be
defined after being used, and by rules of Pascal, the pointer
can be defined first, provided that the record is defined
immediately following it. That is a mouthful but if you just
use the syntax shown in the example, you will not get into
trouble with it.
STILL NO VARIABLES?
____________________________________________________________
It may seem strange, but we still will have no variables
defined, except for our old friend Index. In fact for this
example, we will only define 3 pointers. In the last example
we defined 54 pointers, and had lots of storage room. Before
we are finished, we will have at least a dozen pointers but
they will be stored in our records, so they too will be
dynamically allocated.
Lets look at the program itself now. In line 20, we create
a dynamically allocated record and define it by the pointer
Place_In_List. It is composed of the three data fields, and
another pointer. We define Start_Of_List to point to the
first record created, and we will leave it unchanged
throughout the program. The pointer Start_Of_List will always
point to the first record in the linked list which we are
building up. The data space is as depicted in figure 12-7.
WHAT IS "nil" AND WHAT IS IT USED FOR?
____________________________________________________________
We define the three variables in the record to be any name we
desire for illustrative purposes, and set the pointer in the
Page 12-8
Pointers and Dynamic Allocation
record to nil. The word nil is another reserved word that
doesn't give the pointer an address but defines it as empty.
A pointer that is currently nil cannot be used to manipulate
data because it has no value, but it can be tested in a
logical statement to see if it is nil. It is therefore a
dummy assignment. With all of that, the first record is
completely defined.
DEFINING THE SECOND RECORD
____________________________________________________________
When you were young you may have played a searching game in
which you were given a clue telling you where to find the next
clue. The next clue had a clue to the location of the third
clue. You kept going from clue to clue until you found the
prize. You simply exercised a linked list. We will now build
up the same kind of a list in which each record will tell us
where the next record is at.
In lines 27 through 33 we will define the second record. Our
goal will be to store a pointer to the second record in the
pointer field of the first record. In order to keep track of
the last record, the one in which we need to update the
pointer, we will keep a pointer to it in Temp_Place. Now we
can dynamically allocate another New record and use
Place_In_List to point to it. Since Temp_Place is now
pointing at the first record, we can use it to store the value
of the pointer which points to the new record which we do in
line 29. The 3 data fields of the new record are assigned
nonsense data for our illustration, and the pointer field of
the new record is assigned nil. We have reached the point
when the data space is as depicted in figure 12-8.
Let's review our progress to this point. We now have the
first record with a person's name and a pointer to the second
record, and a second record with a different person's name and
a pointer assigned nil. We also have three pointers, one
pointing to the first record, one pointing to the last record,
and one we used just to get here since it is only a temporary
pointer. If you understand what is happening so far, let's
go on to add some additional records to the list. If you are
confused, go back over this material again.
TEN MORE RECORDS
____________________________________________________________
The next section of code is contained within a for loop so the
statements are simply repeated ten times. If you observe
carefully, you will notice that the statements are identical
to the second group of statements in the program (except of
course for the name assigned). They operate in exactly the
same manner, and we end up with ten more names added to the
Page 12-9
Pointers and Dynamic Allocation
list. You will now see why the temporary pointer was
necessary, but pointers are cheap, so feel free to use them
at will. A pointer generally uses only 4 bytes of memory.
FINALLY, A COMPLETE LINKED LIST
____________________________________________________________
We now have generated a linked list of twelve entries. We
have a pointer pointing at the first entry, and another
pointer pointing at the last. The only data stored within the
program itself are three pointers, and one integer, all of the
data is on the heap. This is one advantage to a linked list,
it uses very little local memory, but it is costly in terms
of programming. (Keep in mind that all of the data must be
stored somewhere in memory, and in the case of the linked
list, it is stored on the heap.) You should never use a
linked list simply to save memory, but only because a certain
program lends itself well to it. Some sorting routines are
extremely fast because of using a linked list, and it could
be advantageous to use in a database. Figure 12-9 is a
graphical representation of what the linked list looks like.
HOW DO WE GET TO THE DATA NOW?
____________________________________________________________
Since the data is in a list, how can we get a copy of the
fourth entry for example? The only way is to start at the
beginning of the list and successively examine pointers until
you reach the desired one. Suppose you are at the fourth and
then wish to examine the third. You cannot back up, because
you didn't define the list that way, you can only start at the
beginning and count to the third. You could have defined the
record with two pointers, one pointing forward, and one
pointing backward. This would be a doubly-linked list and you
could then go directly from entry four to entry three.
Now that the list is defined, we will read the data from the
list and display it on the video monitor. We begin by
defining the pointer, Place_In_List, as the start of the list.
Now you see why it was important to keep a copy of where the
list started. In the same manner as filling the list, we go
from record to record until we find the record with nil as a
pointer.
There are entire books on how to use linked lists, and most
Pascal programmers will seldom, if ever, use them. For this
reason, additional detail is considered unnecessary, but to
be a fully informed Pascal programmer, some insight is
necessary.
Page 12-10
Pointers and Dynamic Allocation
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program to store a few names dynamically, then
display the stored names on the monitor. As your first
exercise in dynamic allocation, keep it very simple.
Page 12-11
CHAP13.TXT
Chapter 13
UNITS IN TURBO PASCAL 4.0 AND 5.X
THIS IS NEW
____________________________________________________________
If you are using TURBO Pascal version 3.0 or earlier, you will
find that none of the programs in this chapter can be compiled
or run with your system, but it would be to your advantage to
read this material anyway.
When Nicklaus Wirth originally defined Pascal, it was intended
to be a very small language to be used primarily for teaching
programming concepts to computer neophytes. A program would
be contained in a single file and compiled in its entirety
each time it was compiled. There was no provision for
splitting a program up into smaller parts, compiling each part
separately, and linking all of the parts together into a final
completed package.
Since human beings make mistakes, and because the entire
program must be recompiled each time any mistake is
discovered, pure Pascal is unsuitable for very large programs.
Seeing this problem, many compiler writers have defined some
method by which a large program could be broken down into
smaller parts and separately compiled.
This chapter will define and illustrate the way Borland
International has chosen to do so.
PART OF A PROGRAM
____________________________________________________________
Load the program named AREAS.PAS and =================
display it on your monitor. This is the AREAS.PAS
first example of a TURBO Pascal unit and =================
although it is similar to a program in
many ways, it has a few differences which
must be pointed out. We will start by pointing out the major
sections, then get into the details of each section.
You will first notice that this program begins with the
reserved word unit instead of our usual program, followed by
the unit name, Areas. In line 10, the reserved word interface
is used and all of the statements following it down to the
next reserved word implementation, are part of the interface
with any program outside of this unit. The reserved word,
implementation, defines the beginning of the definitions and
executable parts of the private portion of the unit.
Page 13-1
Units in TURBO Pascal 4.0 and 5.X
Finally, in lines 48 through 50, we find what appears to be
a program block just like we have been using all through this
tutorial, but actually is not. We will see in a few
paragraphs that this is the initialization section and does
a very specific job for us even though somewhat different than
what we have become used to.
THE INTERFACE PART
____________________________________________________________
Following the unit name we have a section of code in lines 10
through 15 that define the interface of this module to the
outside world. Anything defined here is available to the
outside world and can be used by any other program provided
it has a "uses Areas;" statement in it. Constants, types, and
variables could also be defined here, and if they were, they
too would be available to any user program, but in this case,
only the four functions are made available. It should be
fairly obvious that the functions calculate the areas of four
different geometric shapes.
THE IMPLEMENTATION PART
____________________________________________________________
From line 16 through line 47 we have the implementation part
as delineated by the reserved word implementation and the
beginning of the initialization block. The implementation
part is the actual workhorse of the unit since it contains all
of the executable code for the four functions defined above.
Lines 26 through 31 contain the code needed to generate the
area of a circle, and this code is no different than the code
that would be used if this function were placed in the
declaration part of any Pascal program. There is a difference
in the function header since the formal parameters are not
repeated here. TURBO Pascal allows you to either drop the
formal parameters here or include them if you think the code
would be more readable. If you include them, they must be
exactly as shown in the interface part or you will get a
compile error.
A LOCAL PROCEDURE
____________________________________________________________
In lines 20 through 24, we have a procedure that is used
within one of the four functions, namely the first. It is
really a stupid procedure since it really wastes time setting
up linkage for the procedure call and does nothing that
Page 13-2
Units in TURBO Pascal 4.0 and 5.X
couldn't be done just as easy with a simple multiply, but it
does illustrate that you can use another procedure within the
unit body. The procedure Mult_Two_Numbers cannot be used
outside of this unit because it is not included in the
interface part of the unit. It is, in effect, invisible to
the outside world.
The variable My_Pi would be more correctly represented as a
constant but it is defined as a variable to illustrate use of
the body of the unit later. Since My_Pi is not defined in the
interface part of the unit, it also is invisible to the
outside world and in fact protected from accidental corruption
by a misplaced statement in another program. The procedure
and the variable for all practical purposes have an
impenetrable barrier around them protecting them from
unauthorized use or modification by the outside world, but the
functions internal to this unit have free access to them just
as in any other program.
WHAT IS THE BODY USED FOR?
