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Information about “C TUTOR 1 OF 2 (ALSO 578)”
C TUTOR is a comprehensive instructional course for the C programming
language which assumes the user has a moderate amount of programming
experience.
The C source code is included for each of several examples discussed
throughout the tutorial. These examples are meant to be studied and
then compiled and run by the student. The examples are short and focus
on specific topics.
All of the points of C language, including properly-structured
programming techniques, are covered at the elementary level. The
description and instruction are applicable to most compilers with some
slight differences.
CHAP01.TXT
Chapter 1
GETTING STARTED
WHAT IS AN IDENTIFIER
____________________________________________________________
Before you can do anything in any language, you must at least
know how to name an identifier. An identifier is used for any
variable, function, data definition, etc. In the programming
language C, an identifier is a combination of alphanumeric
characters, the first being a letter of the alphabet or an
underline, and the remaining being any letter of the alphabet,
any numeric digit, or the underline. In the case of some
compilers, a dollar sign is permitted but not as the first
character of an identifier. It should be pointed out that
even though a dollar sign may be permitted by your C compiler,
it is not used anywhere in this tutorial since it is not in
general use by C programmers, and is not even allowed by most
compilers. If you do not plan to write any portable code, you
can use it at will if you feel it makes your code more
readable.
Two rules must be kept in mind when naming identifiers.
1. The case of alphabetic characters is significant. Using
INDEX for a variable name is not the same as using index
and neither of them is the same as using InDeX for a
variable name. All three refer to different variables.
2. According to the ANSI-C standard, at least 31 significant
characters can be used and will be considered significant
by a conforming ANSI-C compiler. If more than 31 are
used, they will be ignored by the compiler.
WHAT ABOUT THE UNDERLINE?
____________________________________________________________
Even though the underline can be used as part of a variable
name, and adds greatly to the readability of the resulting
code, it seems to be used very little by experienced C
programmers. A few underlines are used for illustration in
this tutorial. Since most compiler writers use the underline
as the first character for variable names internal to the
system, you should refrain from using the underline to begin
a variable to avoid the possibility of a name clash.
It adds greatly to the readability of a program to use
descriptive names for variables and it would be to your
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Chapter 1 - Getting started
advantage to do so. Pascal programmers tend to use long
descriptive names, but most C programmers tend to use short
cryptic names. Most of the example programs in this tutorial
use very short names for that reason.
KEYWORDS
____________________________________________________________
There are 32 words defined as keywords in C. These have
predefined uses and cannot be used for any other purpose in
a C program. They are used by the compiler as an aid to
compiling the program. They are always written in lower case.
A complete list follows;
auto double int struct
break else long switch
case enum register typedef
char extern return union
const float short unsigned
continue for signed void
default goto sizeof volatile
do if static while
In addition to this list of keywords, your compiler may use
a few more. If it does, they will be listed in the
documentation that came with your compiler. Each of the above
keywords will be illustrated and used in this tutorial.
WE NEED DATA AND A PROGRAM
____________________________________________________________
Any computer program has two entities to consider, the data,
and the program. They are highly dependent on one another and
careful planning of both will lead to a well planned and well
written program. Unfortunately, it is not possible to study
either completely without a good working knowledge of the
other. For this reason, this tutorial will jump back and
forth between teaching methods of program writing and methods
of data definition. Simply follow along and you will have
a good understanding of both. Keep in mind that, even though
it seems expedient to sometimes jump right into the program
coding, time spent planning the data structures will be well
spent and the final program will reflect the original
planning.
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Chapter 1 - Getting Started
HOW THIS TUTORIAL IS WRITTEN
____________________________________________________________
As you go through the example programs, you will find that
every program is complete. There are no program fragments
that could be confusing. This allows you to see every
requirement that is needed to use any of the features of C as
they are presented. Some tutorials I have seen give very few,
and very complex examples. They really serve more to confuse
the student. This tutorial is the complete opposite because
it strives to cover each new aspect of programming in as
simple a context as possible. This method, however, leads to
a lack of knowledge in how the various parts are combined.
For that reason, the last chapter is devoted entirely to using
the features taught in the earlier chapters. It will
illustrate how to put the various features together to create
a usable program. They are given for your study, and are not
completely explained. Enough details of their operation are
given to allow you to understand how they work after you have
completed all of the previous lessons.
Throughout this tutorial, keywords, variable names, and
function names will be given in boldface as an aid to the
student.