____________________________________________________________
Lines 48 through 50 constitute the body of the unit and
although they appear to consist of another executable program
that can be called and used, they actually perform another
very specific and useful purpose. This is actually an
initialization section and all of the statements in this part
of the unit are executed once and only once, and they are
executed when the main program is loaded. This is done
automatically for you by the system. There is no way provided
for you to call the statements in the body after the program
has begun execution. This is why the variable My_Pi was
defined as a variable, so we could use this section to
initialize it to a useful value.
The body can actually have function and procedure calls that
are executed when the program is loaded, as well as loops or
conditional statements.
If you would like to execute some statements during
initialization and again during the execution of the program
one or more times, you can write a procedure or function to
accomplish your desires and call it at the appropriate times
in the main program.
SELECTIVE NAMING OF FUNCTIONS AND PROCEDURES
____________________________________________________________
If you will study the interface part of this unit you will
find that everything you need to use this unit is contained
within it, provided that you know enough about plane geometry
to understand the functions. You should strive for this
Page 13-3
Units in TURBO Pascal 4.0 and 5.X
understanding in all of your interfaces so that the
implementation doesn't even require consultation. Keep in
mind, that if you need to, you can include comments to further
define the functions in the interface part of the unit.
At this time, you should compile this unit. You will have to
compile it to disk rather than only to memory so it will be
available for use later in this chapter. You do this by using
the menus to change the Compile/Destination to the Disk
option. Note that it will not generate an .EXE file but
instead a .TPU file. This is Borland's filename extension for
a unit.
ANOTHER UNIT
____________________________________________________________
Load the file named PERIMS.PAS for another ================
example of a unit. This is similar to the PERIMS.PAS
last except that it does not contain an ================
internal procedure, and it is composed of
three procedures that calculate the
perimeters of geometric shapes, all of which are visible to
the outside world because they are included in the interface
part of the unit. Once again, we have a private variable
named My_Pi and a block of code (actually a single statement)
to initialize the value of My_Pi when the unit is loaded.
Be sure you compile this unit to disk in the same manner as
the last and they will be ready for use.
Now that we have several functions and procedures that can be
used to calculate the areas or perimeters of several different
shapes, we need a program to illustrate their use, so if you
load and display the program named GARDEN.PAS you will have
an example of their use.
HOW DO WE USE OUR DEFINED UNITS?
____________________________________________________________
GARDEN.PAS is a very simple program that ================
uses one of the functions and one of the GARDEN.PAS
procedures. The only thing you must do is ================
add the names of the units prior to using
the external functions or procedures.
Lines 16 and 17 each use one of our newly defined routines.
As you can see, there is nothing magic about the new routines,
and once you include the unit names in a uses statement, the
new routines are in a sense, an extension to the Pascal
language. Compile and run this program and see that it really
does what you expect it to do.
Page 13-4
Units in TURBO Pascal 4.0 and 5.X
ONE MORE EXAMPLE OF UNIT USE
____________________________________________________________
Load and display the program named =================
SHAPES4.PAS for another example of using a SHAPES4.PAS
predefined unit. In line 3, this program =================
includes our new unit named Areas so all
four of the area functions are available,
and in fact, all four are used within the body of the program.
This program should not be difficult for you to understand and
you will be left to study it on your own. You should observe
that this program is repeated in chapter 14 in a different
form for users of TURBO Pascal 3.0.
MULTIPLE USES OF AN IDENTIFIER
____________________________________________________________
Suppose we wanted to move the variable named My_Pi to the
interface section in both of the units we defined earlier.
Then in the program named GARDEN.PAS when we included both of
the units in the uses statement, both variables named My_Pi
would be available for use so we would have a bit of a problem
defining which one we really meant to use. TURBO Pascal has
a way to tell the system which one you wish to use by using
a qualifier in much the same way that you use a field of a
record. The variable name Areas.My_Pi would refer to that
variable from the unit named Areas, and the name Perims.My_Pi
would refer to the variable from the unit named Perims.
You could even define a new variable of the same name in your
main program and refer to it by the qualified name
Garden.My_Pi if you chose to. This is not recommended as it
would get very confusing to you. The compiler would be very
happy to compile and run such a program, because it would not
get confused.
It is not illustrated in the example program, but this
technique applies to procedure and function names as well.
If you used the same procedure name in two different units,
you could specify which procedure you intend to use by using
the dot notation with the unit name and the procedure name.
Unit_Name.Procedure_Name would therefore refer to the
procedure named Procedure_Name that is a part of the unit
named Unit_Name.
WHY USE UNITS?
____________________________________________________________
There are basically three reasons to use units in your
programming. First, some programs are so large that they
should be broken up into smaller chunks for ease of handling
and reasonable compilation size. In fact some are so large
Page 13-5
Units in TURBO Pascal 4.0 and 5.X
that they cannot be compiled all at one time since TURBO
Pascal has an upper limit of 64K of code which can be compiled
at once. Most other compilers have a similar limit also.
Secondly, once you complete the code to perform a certain job,
you may wish to use the same code in another program to do the
same job. If you put the code in a unit, it is ready to
simply call and use again. This is becoming a rather
important topic in software engineering usually referred to
as "Reusable Software".
THIS IS INFORMATION HIDING
____________________________________________________________
Finally, it is sometimes important to hide a portion of code
from the rest of the program to assure that it cannot be
unduly modified by an error somewhere else in the program.
This too is becoming an important area of software engineering
and is usually referred to as information hiding.
PROGRAMMING EXERCISE
____________________________________________________________
1. Move My_Pi to the interface in both units and change one
of the values slightly to see if you can read in the
right one at the right time. Define another variable of
the same name in your main program and see if you can
differentiate between all three values. Note that, due
to the nature of this exercise, no answer is given for
it on the distribution disk.
Page 13-6
CHAP14.TXT
Chapter 14
COMPLETE SAMPLE PROGRAMS
Prior to this point, this tutorial has given you many example
programs illustrating a point of some kind, but these have all
been "nonsense" programs as far as being useful. It would be
a disservice to you to simply quit with only tiny programs to
study, so the following programs are offered to you as
examples of good Pascal programming practice. They are useful
programs, but they are still short enough to easily grasp
their meaning. We will discuss them one at a time.
AMORTIZATION TABLE GENERATOR
____________________________________________________________
This is not one program, but five. Each one is an improvement
on the previous one, and the series is intended to give you
an idea of program development.
AMORT1.PAS - This is the bare outline of the amortization
program. Although it is an operating program, it doesn't
do very much. After some thought and planning, the main
program was written to allow for an initialization, then
an annual repeating loop. The annual loop would require
a header, a monthly calculation, and an annual balance.
Finally, a procedure was outlined for each of these
functions with a minimum of calculations in each
procedure. This program can be compiled and run to see
that it does do something for each month and for each
year. It has a major problem because it does not stop
when the loan is payed off but keeps going to the end of
that year. The primary structure is complete.
AMORT2.PAS - This is an improvement over AMORT1. The monthly
calculations are correct but the final payment is still
incorrectly done. Notice that for ease of testing, the
loan variables are simply defined as constants in the
initialize procedure. To make the procedures easier to
find, comments with asterisks were added. This program
is nearly usable. Compile and run it.
AMORT3.PAS - Now we calculate the final payment correctly and
we have a correct annual header with column headings. We
have introduced a new variable to be used for an annual
interest accumulation. This is neat to have at income tax
time. This program can also be compiled and run.
AMORT4.PAS - This program does nearly everything we would like
it to do. All of the information needed to build the
table for any loan is now read in from the keyboard,
Page 14-1
Complete Sample Programs
greatly adding to the flexibility. After the information
is available, the monthly payment is calculated in the
newly added procedure Calculate_Payment. The annual
header has a new line added to include the original loan
amount and the interest rate in the information. Compile
and run this program to see its operation.
AMORT5.PAS - The only additional feature in this program is
the addition of a printout of the results. Examining the
program, you will notice that many of the output
statements are duplicated with the Lst included for the
device selection. Compile and run this program, but be
sure to turn your printer on to get a printout of the
amortization table you ask for. If you are using TURBO
Pascal version 3.0, you will need to either comment out
line 3 or remove it altogether.
TOP DOWN PROGRAMMING
____________________________________________________________
The preceding example is an example of a top-down approach to
programming. This is where the overall task is outlined, and
the details are added in whatever fashion makes sense to the
designer. The opposite is a bottom-up programming effort, in
which the heart of the problem is defined and the rest of the
program is built up around it. In this case, the monthly
payment schedule would probably be a starting point and the
remainder of the program slowly built up around it. Use
whichever method works best for you.
The final program AMORT5.PAS is by no means a program which
can never be improved upon. Many improvements can be thought
of. These will be exercises for you if you so desire.
1. In the data input section, ask if a printout is desired,
and only print if it was requested. This would involve
defining a new variable and if statements controlling all
write statements with Lst as a device selector.
2. Format the printout with a formfeed every three years to
cause a neater printout. The program presently prints
data right across the paper folds with no regard to the
top of page.
3. Modify the program to include semimonthly payments.
Payments twice a month are becoming popular, but this
program cannot handle them.
4. Instead of listing the months as numbers, put in a case
statement to cause the months to be printed out as three
letter names. You could also include the day of the month
when the payment is due.
Page 14-2
Complete Sample Programs
5. Any other modification you can think up. The more you
modify this and other programs, the more experience and
confidence you will gain.
LIST.PAS, to list your Pascal programs
____________________________________________________________
Since the differences between TURBO Pascal versions are
significant, two files are included here. If you are using
TURBO Pascal 3.0, rename LIST3.PAS to LIST.PAS, and if you are
using TURBO Pascal 4.0 or 5.x, rename LIST4.PAS to LIST.PAS
before continuing on to the next section.
LIST.PAS is a very useful program that you can use to list
your Pascal programs on the printer. It can only be compiled
with TURBO Pascal because it uses TURBO extensions. The two
extensions it uses are the string type variable and (in the
case of TURBO Pascal version 3.0), the absolute type variable.
The absolute type variable in line 14 and the coding in the
Initialize procedure is an example of how you can read in the
parameters given on the command line.
If you are using TURBO Pascal 4.0 or 5.x, a completely
different method is used in the Initialize procedure which
should be no problem for you to understand at this point. To
use this program to print out the last program, for example,
you would enter the following at the DOS prompt LIST
AMORT5.PAS. This program reads in the AMORT5.PAS from the
command line and uses it to define the input file. It should
be pointed out that this program cannot be run from a
"compiled in memory" compilation with the TURBO Pascal
compiler. It must be compiled to a Disk file, and you must
quit TURBO Pascal in order to run it from the DOS command
level.
The parameter read from the command line, AMORT5.PAS, is
stored at computer memory location 80(hexadecimal) referred
to the present code segment. If you didn't understand that,
don't worry, you can still find the input parameter in any
program using the method given in the initialize procedure for
your version of TURBO Pascal.
If you do not have TURBO Pascal, but you are using MS-DOS or
PC-DOS, you can still use this program because it is on your
disk already compiled as LIST.EXE, and can be run like any
other .COM or .EXE program. Note that LIST4.PAS is the
version that is compiled and copied on the distribution disk.
Page 14-3
Complete Sample Programs
TIMEDATE.PAS, to get today's time and date
____________________________________________________________
This is a very useful program as an example of using some of
the extensions of TURBO Pascal if you are using TURBO Pascal
3.0. It interrogates the inner workings of DOS and gets the
present time and date for you, provided you entered them
correctly when you turned your computer on. The procedure
Time_And_Date can be included in any TURBO Pascal version 3.0
program you write to give you the time and date for your
listings. As an exercise in programming, add the time and
date to the program LIST to improve on its usefulness.
The program named TIMEDAT4.PAS does the same thing as the
last, but it works with TURBO Pascal 4.0 or 5.x using the
means of defining a DOS call as it has been revised for the
newer versions. It turns out to be an almost trivial program
but is still a good illustration of how to use some of the
newer Borland extensions to Pascal. The observant student
will notice that the time and date procedures have already
been added to LIST4.PAS.
SETTIME.PAS, a useful utility program
____________________________________________________________
This program is very interesting in that it changes the date
and time stamp on any file in the current directory. It is
the program used to set the time and date on all of the files
on the distribution disks included with this tutorial. It
sets the time to 12:00:00 and the date to Dec 1, 1989 but you
can use it to set any desired time.
SHAPES3.PAS, an example of menus
____________________________________________________________
This program is not very useful, but it illustrates one way
to handle menus in a Pascal program, but only if you are using
version 3.0 of TURBO Pascal. Chapter 13 included the
identical program done slightly differently for use with the
TURBO Pascal 4.0 or 5.x compilers. You can study the
structure and imagine many ways a menu can be used to improve
the usefulness of your own programs.
OT.PAS, The OAKTREE directory program
____________________________________________________________
This program should be very useful to you, especially if you
have a hard disk. It will list the entire contents of your
hard disk (or floppy) in a very easy to read and easy to use
form. The program is documented in the file named OT.DOC.
It uses many of the TURBO Pascal extensions and will probably
Page 14-4
Complete Sample Programs
not compile with any other Pascal compiler without extensive
modifications.
You will find two versions of the source code for this
program, one named OT3.PAS for use with TURBO Pascal version
3.0, and another named OT4.PAS for use with versions 4.0 or
5.x of the TURBO Pascal compiler. You should rename one of
them OT.PAS for use with your particular compiler.
The two versions are different in a number of ways. The first
version was written for TURBO Pascal version 3.0 and was only
slightly modified for this new version of the tutorial. The
newer version, OT4.PAS, was modified extensively to use some
of the procedures provided by Borland such as GetDate,
GetTime, FindFirst, and FindNext. The program for version 4.0
or 5.x is somewhat smaller since the predefined procedures use
fewer characters to perform a given job, and the executable
version shows an even greater reduction in size. Apparently
Borland has done a very good job in code size reduction with
the introduction of each new version of TURBO Pascal.
It would benefit you greatly to study the two versions of
OT.PAS side by side and compare the benefits of using the
predefined procedures.
Examine the procedure named Count_Print_Lines in each of the
OT.PAS programs, and you will see a marked difference between
them. OT3.PAS counts the lines of output, and when it detects
that 55 lines have been output, it makes 11 Writeln calls to
space the paper up to 66 lines. This works fine for a printer
using continuous form paper, but it doesn't work with a laser
printer that happens to be set to some number other than 66
lines. For that reason, OT4.PAS uses a form feed after it
detects that 55 lines have been output. You can choose the
proper OT.PAS program for your compiler and use whichever
paper advance method suits you best. This is a very useful
program, so you should spend the time necessary to both
understand it and modify it for your own needs.
You will find either program to be a good example of linked
lists because they include a sort routine using a dynamically
allocated B-TREE and another sorting routine that uses a
dynamically allocated linked list with a bubble sort. These
methods are completely defined in Niklaus Wirth's book,
"Algorithms + Data Structures = Programs", a highly
recommended book if you are interested in advanced programming
techniques.
It might also be pointed out that both OT3.PAS and OT4.PAS
also makes use of recursive methods for both sorting and
handling subdirectories. It is definitely an example of
advanced programming methods, and it would be a good vehicle
for your personal study.
Page 14-5
Complete Sample Programs
MOST IMPORTANT - Your own programs
____________________________________________________________
Having completed this tutorial on Pascal, you are well on your
way to becoming a proficient Pascal programmer. The best way
you can improve your skills now is to actually write Pascal
programs. Another way to aid in your building of skill and
confidence is to study other Pascal programs. Many
programming examples can be found in computing magazines and
books. There are many books available devoted entirely to
TURBO Pascal and you would do well to visit your local
bookstore and review a few of them.
You already own one of the best books available for reference
if you are using TURBO Pascal. Although the TURBO Pascal
reference manual is worth very little as a learning tool, it
is excellent as a language reference manual. Now that you
have completed all 14 chapters of this tutorial, you have a
good grasp of the terminology of Pascal and should have little
trouble reading and understanding your reference manual. Your
only limitation at this point is your own perseverance and
imagination.
OBJECT ORIENTED PROGRAMMING
____________________________________________________________
If you are relatively new to programming, it would be good for
you to completely ignore the last two chapters of this
tutorial until you gain some experience with standard Pascal.
Object oriented programming is a relatively new development,
and although it is a meaningful study, it is totally
unnecessary for small programming projects. Once your
programs grow to the point where you feel comfortable with a
1000 line Pascal program, you are ready for a study of this
new packaging technique. Until then, it may only serve to
confuse you.
Whatever your programming level or needs may be, Pascal can
fulfill them and do so in a very elegant way,
Happy Programming.
Page 14-6
CHAP15.TXT
Chapter 15
ENCAPSULATION & INHERITANCE
Encapsulation is the cornerstone upon which object oriented
programming is built, and without which it would not exist.
We will cover the topic of encapsulation in this chapter in
enough depth to illustrate its use and what it can do for you
in software development. Because there are many new terms in
this chapter, you could very easily become intimidated, and
wish to simply give up on this new topic. You can be assured
that the time spent studying encapsulation will be greatly
rewarded as you apply this new technique in your software
development efforts.
Object oriented programming is not a panacea to solve all of
your software problems, but it is a new and improved way of
programming. In fact it is really more of a software
packaging technology than a new method of programming. You
will find that your software will be easier to write and debug
as you gain experience using this new packaging method. Like
any new endeavor however, it will require some effort on your
part to master these concepts.
OUR FIRST ENCAPSULATION
____________________________________________________________
The example program named ENCAP1.PAS contains ==============
our first example of encapsulation. In order ENCAP1.PAS
to keep it easy to understand, it was kept ==============
very short. This results in a program that
does not illustrate the advantage of using
object oriented programming, but it does give us a start in
the right direction. With this in mind, load ENCAP1.PAS and
we will study the code contained in it.
Line 5 has our first new reserved word, object. This is used
in much the same way that the reserved word record is used,
but it has a much different meaning. An object is permitted
to have not only data embedded within it, but also procedures,
functions, and constructors. Constructors will be described
in detail later. Since data plus procedures and functions can
be grouped together in this fashion, the object is said to be
encapsulated. An object is therefore a group of related data
and the subprograms that operate on that data, all entities
being very closely coupled together.
WHAT IS A METHOD?
____________________________________________________________
A method is a term used with object oriented programming, and
for the time being we will simply say that a method is either
Page 15-1
Encapsulation and Inheritance
a function or a procedure (including a constructor). A method
is therefore a method for doing an operation on some data.
Lines 8 through 10 are method headers and give the pattern for
all calls to these methods which can be used by the compiler
to check for the correct number and types of parameters.
Once again, we promise to discuss the constructor soon. For
the time being, simply think of it as another procedure.