RESULT OF EXECUTION
____________________________________________________________
The result of executing each program will be given in comments
at the end of the program listing, after the comment is
defined in about the fourth program of chapter 2. If you feel
confident that you completely understand the program, you can
simply refer to the result of execution to see if you
understand the result. In this case, it will not be necessary
for you to compile and execute every program. It would be a
good exercise for you to compile and execute some of them
however, because all C compilers will not generate exactly the
same results and you need to get familiar with your own
compiler.
At this point, you should load and run =============
FIRSTEX.C if you have not yet done so, to see FIRSTEX.C
that your C compiler is properly loaded and =============
operating. Don't worry about what the
program does yet. In due time you will
understand it completely.
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Chapter 1 - Getting Started
A DISCUSSION OF SOME OF THE FILES
____________________________________________________________
LIST.EXE
This file will list the source files for you with line numbers
and filename. To use it, simply type LIST followed by the
appropriate filename. At the user prompt, enter the command
LIST FIRSTEX.C now for an example. C source code is given in
Chapter 14 for a similar listing program along with a brief
description of its operation. After you have completed your
study of C, you will have the ability to read and understand
the source code for this program.
PRINTALL.BAT
This is a batch file that will call the above LIST.EXE file
once for each of the example C programs, printing all of the
files out. If you want a hardcopy of all of the files, enter
the command PRINTALL at the user prompt and watch as your
printer fills about 70 sheets of paper with C programs.
THE \ANSWER DIRECTORY
____________________________________________________________
There is a directory on the distribution disk named ANSWER
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.C where nn is the chapter number, and m is the exercise
number. If more than one answer is required, an A, B, or C
is included following the exercise number.
Page 1-4
CHAP02.TXT
Chapter 2
GETTING STARTED IN C
YOUR FIRST C PROGRAM
____________________________________________________________
The best way to get started with C is to =============
actually study a program, so load the file TRIVIAL.C
named TRIVIAL.C and display it on the =============
monitor. You are looking at the simplest
possible C program. There is no way to
simplify this program or to leave anything out.
Unfortunately, the program doesn't do anything.
The word main is very important, and must appear once, and
only once in every C program. This is the point where
execution is begun when the program is run. We will see later
that this does not have to be the first statement in the
program but it must exist as the entry point. Following the
main program name is a pair of parentheses which are an
indication to the compiler that this is a function. We will
cover exactly what a function is in due time. For now, I
suggest that you simply include the pair of parentheses.
The two curly brackets, properly called braces, are used to
define the limits of the program itself. The actual program
statements go between the two braces and in this case, there
are no statements because the program does absolutely nothing.
You can compile and run this program, but since it has no
executable statements, it does nothing. Keep in mind however,
that it is a valid C program.
A PROGRAM THAT DOES SOMETHING
____________________________________________________________
For a much more interesting program, load the =============
program named WRTSOME.C and display it on WRTSOME.C
your monitor. It is the same as the previous =============
program except that it has one executable
statement between the braces.
The executable statement is a call to a function supplied as
a part of your C library. Once again, we will not worry about
what a function is, but only how to use this one named
printf(). In order to output text to the monitor, the desired
text is put within the function parentheses and bounded by
quotation marks. The end result is that whatever is included
between the quotation marks will be displayed on the monitor
when the program is run.
Notice the semi-colon at the end of the line. C uses a
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Chapter 2 - Getting Started in C
semi-colon as a statement terminator, so the semi-colon is
required as a signal to the compiler that this line is
complete. This program is also executable, so you can compile
and run it to see if it does what you think it should.
ANOTHER PROGRAM WITH MORE OUTPUT
____________________________________________________________
Load the program WRTMORE.C and display it on =============
your monitor for an example of more output WRTMORE.C
and another small but important concept. You =============
will see that there are four program
statements in this program, each one being a
call to the function printf(). The top line will be executed
first, then the next, and so on, until the fourth line is
complete. The statements are executed in order from top to
bottom.
Notice the funny character near the end of the first line,
namely the backslash. The backslash is used in the printf()
statement to indicate that a special control character is
following. In this case, the "n" indicates that a newline is
requested. This is an indication to return the cursor to the
left side of the monitor and move down one line. It is
commonly referred to as a carriage return/line feed. Any
place within text that you desire, you can put a newline
character and start a new line. You could even put it in the
middle of a word and split the word between two lines. The
C compiler considers the combination of the backslash and
letter n as one character.