The entire object type definition is given in lines 5 through
11. This object contains two variables named length and
width, each of type integer, and three methods which can be
used to operate on the two variables. In the same manner that
the definition of a type in Pascal does not actually give you
a variable to use, only a pattern, the definition of an object
type does not give you an object. We will declare the objects
when we get to line 30 of this program.
You will note that we are already using new terminology, but
this is necessary. The field of object oriented programming
has its own vocabulary and in order for you to understand
technical articles in this field, you must begin now to learn
the new terminology. It won't be too long until you feel
somewhat comfortable with it.
THE METHOD IMPLEMENTATION
____________________________________________________________
The object type definition describes in detail what we can do
with the object but we must now describe what actions will
take place when each of the methods is called. The
implementation for each method will define the operations for
that method and are defined in lines 13 through 28 of this
example program. The only thing that is really different
about these methods is the way their headers are defined,
namely the inclusion, in the header, of the object type name
Box dotted to the method name. This is required, but we will
wait until the next example program to define why it is
needed.
The observant student will also notice that we are referring
to the object variables within the methods of that object
without the object name dotted to the variable name. This is
because the implied object name is automatically "with"ed to
the variable names within the method implementations, allowing
the variables to be directly referred to within the objects
because of the definition of object oriented programming. It
should be obvious that any mathematics or logical operations
can be done within the implementations of the methods. In
fact, you can perform any legal Pascal operations within the
methods, just like you can in any Pascal function or
procedure. Very short operations were selected here because
Page 15-2
Encapsulation and Inheritance
we wish to illustrate the interfaces to the methods at this
point in the tutorial.
AN INSTANCE OF AN OBJECT
____________________________________________________________
We need another new term at this point. When we use the
object type to declare variables of that type as we do in line
30, we are creating instances of that object type. An
instance is like a variable in conventional Pascal (non object
oriented), except that it can do more and has some very
interesting properties that a simple variable does not have.
In line 30 we have created three instances of the object type
named Box and each has two simple variables associated with
it. Three methods are available which can be called to
operate on these variables. We therefore have three objects
named Small, Medium, and Large. In order to initialize the
values stored within the objects we call the three objects in
lines 34 through 36 to store values in their internal
variables by dotting the name of the object to the name of the
method we wish to call. You will note that this looks like
the same technique we use to refer to the fields of a record.
We display the area of the three boxes in lines 38 through 40
using the same technique used to initialize the values stored,
and the program is complete.
We seem to have accomplished very little with this program
that we could not have more easily accomplished with an even
shorter standard Pascal program, and that is true. This
program is only meant to introduce some of the mechanics of
object oriented programming and additional programs will be
used to illustrate some of the uses of this new technique.
NEW TERMINOLOGY
____________________________________________________________
You may note that we switched terminology halfway through the
above paragraphs. We began by referring to the object types
as object types and calling the variables declared in line 30
instances. Later we began calling the instances objects. In
this tutorial we will refer to the types as object types and
the variables either as objects or instances. This
terminology is consistent with current practice and should
help you learn the new terminology.
Another very important point is the fact that we pass a
message to a method rather than call a subprogram as in
conventional Pascal. The difference is rather subtle, but
there really is a difference as we will see a little later in
this tutorial.
Page 15-3
Encapsulation and Inheritance
WHAT DID WE ACCOMPLISH?
____________________________________________________________
In this program we defined an object type, then declared
several instances of that type, one of which was named Small.
The object named Small has two internal variables that should
only be accessed via its methods, so we will refer to them as
private data points. (Actually, they should be unavailable
to any user outside of the method implementations but Borland
chose not to make them private. It is therefore up to you to
discipline yourself to not refer to them directly.) The
proper way to use the object is to send a message to the
object telling it to do something to itself. In the case of
the Init method, we are telling it to store the two values in
its private variables named length and width, and in the case
of the Get_Area method, we are telling it to give us the
product of its own internally stored width and length which
we can then print out.
Remember that in the beginning of this chapter we said that
object oriented programming is a code packaging technique.
That should help to explain some of the strange things we did
in this program. As we continue through the example programs,
we will see that everything here was done for a reason and you
will eventually learn to use and prefer object oriented
programming methods over the old familiar procedural
programming method you have been using.
Be sure to compile and execute this program to see if it does
what the comments say it will do.
DATA & CODE PROTECTION
____________________________________________________________
The data and methods are protected from outside influence
because they are packaged together with an object. Of even
more importance is the fact that because they were to be
packaged together, they were probably carefully thought out
together during the design stage. This would probably result
in a much more understandable program. The object keeps the
data and methods together and keeps them working in close
synchronization.
An object type is sometimes referred to as an abstract data
type in the technical literature discussing object oriented
programming.
Page 15-4
Encapsulation and Inheritance
MORE ENCAPSULATION
____________________________________________________________
The example program named ENCAP2.PAS uses ================
most of the same techniques as the last ENCAP2.PAS
program but this is much more meaningful ================
since it illustrates one of the simplest
advantages of using object oriented
programming.
In this program, we define two object types in lines 5 through
19, Box and Pole. Each has its own unique kinds of variables
associated with it, and each has three methods that can be
used with these kinds of data. The method definitions in
lines 33 and 44 clearly illustrate why the object name must
be associated with the method name in the method
implementation. This allows you to use the same method name
in more than one object definition. In addition to the two
method definitions named Set_Data given here, we could also
define and use another procedure with the name Set_Data that
was a normal Pascal procedure just like any others we have
used in prior chapters of this tutorial. Using the same
method name in several places is often referred to as name
overloading in object oriented programming terminology.
You will note that in lines 5 through 19 we define the object
types which define what the object will do. In lines 21
through 53 we define the method implementations which define
how we do it. It is assumed that you know enough Pascal at
this point to understand what each method does, so nothing
more will be said about the details of this program.
In lines 55 and 56, we declare several objects of the defined
object types and use some of them in the main program. We can
finally illustrate one of the biggest advantages of object
oriented programming.
We should all agree that it would be silly and meaningless to
multiply the height of one of the poles by the width of a box.
If we were using standard procedural programming with all
variables defined globally, it would be a simple matter to
accidentally write height*width and print out the result
thinking we had a meaningful answer. By encapsulating the
data within the objects, we would have to really work at it
to get that meaningless answer because the system itself would
prevent us from accidentally using the wrong data. This is
true only if we have agreed not to use any of the data
directly but to do all data access through the available
methods.
Encapsulation is a form of information hiding, but it is a
rather weak form of it in TURBO Pascal because, as mentioned
earlier, Borland chose not to make the variables within the
object private. If the variables were defined as private
Page 15-5
Encapsulation and Inheritance
variables, they would be unaccessible outside of the
implementation for the object and the client would be forced
to use only the methods provided by the author of the object
to access the contained data. This would be true information
hiding and would add some degree of protection to the internal
data. It is up to you to never refer to the data within the
object directly as stated by Borland in the OOP GUIDE included
with the compiler.
The careful student will notice that since all data is
carefully tied up within the objects, inadvertent mixing of
the wrong data is impossible provided a few simple rules are
followed as discussed above. Once again, this is such a small
program that it is difficult to see the advantage of going to
all of this trouble. In a larger program, once the objects
are completed, it is a simple matter to use them knowing that
they are debugged and working.
After the data are all printed out, some of the variables are
changed in lines 78 through 80, and the same output statements
are used to reprint the same data so you can observe the
changes.
A FEW RULES ARE NEEDED
____________________________________________________________
As with any new topic, there are a few rules we must follow
to use this new technique. The variables must all be declared
first in the object followed by the method definitions. The
names of all variables within an object must be unique and
may not be repeated as the names of any of the formal
variables in any of the methods. Thus length, width, len, and
wid must be unique as used in lines 6, 7, and 8. The names
of formal variables may be reused in other methods however,
as illustrated in lines 8 and 9, and all names may be reused
in another object. It should be obvious that all object type
names must be unique within a given file and all objects must
have unique names.
All of the above rules are obvious if you spend a little time
thinking about them. They should therefore not be a stumbling
block to anyone with some procedural programming experience.
WHAT IS A CONSTRUCTOR?
____________________________________________________________
It is time to keep our promise and define just what a
constructor is. In this present context, that of simple
objects, the constructor does very little for us, but we will
include one for nearly every object to illustrate its use.
Page 15-6
Encapsulation and Inheritance
The constructor can be named anything desired but it would be
best to stick with the convention and name every constructor
Init as suggested by Borland. The constructor is used to
initialize all values within an object and do any other setup
that must be done to use an object. The constructor should
be called once for every declared object. When we get to the
topic of virtual functions, constructors will be absolutely
required for every object, but for the simple objects we are
using here, they are optional.
It would be best to include a constructor in every object
type, use the constructor to initialize all variables within
the object, and call the constructor once for each instance
of the object type. Until we get to virtual methods, none of
this is required, but it would be good practice to get in the
habit of doing it.
WHAT IS A DESTRUCTOR?
____________________________________________________________
A destructor is another method that can be used for cleanup
when you are finished with an object. It is usually used in
conjunction with dynamic allocation to assure that all
dynamically allocated fields associated with the object are
deallocated prior to leaving the scope of the object. A
destructor is not illustrated in this tutorial but it should
be easy for you to define and use one when you have a need for
one.