A complete description of this program is now possible. The
first printf() outputs a line of text and returns the
carriage. The second printf() outputs a line but does not
return the carriage so that the third line is appended to the
second, then followed by two carriage returns, resulting in
a blank line. Finally the fourth printf() outputs a line
followed by a carriage return and the program is complete.
After compiling and executing WRTMORE.C, the following text
should be displayed on your monitor;
This is a line of text to output.
And this is another line of text.
This is a third line.
Compile and run this program to see if it gives you this
output. It would be a good idea at this time for you to
experiment by adding additional lines of printout to see if
you understand how the statements really work.
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Chapter 2 - Getting Started in C
LET'S PRINT SOME NUMBERS
____________________________________________________________
Load the file named ONEINT.C and display it ==============
on the monitor for our first example of how ONEINT.C
to work with data in a C program. The entry ==============
point main should be clear to you by now as
well as the beginning brace. The first new
thing we encounter is line 3 containing int index;, which is
used to define an integer variable named index. The word int
is a keyword in C, and can not be used for anything else. It
defines a variable that can have a value from -32768 to 32767
in most C compilers for microcomputers. The variable name,
index, can be any name that follows the rules for an
identifier and is not one of the keywords for C. The final
character on the line, the semi-colon, is the statement
terminator.
Note that, even though we have defined a variable, we have not
yet assigned a value to it. We will see in a later chapter
that additional integers could also be defined on the same
line, but we will not complicate the present situation.
Observing the main body of the program, you will notice that
there are three statements that assign a value to the variable
index, but only one at a time. The statement in line 4
assigns the value of 13 to index, and its value is printed out
by line 5. (We will see how shortly.) Later, the value of
27 is assigned to index, and finally 10 is assigned to it,
each value being printed out. It should be intuitively clear
that index is indeed a variable and can store many different
values but only one value at a time of course.
Please note that many times the words "printed out" are used
to mean "displayed on the monitor". You will find that in
many cases experienced programmers take this liberty, probably
due to the printf() function being used for monitor display.
HOW DO WE PRINT NUMBERS?
____________________________________________________________
To keep our promise, let's return to the printf() statements
for a definition of how they work. Notice that they are all
identical and that they all begin just like the printf()
statements we have seen before. The first difference occurs
when we come to the % character. This is a special character
that signals the output routine to stop copying characters to
the output and do something different, namely output the value
of a variable. The % sign is used to signal the output of
many different types of variables, but we will restrict
ourselves to only one for this example. The character
following the % sign is a d, which signals the output routine
to get a decimal value and output it. Where the decimal value
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Chapter 2 - Getting Started in C
comes from will be covered shortly. After the d, we find the
familiar \n, which is a signal to return the video "carriage",
and the closing quotation mark.
All of the characters between the quotation marks define the
pattern of data to be output by this statement, and after the
pattern, there is a comma followed by the variable name index.
This is where the printf() statement gets the decimal value
which it will output because of the %d we saw earlier. We
could add more %d output field descriptors within the brackets
and more variables following the description to cause more
data to be printed with one statement. Keep in mind however,
that the number of field descriptors and the number of
variable definitions must be the same or the runtime system
will get confused and probably quit with a runtime error.
Much more will be covered at a later time on all aspects of
input and output formatting. A reasonably good grasp of these
fundamentals are necessary in order to understand the
following lessons. It is not necessary to understand
everything about output formatting at this time, only a fair
understanding of the basics.
Compile and run ONEINT.C and observe the output.
HOW DO WE ADD COMMENTS IN C?
____________________________________________________________
Load the file named COMMENTS.C and observe it ==============
on your monitor for an example of how COMMENTS.C
comments can be added to a C program. ==============
Comments are added to make a program more
readable to you but the compiler must ignore
the comments. The slash star combination is used in C for
comment delimiters. They are illustrated in the program at
hand. Please note that the program does not illustrate good
commenting practice, but is intended to illustrate where
comments can go in a program. It is a very sloppy looking
program.
The first slash star combination introduces the first comment
and the star slash at the end of the first line terminates
this comment. Note that this comment is prior to the
beginning of the program illustrating that a comment can
precede the program itself. Good programming practice would
include a comment prior to the program with a short
introductory description of the program. The next comment is
after the main program entry point and prior to the opening
brace for the program code itself.