OUR FIRST INHERITANCE
____________________________________________________________
Load the example program named INHERIT1.PAS ================
for our first example of a program with INHERIT1.PAS
inheritance. As always, our first encounter ================
with this new topic will be very simple.
In lines 7 through 14 we define a simple object type defining
a vehicle and a few characteristics about the vehicle. We
have the ability to store a few values and read them out in
several ways. Of course, most of the interest is in the
interfaces, so the implementations are purposely kept very
small.
In lines 17 through 35, we declare two additional object types
that use the Vehicle type as a base for the new types as
indicated by the previously defined name Vehicle in
parentheses in the object definitions in lines 17 and 26. The
Vehicle object is said to be the ancestor type and the two new
object types are called descendant types. The descendant
Page 15-7
Encapsulation and Inheritance
types inherit some information from the ancestor types
according to well defined rules. The variables in the
ancestor type are all included within the descendant types and
are available in objects of the descendant types just as if
they had been defined within the descendant types. For that
reason, all variable names must be unique within the ancestor
type and within each of the descendant types. A name can be
reused in one or more descendants however, as is illustrated
in lines 18 and 27 where the variable name Passenger_Load is
used in both object types.
The method names from the ancestor object types can be
repeated in the descendant object types but this has the
effect of overriding the method of the same name in the
ancestor making the ancestor method unavailable for use in
objects of the descendant types. Objects instantiated of the
type Car therefore, have the three methods available in lines
11 through 13 of the ancestor type, the constructor in line
19 which overrides the constructor in line 10 of the ancestor
type, and the function given in line 22. This object
therefore has five different methods to perform its required
operations.
Objects of type Truck have five methods available also, the
two in lines 11 and 12 and the one in line 33. The two in
lines 29 and 34 of the descendant overrides the two in lines
10 and 13 of the ancestor object type.
You should note that even though some of the methods were
overridden in the descendant object type, they do not affect
the ancestor, and instances of the Vehicle type have two
variables and four methods available.
In effect we have an object hierarchy which can be extended
to as many levels as necessary to complete the task at hand.
The most important part of object oriented programming is the
definition of the objects in a meaningful manner, but it is
not something you will learn to do overnight. It will take
a great deal of practice until you can see the objects in any
given project in such a way that a clear solution can be
found. I was somewhat intimidated by the clever examples
found in a classic text on object oriented programming until
I talked to a man that had shared an office with the author
at the time he was writing that particular book. I learned
that what was finally put in the book was at least the fourth
iteration of each problem and in some cases the seventh before
he finally arrived at a good solution. We will have more to
say about this as we progress through this tutorial.
Page 15-8
Encapsulation and Inheritance
HOW DO WE USE THE OBJECTS?
____________________________________________________________
The implementations of the methods are given in lines 39
through 92 and should be self explanatory, except for a few
notable exceptions. You will note that in line 81 we send a
message to the Vehicle.Init method to initialize some data.
A change to the Vehicle type will be reflected in the Truck
type also because of this call. In lines 64 and 65 we are
using some inherited variables just as if they had been
defined as part of the descendant object types.
In lines 96 through 98 we instantiate one of each and send a
message to their constructors in lines 102 through 104 then
print out a few of the stored values.
Lines 113 through 116 are repeated in lines 120 through 123
where they are placed within a with section to illustrate that
the with can be used for the calls to the methods in the same
manner that it is used for accessing the fields of a record.
Any other details of this program can be gleaned by the
diligent student. Be sure to compile and execute this program
so you can verify the given result.
AN OBJECT IN A UNIT
____________________________________________________________
Load the example program named VEHICLES.PAS ================
for an example of the proper way to package VEHICLES.PAS
the object so it can be conveniently reused ================
for another project.
The object type definition is given in the public part of the
unit so it is available to any Pascal program which needs to
use it. The implementation of the methods are hidden in the
implementation part of the unit where they are not directly
available to any calling program. Note that it is also
possible to define a few local methods within the
implementation for use only within the implementation but none
are illustrated here. There is no body to this unit, only the
end statement in line 39 so there is no initialization code
to be executed during loading. It would be perfectly legal
to include an initialization body, but if you do, be sure to
include a constructor to be called once for each object. This
is to prepare you for the use of virtual functions which we
will study in the next chapter.
It will be necessary for you to compile this unit to disk so
it can be used with the last example program in this chapter.
Page 15-9
Encapsulation and Inheritance
ANOTHER OBJECT IN A UNIT
____________________________________________________________
The example program named CARTRUCK.PAS ================
continues the new packaging scheme by CARTRUCK.PAS
including the two descendant object types in ================
its interface after telling the system that
it uses the Vehicles unit.
The remainder of this unit is constructed just like the last
one so nothing more needs to be said about it. Be sure to
compile this unit to disk so it will be available for use with
the next example program.
Note that this unit could have been further divided into two
separate units, one for each object type, but it was felt that
it was important to illustrate that several can be combined
in this manner if desired. In like manner, the last unit
could have been combined with this unit, but once again, it
was desired to illustrate the generality of program
decomposition and packaging.
USING THE OBJECTS DEFINED IN UNITS
____________________________________________________________
Load the program named INHERIT2.PAS for an ================
example that uses the units of the last two INHERIT2.PAS
example programs and is identical to the ================
program named INHERIT1.PAS.
The only difference in these two programs is in the way the
code was packaged. The second way is much more general and
more conducive to good software engineering practices because
it allows separate development of each of the three program
units. Each can be refined independently of the other two and
the overall package can be simpler to debug and maintain. It
should be clear that any changes to the Car object, for
example, will be localized to that single unit and not
scattered all over the software terrain.
AN ARRAY AND A POINTER
____________________________________________________________
Examine the example program named ================
INHERIT3.PAS for an example of the use of a INHERIT3.PAS
pointer to an object and the use of an array ================
of objects.
This program is nearly identical to INHERIT2.PAS except for
the addition of an array of Car type objects named Sedan[1]
to Sedan [3], and the definition of a pointer to the Truck
type object named Semi_Point. Lines 16 and 17 illustrate the
Page 15-10
Encapsulation and Inheritance
initialization of the array of Sedan, and lines 23 through 26
illustrates its use when the data is printed out. An object
is dynamically allocated in line 18 and it is then initialized
in the next line. Its use is illustrated in lines 28 through
40 and it is deallocated in line 41.
TURBO Pascal 5.5 has an extension to the New procedure
allowing the dynamic allocation and the initialization to take
place in the same procedure call. The line
New(Semi_Point, Init(1, 25000.0, 18, 5000.0));
can be used to replace lines 18 and 19 in this program if you
desire to do so.
This program should illustrate that objects can be used with
arrays and pointers just like a record. Be sure to compile
and execute this program.
WHAT IS MULTIPLE INHERITANCE?
____________________________________________________________
Multiple inheritance allows the programmer to inherit data and
methods from two or more ancestor objects. When this is done
however, there is a real problem if there are two variables
or methods of the same name and it is up to the programmer to
somehow define which will be used by the descendent. Some
object oriented programming languages allow multiple
inheritance, but most do not. TURBO Pascal (version 5.5) has
no provision for multiple inheritance, and Borland has made
no indication at this time whether future versions will allow
it.
WHAT SHOULD YOU DO NOW?
____________________________________________________________
You have reached a major point in your excursion of object
oriented programming, because you now have most of the
knowledge you need to do some serious object oriented
programming. The best thing for you to do at this point is
stop studying and get busy programming and using some of these
techniques for your projects. The only topic left is the use
of virtual methods and you can easily defer its use for a long
time.
One point should be made before you begin a serious
programming project. Your first program could have too many
objects and be nearly unreadable unless you strive to use only
a few objects until you gain experience. Define only a few
objects and write the majority of the program in standard
Page 15-11
Encapsulation and Inheritance
procedural programming methods. Add a few more objects to
your next project and as you gain experience, you will feel
very comfortable with the use of objects and your programming
methods will be very clear.
Now is the time to begin using this new knowledge but enter
the water slowly the first time.
PROGRAMMING EXERCISES
____________________________________________________________
1. Modify ENCAP2.PAS in such a way to multiply the height
of the short pole times the length of the medium box and
print the result out. Even though this is possible to
do, it requires you to expend a bit of effort to
accomplish. Remember that you should not use the
components of an object directly, only through use of the
available methods.
2. Add an object named Pick_Up to INHERIT2.PAS of type Truck
and initialize it to some reasonable values. Print out
its loading and efficiency in a manner similar to the
Semi.
Page 15-12
CHAP16.TXT
Chapter 16
VIRTUAL METHODS
Since we covered encapsulation and inheritance in the last
chapter, we are left with only virtual methods to complete the
major topics of object oriented programming. Virtual methods,
as they are called in TURBO Pascal, have several other names
in the literature to describe the same technique. This
technique is sometimes called run-time binding or late binding
referring to when the decision is made as to what method will
respond to the message. The use of virtual methods moves the
responsibility of selection from the client (the logic sending
the message) to the supplier (the methods responding to the
message). We will begin with a skeleton of a program without
a virtual method and add one to show the effect of adding a
virtual method.
WITHOUT A VIRTUAL METHOD
____________________________________________________________
The example program named VIRTUAL1.PAS will ================
be used as the starting point for the study VIRTUAL1.PAS
of virtual functions. We must state that ================
this program does not contain a virtual
function, it is only the starting point for
studying them.
The objects included here are very similar to the objects
describing vehicles which we were working with in the last
chapter. You will notice that all three objects contain a
method named Message in lines 11, 20, and 31. The Message
method will be the center of our study in the first three
example programs. It should be pointed out that the
constructors for the three objects are called in lines 98
through 100 even though a constructor call is still not
absolutely necessary in this case. We will have more to say
about the constructor calls during the next example program.
Compile and execute the program and you will find that even
though it is legal to pass the objects of type Car and Truck
to the method named Output_A_Message in lines 109 and 110, the
method that is called from line 86 is the method named Message
in the parent type Vehicle. This is probably no surprise to
you since we defined an object of type Vehicle as a formal
parameter of the method Output_A_Message. We need only one
small change and we will have a virtual procedure call.
Even though this program seems to do very little, it will be
the basis of our study of virtual methods so you should study
the code in detail.
Page 16-1
Virtual Methods
NOW TO MAKE IT A VIRTUAL METHOD
____________________________________________________________
Examine the example program named ================
VIRTUAL2.PAS, and you will find only one VIRTUAL2.PAS
small change in the code but a world of ================
difference in the way it executes.
The careful student will notice the addition of the reserved
word virtual in lines 13, 22, and 33. This makes the method
named Message a virtual method which operates a little
differently from the way it did in the last program. Once
again, we call the three constructors in lines 100 through 102
and this time the constructor calls are absolutely essential.
We will discuss why in a couple of paragraphs.
Once again we send a message to Output_A_Message three times
in lines 110 through 112 and line 88 is used to send a message
to the Message method. When we compile and execute this
program, we find that even though the method Output_A_Message
only uses the parent type Vehicle, the system calls the
correct procedure based on the type of the actual object
passed to this method. The system sends a message to the
objects of the correct type instead of to the parent type as
may be expected. It should be clear to you that the object
that is to receive the message is not known at compile time
but must be selected at run time when the object arrives at
the method Output_A_Message. This is known as late binding
since the type is not known until run time as opposed to early
binding where the type is known at compile time. Every
subprogram call in this entire tutorial, up to this point, has
been early binding.
You will note that even though the method Output_A_Message
only knows about the objects of type Vehicle, it has the
ability to pass through other types, provided of course that
they are descendant types of Vehicle. The method
Output_A_Message only passes the message through, it does not
do the selection. The selection is done by the objects
themselves which answer the messages passed to them. This
means that the sender does not know where the message will be
answered from, and it is up to the receiver to find that a
message is being sent its way and to respond to it. It is
often said that the supplier (the method doing the work) must
make the decision to answer the message, rather than the
client (the user of the work done). The burden is placed on
the supplier to do the right thing.
If a method is declared virtual, all methods of that name must
also be virtual including all ancestors and all descendants.
It is not possible to declare part of the methods of the same
name virtual and part standard. All parameter lists for all
virtual methods of the same name must also be identical since
Page 16-2
Virtual Methods
they must all be capable of being called by the same method
call.
ASSIGNING DESCENDANTS TO ANCESTORS?
____________________________________________________________
It is legal in any object oriented language to assign a
descendant object to an ancestor variable but the reverse is
not true. A vehicle, for example, can be used to define a
car, a truck, a bus, or any number of other kinds of vehicles
so it can be assigned any of those values. A car on the other
hand, is too specific to be used for the definition of
anything but a car, so it cannot have any other value assigned
to it. A vehicle is very general and can cover a wide range
of values, but a car is very specific and can therefore only
define a car.
WHY USE A CONSTRUCTOR?
____________________________________________________________
The constructor is absolutely required in this case because
of the way the authors of TURBO Pascal defined the use of
virtual functions. The constructor sets up a pointer to a
virtual method table (VMT) which is used to find the virtual
methods. If there is no pointer, the system jumps off to some
unknown location and tries to execute whatever happens to be
there and could do almost anything at that unknown and
undefined point in the code. So it is important to call a
constructor once for each object as is done here so the
pointer to the VMT can be initialized to the proper value.
If you make several objects of one type, it is not enough to
call a constructor for one object and copy that object into
each of the other objects. Each object must have its own
constructor call in order to prevent a system crash.
The strange looking code in line 6 tells the system to check
each call to a virtual function to see if the constructor has
been called. This slows the program down slightly but will
result in an error message if a virtual method is called prior
to its VMT being properly set up with a constructor call.
After a program is thoroughly tested, the code can be removed
from line 6 to speed up the program slightly by eliminating
the checks. Be warned however, that a call to a virtual
method without A VMT will probably result in the computer
hanging up.
Page 16-3
Virtual Methods
VIRTUALS AND POINTERS
____________________________________________________________
The example program named VIRTUAL3.PAS is ================
nearly identical to the last program except VIRTUAL3.PAS
that this program uses pointers to objects ================
instead of using the objects directly.
You will notice that once again, the methods named Message are
all defined as virtual and a pointer type is defined for each
object type. In lines 99 through 101, three pointers are
declared and memory is dynamically allocated on the heap for
the objects themselves. The objects are all sent a
constructor message to initialize the stored data within the
objects and to set up the VMT for each. The rest of the
program is nearly identical to the last program except that
Dispose procedures are called for each of the dynamically
allocated objects. The code used in line 6 of the last
program to force a check of each virtual method call has been
removed to illustrate that it doesn't have to be there if you
are sure a message is sent to a constructor once for each
object with a virtual method.
Compiling and executing this program will give the same result
as the last program indicating that it is perfectly legal to
use pointers to objects as well as the objects themselves.
AN ANCESTOR OBJECT
____________________________________________________________
The example program PERSON.PAS is not a ================
complete program at all but only an object PERSON.PAS
definition within a unit. This unit should ================
pose no problem for you to understand so we
will not say much except to point out that
the method named Display is a virtual method.
This example program, as well as the next two example
programs, have been carefully selected to illustrate the
proper way to package objects for use in a clear
understandable manner.
Compile this unit to disk in order to make it available for
use in the remainder of this chapter.
SOME DESCENDENT OBJECTS
____________________________________________________________
The example program named SUPERVSR.PAS is another unit which
contains three descendants of the previously defined object
Page 16-4
Virtual Methods
named Person. You will notice that each of ================
the objects have a method named Display which SUPERVSR.PAS
is virtual just as the same method in the ================
ancestor object was.
The interface for each object has been purposely kept very
simple in order to illustrate the use of objects. The
implementation has also been kept as simple as possible for
the same reason so the diligent student should have no trouble
in understanding this unit completely.
Once again, be sure to compile this unit to disk in order to
make it available for use in the next few example programs.
A COMPLETE EMPLOYEE PROGRAM
____________________________________________________________
Although the program named EMPLOYEE.PAS is a ================
very short program that does very little, it EMPLOYEE.PAS
is a complete program to handle a very small ================
amount of data about your employees.
You will notice that we declare an array of ten pointers to
the Person_ID object and one pointer to each of the three
descendant objects. In the main program we send a message to
the constructor for each of the array elements. Inspection
of the Person_ID.Init code will reveal that this
initialization does nothing. It is used to initialize the
pointer to the VMT for each object, so the message must be
sent. We then dynamically allocate six objects of assorted
descendant objects being careful to send a message to the
constructor for each object. This is done to generate a VMT
for each object as it is allocated. Finally, we send a
message to the first six objects pointed to by the array of
pointers instructing them to display their values.
When the program is compiled and executed, we find that the
virtual methods were called as explained in the last example
program. Even though only one kind of pointer was passed to
the Display method, three different messages were actually
displayed, each message being of the proper kind based on the
type of pointer used.
You will notice how clean and neat the main program is. It
is extremely easy to follow because all of the implementation
details have been moved to the objects themselves. Once the
objects are carefully defined and debugged, the main program
is usually a snap to write and debug.
Object oriented programming requires a whole new mindset over
the procedural methods you have been using but after you catch
on to the technique, you will find your programs much easier
Page 16-5
Virtual Methods
to debug and maintain. The one thing you should avoid is the
use of too many objects in your first program. It is best to
define a few simple objects for your first attempt at object
oriented programming and write the rest of the program using
standard procedural methods. Then as you gain experience, you
can begin using more and more objects until you finally write
a program that is essentially all objects. Of course, you
will find that you will always write at least part of your
program in a standard procedural format as was done in
EMPLOYEE.PAS in this chapter.
PROGRAMMING EXERCISE
____________________________________________________________
1. Add a new object type to SUPERVSR.PAS to define a
Consultant defining appropriate data fields for him, then
add a couple of Consultant type objects to EMPLOYEE.PAS
to use the new object type.
Page 16-6
FILE0579.TXT
Disk No: 579
Disk Title: Pascal Tutor 1 of 2 (580 2nd disk) (Disk 1 of 2)
PC-SIG Version: S2.2
Program Title: Pascal Tutor
Author Version: 2.4
Author Registration: $15.00 to $39.95.
Special Requirements: Printer.
PASCAL TUTOR is a text-based tutorial on programming with the Pascal
language. The assumption is that you know nothing about Pascal in
particular and programming in general.
It begins with the most elementary aspects of programming and takes
you through each step from dynamic allocation to the use of linked
lists. The program was specifically written for use with Turbo Pascal
versions 3.0, 4.0, and 5.0. The Pascal source code used for examples
throughout the tutorial is included.