The third comment starts after the first executable statement
in line 5 and continues for four lines. This is perfectly
legal because a comment can continue for as many lines as
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Chapter 2 - Getting Started in C
desired until it is terminated. Note carefully that if
anything were included in the blank spaces to the left of the
three continuation lines of the comment, it would be part of
the comment and would not be compiled. The last comment is
located following the completion of the program, illustrating
that comments can go nearly anywhere in a C program.
Experiment with this program by adding comments in other
places to see what will happen. Comment out one of the
printf() statements by putting comment delimiters both before
and after it and see that it does not get executed causing a
line of printout.
Comments are very important in any programming language
because you will soon forget what you did and why you did it.
It will be much easier to modify or fix a well commented
program a year from now than one with few or no comments.
You will very quickly develop your own personal style of
commenting.
Some C compilers will allow you to "nest" comments which can
be very handy if you need to "comment out" a section of code
during debugging. Since nested comments are not a part of the
ANSI standard, none will be used in this tutorial. Check the
documentation for your compiler to see if they are permitted
with your implementation of C.
GOOD FORMATTING STYLE
____________________________________________________________
Load the file GOODFORM.C and observe it on ==============
your monitor. It is an example of a well GOODFORM.C
formatted program. Even though it is very ==============
short and therefore does very little, it is
very easy to see at a glance what it does.
With the experience you have already gained in this tutorial,
you should be able to very quickly grasp the meaning of the
program in it's entirety. Your C compiler ignores all extra
spaces and all carriage returns giving you considerable
freedom in formatting your program. Indenting and adding
spaces is entirely up to you and is a matter of personal
taste. Compile and run the program to see if it does what
you expect it to do.
Now load and display the program UGLYFORM.C ==============
and observe it. How long will it take you to UGLYFORM.C
figure out what this program will do? It ==============
doesn't matter to the compiler which format
style you use, but it will matter to you when
you try to debug your program. Compile this program and run
it. You may be surprised to find that it is the same program
as the last one, except for the formatting. Don't get too
worried about formatting style yet. You will have plenty of
Page 2-5
Chapter 2 - Getting Started in C
time to develop a style of your own as you learn the language.
Be observant of styles as you see C programs in magazines,
books, and other publications.
This should pretty well cover the basic concepts of
programming in C, but as there are many other things to learn,
we will forge ahead to additional program structure.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program to display your name on the monitor.
2. Modify the program to display your address and phone
number on separate lines by adding two additional
printf() statements.
Page 2-6
CHAP03.TXT
Chapter 3
PROGRAM CONTROL
THE WHILE LOOP
____________________________________________________________
The C programming language has several structures for looping
and conditional branching. We will cover them all in this
chapter and we will begin with the while loop.
The while loop continues to loop while some condition is true.
When the condition becomes false, the looping is discontinued.
It therefore does just what it says it does, the name of the
loop being very descriptive.
Load the program WHILE.C and display it for =============
an example of a while loop. We begin with a WHILE.C
comment and the program entry point main(), =============
then go on to define an integer variable
count within the body of the program. The
variable is set to zero and we come to the while loop itself.
The syntax of a while loop is just as shown here. The keyword
while is followed by an expression of something in
parentheses, followed by a compound statement bracketed by
braces. As long as the expression in parenthesis is true, all
statements within the braces will be repeatedly executed. In
this case, since the variable count is incremented by one
every time the statements are executed, it will eventually
reach 6. At that time the statement will not be executed, and
the loop will be terminated. The program control will resume
at the statement following the statements in braces.
We will cover the compare expression, the one in parentheses,
in the next chapter. Until then, simply accept the
expressions for what you think they should do and you will
probably be correct.
Several things must be pointed out regarding the while loop.
First, if the variable count were initially set to any number
greater than 5, the statements within the loop would not be
executed at all, so it is possible to have a while loop that
never is executed. Secondly, if the variable were not
incremented in the loop, then in this case, the loop would
never terminate, and the program would never complete.
Finally, if there is only one statement to be executed within
the loop, it does not need delimiting braces but can stand
alone.
Compile and run this program after you have studied it enough
to assure yourself that you understand its operation
completely. Note that the result of execution is given for
this program, (and will be given for all of the remaining
Page 3-1
Chapter 3 - Program Control
example programs in this tutorial) so you do not need to
compile and execute every program to see the results. Be sure
to compile and execute some of the programs however, to gain
experience with your compiler.