PC-SIG
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GO.TXT
╔═════════════════════════════════════════════════════════════════════════╗
║ <<<< Disk #579 PASCAL TUTORIAL (Disk 1 of 2) >>>> ║
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╚═════════════════════════════════════════════════════════════════════════╝
INTRO.TXT
INTRODUCTION
IF YOU KNOW NOTHING ABOUT PASCAL
____________________________________________________________
Assuming you know nothing at all about Pascal, and in fact,
that you may know nothing about programming in general, we
will begin to study Pascal. If you are already somewhat
familiar with programming and especially Pascal, you will
probably want to skip very quickly through the first few
chapters. You should at least skim these chapters, and you
should read the remainder of this introduction.
A few comments are in order to get us started in the right
direction. The sample programs included on the disks are
designed to teach you the basics of Pascal and they do not
include any clever or tricky code. Nearly all of the programs
are really quite dumb as far as being useful programs, but all
will teach one or more principles of Pascal. I have seen one
tutorial that included a 12 page program as the first example.
In fact there were only 2 example programs in the entire
tutorial, and it was impossible to glean the essentials of
programming from that system. For this reason, I will
completely bypass any long programs until the very end of this
tutorial. In order to illustrate fundamental concepts used
in Pascal programming, all programs will be very short and
concise until we reach the last chapter.
LARGER PASCAL PROGRAMS
____________________________________________________________
Chapter 14 has some rather large programs to illustrate to you
how to write a large program. It would be a disservice to you
to show you all of the constructs of Pascal and not show you
how to put them together in a meaningful way to build a large
program. After completing all of the fundamentals of Pascal,
it will then be very easy for you to use the tools learned to
build as large a program as you desire or require for your
next programming project.
Another problem I have noticed in example programs is the use
of one word for all definitions. For example, a sort program
is stored in a file called SORT, the program is named Sort,
and various parts of the program are referred to as Sort1,
Sort2, etc. This can be confusing since you have no idea if
the program name must be the same as the filename, or if any
of the other names were chosen to be the same because of some
obscure rule not clearly documented. For this reason, the
example programs use completely arbitrary names whenever the
choice of a name adds nothing to the readability or clarity
of a program. As an illustration of this, the first program
is named Puppy_Dog. This adds nothing to the understanding
Page I-1
Introduction
of the program but does illustrate that the program name means
nothing to the Pascal compiler concerning what the program
does.
Due to the fundamental design of the Pascal language, certain
words are "reserved" and can only be used for their defined
purposes. These are listed in your TURBO Pascal reference
manual (page 37 - version 3.0; page 196 - version 4.0;
reference guide page 11 - version 5.x). All of the sample
programs in this tutorial are written with the reserved words
in all lower-case letters, and the user variables in lower
case with the first letter capitalized since this is becoming
the accepted industry standard. Don't worry about what
reserved words are yet, they will be completely defined later.
In this tutorial, all reserved words, type names, variable
names, and procedure and function names will be listed in
boldface type within the text as an aid to the student.
Because it would add little and could possibly be confusing,
the simple predefined types will not be listed in boldface
type.
WHAT IS A COMPILER?
____________________________________________________________
There are two methods used to run any computer program that
is written in a readable form of English. The first method
is to use an interpreter. An interpreter is a program that
looks at each line of the "English" program, decides what the
"English" on that line means, and does what it says to do.
If one of the lines is executed repeatedly, it must be scanned
and analyzed each time, greatly slowing down the solution of
the problem at hand. A compiler, on the other hand, is a
program that looks at each statement one time and converts it
into a code that the computer understands directly. When the
compiled program is actually run, the computer does not have
to figure out what each statement means, it is already in a
form that the computer can run directly, hence a much faster
execution of the program.
This tutorial is written especially for Borland
International's TURBO Pascal compilers version 3.0, 4.0, or
5.x. These are very high quality compilers that can do nearly
anything you will ask them to do since they are so flexible.
The original intent of this tutorial was to write it in such
a way that it would be completely generic and usable with any
good Pascal compiler. The programmers at Borland included a
great many nonstandard aids for the Pascal language and
resulted in a very good product that has dominated the market
for microcomputers. To completely omit all of the extensions
would do those of you with the Borland compiler a real
disservice, and to include the extensions would not allow
other compilers to be used effectively with this tutorial.
Page I-2
Introduction
The decision was made to use the Borland extensions and make
the tutorial very difficult to use with other compilers. If
you have a need to use Pascal with some other compiler, TURBO
Pascal is so inexpensive that it would be a wise decision to
purchase a copy solely for the purpose of learning the Pascal
programming language, then moving to a larger compiler on a
minicomputer or a mainframe using the accumulated knowledge
to very quickly learn the extensions provided by that
particular compiler. At any rate, this tutorial will not
teach you everything you will ever need to know about Pascal.
It will, however, teach you the fundamentals and the advanced
features of Pascal, but of even more importance is the
definition of Pascal terminology needed to progress on your
own into more advanced topics of Pascal and programming in
general. You will find that experience will be your best
teacher.
WHICH VERSION OF TURBO PASCAL?
____________________________________________________________
Some of the example files will only work properly with TURBO
Pascal version 3.0 and some will only work properly with
version 4.0 and later, but most will work with either. It
will be clearly indicated to you which files will work with
any of the versions of TURBO Pascal. It should be pointed out
that each succesive version of TURBO Pascal has been an
improvement over the previous version since additional
capabilities have been added, and each new one compiles a
little faster and results in smaller but faster executable
code than the previous version. Any of the versions of TURBO
Pascal can be used to learn to program in Pascal, so whichever
version you have on hand will be adequate. Later, when you
become more versed in programming techniques, you may wish to
upgrade to the absolute latest version.
WHAT ABOUT TURBO PASCAL VERSION 2.0?
____________________________________________________________
Most of the files will compile properly with TURBO Pascal
version 2.0, but no warning will be given since that version
has been superseded for so long. It will pay you to purchase
a newer version because of the flexibility. If you choose not
to however, this tutorial will work fine in most cases if you
follow the instructions for TURBO Pascal version 3.0.
WHAT ABOUT TURBO PASCAL VERSION 5.5?
____________________________________________________________
Chapters 15 and 16 of this tutorial are written especially for
TURBO Pascal version 5.5 to discuss the use of object oriented
Page I-3
Introduction
programming and how to use the Borland extensions. Since the
topic of object oriented programming is a very large and
diverse field of study and only a limited space is available
to discuss it in this tutorial, these chapters will give you
only a brief overview of what it is and how to use it. You
will find 13 complete example programs to get you started in
this new and very meaningful endeavor and this introduction
should whet your appetite to continue your study in more
depth.
PREPARATION FOR USE OF THIS TUTORIAL.
____________________________________________________________
Copy the example files onto your TURBO working disk and you
are ready to begin, provided of course that you have already
learned how to start the TURBO system and how to edit a Pascal
file. Be sure you make a backup copy of the Pascal source
disk so you cannot accidentally lose all information on the
distribution disk. If you are using TURBO Pascal version 3.0,
you should read Chapter 1 of the reference manual to be ready
to use this tutorial, and if you are using TURBO Pascal
version 4.0, you should read parts of chapters 1, 2, & 11 of
your reference manual. TURBO Pascal version 5.x (5.0 or 5.5)
users should read chapters 1 and 2 of the User's Guide. You
should be familiar with use of the editor supplied with TURBO
Pascal before beginning.
If you are not using TURBO Pascal, you will still be able to
compile and execute many of these Pascal files, since most of
the examples use standard Pascal syntax. There will be some
statements used which are unique to TURBO Pascal and will not
work with your compiler. This will be especially true when
you come to the chapter on standard input and output since
this is where most compilers differ. Unfortunately, this is
one of the most important aspects of any programming language,
since it is required to get data into and out of the computer
to do anything useful. You will also find that chapter 13,
covering the topic of units, is unique to TURBO Pascal and
will not work with any Pascal compilers other than TURBO
Pascal versions 4.0 and 5.x.
WHAT ABOUT THE PROGRAMMING EXERCISES?
____________________________________________________________
It is highly suggested that you do the programming exercises
after you complete the study for each chapter. They are
carefully selected to test your understanding of the material
covered in that chapter. If you do not write, enter, debug,
and run these programs, you will only be proficient at reading
Pascal. If you do the exercises completely, you will have a
good start at being a Pascal program writer.
Page I-4
Introduction
It should also be mentioned that this tutorial will not teach
you everything you will ever need to know about Pascal. You
will continue to learn new techniques as long as you continue
to write programs. Experience is the best teacher here just
as it is in any endeavor. This tutorial will teach you enough
about Pascal that you should feel very comfortable as you
search through the reference manual for some topic. You will
also be able to read and understand any Pascal program you
find in textbooks or magazines. Although the primary goal of
this tutorial is to teach you the syntax and use of Pascal,
the most important byproduct is the knowledge of Pascal
terminology you will gain. This terminology will enable you
to learn even more about Pascal and programming in general.
THE ANSWERS DIRECTORY
____________________________________________________________
There is a directory on the distribution disk named ANSWERS
which contains an answer to each of the programming exercises
given at the end of the chapters. You should attempt to do
original work on each of the exercises before referring to
these answers, in order to gain your own programming
experience. These answers are given for your information in
case you are completely stuck on how to solve a particular
problem. These answers are not meant to be the only answer,
since there are many ways to program anything, but they are
meant to illustrate one way to solve the suggested programming
problem.
The answers are all in executable files named in the format
CHnn_m.PAS where nn is the chapter number, and m is the
exercise number. If there is more than one answer required,
an A, B, or C is included following the exercise number.
A SPECIAL NOTE FOR THE SHAREWARE VERSION
____________________________________________________________
It is impossible to include the graphics diagrams in chapter
12 in a pure ASCII text. They are therefore omitted from this
version of the tutorial. If you need these diagrams, they can
be purchased directly from Coronado Enterprises along with
your registration. See the READ.ME file on either diskette
for more information.
Page I-5
TABCONT.TXT
CORONADO ENTERPRISES
TURBO PASCAL TUTOR - Version 2.4
This documentation and the accompanying software, including all
of the example Pascal programs and text files, are protected
under United States copyright law to protect them from unauth-
orized commercialization. This version of the tutorial is dis-
tributed under the shareware concept, which means you are not
required to pay for it. You are permitted to copy the disks,
and pass the copies on to a friend, provided that you do not
modify any files or omit any files from the complete package,
and you are in fact encouraged to pass on complete copies to
friends. You are permitted to charge a small fee to cover the
costs of duplication, but you are not permitted to charge any-
thing for the software itself.
If you find the tutorial helpful, you are encouraged to register
with the author and to submit a small fee to help compensate him
for his time and expense in writing it. We will provide you
with a beautifully printed copy of this tutorial if you submit a
full registration. See the READ.ME file on either diskette for
additional details.
Whether or not you send a registration fee, feel free to request
a copy of the latest list of available tutorials and a list of
the authorized Public Domain libraries that distribute our full
line of programming language tutorials.
Gordon Dodrill - Dec 1, 1989
Copyright (c) 1988, 1989 Coronado Enterprises
Coronado Enterprises
12501 Coronado Ave NE
Albuquerque, New Mexico 87122
TURBO PASCAL TUTORIAL - TABLE OF CONTENTS
Introduction to the TURBO Pascal tutorial Page I-1
Chapter 1 - What is a computer program? Page 1-1
Chapter 2 - Getting started in Pascal. Page 2-1
TRIVIAL.PAS The minimum Pascal program. 2-1
WRITESM.PAS Write something out. 2-3
WRITEMR.PAS Write more out. 2-4
PASCOMS.PAS Pascal comments illustration. 2-4
GOODFORM.PAS Good formatting example. 2-6
UGLYFORM.PAS Ugly formatting example. 2-6
Chapter 3 - The simple Pascal data types. Page 3-1
INTVAR.PAS Integer variables. 3-2
INTVAR2.PAS More integer variables. 3-3
ALLVAR.PAS All simple variable types. 3-4
REALMATH.PAS Real variable math example. 3-4
INTMATH.PAS Integer variable math example. 3-4
BOOLMATH.PAS Boolean variable math example. 3-5
CHARDEMO.PAS Character variable demonstration. 3-6
CONVERT.PAS Data type conversion. 3-7
NEWINT4.PAS New integer types, ver 4.0 & 5.x 3-7
NEWREAL4.PAS New real types, ver 4.0 & 5.x 3-8
Chapter 4 - Pascal loops and control structures. Page 4-1
LOOPDEMO.PAS Loop demonstration. 4-1
IFDEMO.PAS Conditional branching. 4-2
LOOPIF.PAS Loops and If's together. 4-3
TEMPCONV.PAS Temperature conversion. 4-4
DUMBCONV.PAS Poor variable names. 4-4
REPEATLP.PAS Repeat until structure. 4-4
WHILELP.PAS While structure. 4-5
CASEDEMO.PAS Case demonstration. 4-5
BIGCASE.PAS Bigger case example 4-6
Chapter 5 - Pascal procedures and functions. Page 5-1
PROCED1.PAS Simple procedures. 5-1
PROCED2.PAS Procedures with variables. 5-4
PROCED3.PAS Multiple variables. 5-6
PROCED4.PAS Scope of variables. 5-7
PROCED5.PAS Procedure calling procedures. 5-8
FUNCTION.PAS An example function. 5-8
RECURSON.PAS An example with recursion. 5-9
FORWARD.PAS The forward reference. 5-10
PROCTYPE.PAS The procedure type 5-10
FUNCTYPE.PAS The function type 5-11
Chapter 6 - Arrays, types, constants, & labels. Page 6-1
ARRAYS.PAS Simple arrays. 6-1
ARRAYS2.PAS Multiple arrays. 6-2
TYPES.PAS Example of types. 6-3
CONSTANT.PAS Example of constants. 6-5
LABELS.PAS Label illustration. 6-6
Chapter 7 - Strings and string procedures. Page 7-1
STRARRAY.PAS Pascal strings. 7-1
STRINGS.PAS TURBO Pascal strings. 7-2
WHATSTRG.PAS What is a string? 7-2
Chapter 8 - Scalars, subranges, and sets. Page 8-1
ENTYPES.PAS Enumerated types. 8-1
SUBRANGE.PAS Scaler operations. 8-2
SETS.PAS Set operations. 8-4
FINDCHRS.PAS Search for characters. 8-5
Chapter 9 - Records. Page 9-1
SMALLREC.PAS A small record example. 9-1
BIGREC.PAS A large record example. 9-2
VARREC.PAS A variant record example. 9-5
Chapter 10 - Standard Input/Output. Page 10-1
WRITELNX.PAS Generalized output statements. 10-1
READINT.PAS Read integers from keyboard. 10-2
READREAL.PAS Read reals from keyboard. 10-3
READCHAR.PAS Read characters from keyboard. 10-4
READARRY.PAS Read an array from keyboard. 10-4
READSTRG.PAS Read a string from keyboard. 10-4
PRINTOUT.PAS Print some data on the printer. 10-5
Chapter 11 - Files. Page 11-1
READFILE.PAS Read and display this file. 11-2
READDISP.PAS Read and display any file. 11-3
READSTOR.PAS Read and store any file. 11-4
READINTS.PAS Read an integer data file. 11-6
INTDATA.TXT Integer data file. 11-6
READDATA.PAS Read a mixed data file. 11-7
REALDATA.TXT Real data file. 11-8
BINOUT.PAS Write a binary file. 11-8
BININ.PAS Read a binary file. 11-10
Chapter 12 - Pointers and dynamic allocation. Page 12-1
POINT.PAS First pointer example 12-1
POINT4.PAS Pointers with ver 4.0 & 5.x 12-3
POINTERS.PAS Example program with pointers. 12-3
DYNREC.PAS Dynamic record allocation. 12-5
LINKLIST.PAS An example linked list. 12-7
Chapter 13 - Units in TURBO Pascal 4.0 & 5.x Page 13-1
AREAS.PAS Areas of geometric shapes. 13-1
PERIMS.PAS Perimeters of geometric shapes. 13-4
GARDEN.PAS User of above units. 13-4
SHAPES4.PAS User of above units. 13-5
Chapter 14 - Complete example programs. Page 14-1
AMORT1.PAS Start of amortization program. 14-1
AMORT2.PAS Better amortization program. 14-1
AMORT3.PAS Useable amortization program. 14-1
AMORT4.PAS Neat amortization program. 14-1
AMORT5.PAS Complete amortization program. 14-2
LIST3.PAS List Pascal programs (ver 3.0). 14-3
LIST4.PAS List Pascal programs (ver 4 & 5). 14-3
LIST.EXE Ready to use list program. 14-3
TIMEDATE.PAS Get time and date (ver 3.0). 14-4
TIMEDAT4.PAS Get time and date (ver 4 & 5). 14-4
SETTIME.PAS Set a file's time and date. 14-4
SHAPES3.PAS Calculate areas of shapes. 14-4
OT3.PAS Directory list (ver 3.0). 14-5
OT4.PAS Directory list (ver 4 & 5). 14-5
OT.DOC How to use OakTree. 14-5
Chapter 15 - Encapsulation and Inheritance Page 15-1
ENCAP1.PAS First Encapsulation. 15-1
ENCAP2.PAS More Encapsulation. 15-5
INHERIT1.PAS First Inheritance. 15-7
VEHICLES.PAS An Object in a Unit. 15-9
CARTRUCK.PAS Descendant Objects. 15-10
INHERIT2.PAS Inheritance in Use. 15-10
INHERIT3.PAS Pointers and Arrays. 15-10
Chapter 16 - Virtual Methods Page 16-1
VIRTUAL1.PAS No Virtual Yet. 16-1
VIRTUAL2.PAS Virtual Methods in Use. 16-2
VIRTUAL3.PAS Virtuals and Pointers. 16-4
PERSON.PAS An Ancestor Object. 16-4
SUPERVSR.PAS Descendant Objects. 16-5
EMPLOYEE.PAS Using Virtual Methods. 16-5
ABOUT THE AUTHOR
___________________________________________________________
The author of this tutorial began programming in 1961 using
FORTRAN on an IBM 1620. Since then, most of his career has
been involved with designing digital logic for satellite
application. In 1983, being somewhat burned out with logic
design, he began a study of some of the more modern
programming languages and has since made a complete career
shift to software development. After learning Pascal, C was
studied, followed by Modula-2 and Ada, and more recently C++.
Rather than simply learning the syntax of each new language,
modern methods of software engineering were studied and
applied to effectively utilize the languages. He is current-
ly employed by a large research and development laboratory
where he continues to study, teach, and apply the newer
programming languages.
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