THE DO-WHILE LOOP
____________________________________________________________
A variation of the while loop is illustrated =============
in the program DOWHILE.C, which you should DOWHILE.C
load and display. This program is nearly =============
identical to the last one except that the
loop begins with the keyword do, followed by
a compound statement in braces, then the keyword while, and
finally an expression in parentheses. The statements in the
braces are executed repeatedly as long as the expression in
parentheses is true. When the expression in parentheses
becomes false, execution is terminated, and control passes to
the statements following this statement.
Several things must be pointed out regarding the do-while
loop. Since the test is done at the end of the loop, the
statements in the braces will always be executed at least
once. Secondly, if the variable i were not changed within the
loop, the loop would never terminate, and hence the program
would never terminate. Finally, just like the while loop, if
only one statement will be executed within the loop, no braces
are required. Compile and run this program to see if it does
what you think it should do.
It should come as no surprise to you that these loops can be
nested. That is, one loop can be included within the compound
statement of another loop, and the nesting level has no limit.
THE FOR LOOP
____________________________________________________________
The for loop is really nothing new, it is =============
simply a new way to describe the while loop. FORLOOP.C
Load and display the file named FORLOOP.C on =============
your monitor for an example of a program with
a for loop. The for loop consists of the
keyword for followed by a rather large expression in
parentheses. This expression is really composed of three
fields separated by semi-colons. The first field contains the
expression "index = 0" and is an initializing field. Any
expressions in this field are executed prior to the first pass
through the loop. There is essentially no limit as to what
can go here, but good programming practice would require it
to be kept simple. Several initializing statements can be
placed in this field, separated by commas.
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Chapter 3 - Program Control
The second field, in this case containing "index < 6", is the
test which is done at the beginning of each loop through the
program. It can be any expression which will evaluate to a
true or false. (More will be said about the actual value of
true and false in the next chapter.)
The expression contained in the third field is executed each
time the loop is exercised but it is not executed until after
those statements in the main body of the loop are executed.
This field, like the first, can also be composed of several
operations separated by commas.
Following the for() expression is any single or compound
statement which will be executed as the body of the loop. A
compound statement is any group of valid C statements enclosed
in braces. In nearly any context in C, a simple statement can
be replaced by a compound statement that will be treated as
if it were a single statement as far as program control goes.
Compile and run this program.
You may be wondering why there are two statements available
that do exactly the same thing because the while and the for
loop do exactly the same thing. The while is convenient to
use for a loop that you don't have any idea how many times the
loop will be executed, and the for loop is usually used in
those cases when you are doing a fixed number of iterations.
The for loop is also convenient because it moves all of the
control information for a loop into one place, between the
parentheses, rather than at both ends of the code. It is your
choice as to which you would rather use.
THE IF STATEMENT
____________________________________________________________
Load and display the file IFELSE.C for an ==============
example of our first conditional branching IFELSE.C
statement, the if. Notice first, that there ==============
is a for loop with a compound statement as
its executable part containing two if
statements. This is an example of how statements can be
nested. It should be clear to you that each of the if
statements will be executed 10 times.
Consider the first if statement. It starts with the keyword
if followed by an expression in parentheses. If the
expression is evaluated and found to be true, the single
statement following the if is executed, and if false, the
following statement is skipped. Here too, the single
statement can be replaced by a compound statement composed of
several statements bounded by braces. The expression "data
== 2" is simply asking if the value of data is equal to 2,
this will be explained in detail in the next chapter. (Simply
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Chapter 3 - Program Control
suffice for now that if "data = 2" were used in this context,
it would mean a completely different thing.)
NOW FOR THE IF-ELSE
____________________________________________________________
The second if is similar to the first with the addition of a
new keyword, the else in line 15. This simply says that if
the expression in the parentheses evaluates as true, the first
expression is executed, otherwise the expression following the
else is executed. Thus, one of the two expressions will
always be executed, whereas in the first example the single
expression was either executed or skipped. Both will find
many uses in your C programming efforts. Compile and run this
program to see if it does what you expect.
THE BREAK AND CONTINUE
____________________________________________________________
Load the file named BREAKCON.C for an ==============
example of two new statements. Notice that BREAKCON.C
in the first for loop, there is an if ==============
statement that calls a break if xx equals
8. The break will jump out of the loop you
are in and begin executing statements following the loop,
effectively terminating the loop. This is a valuable
statement when you need to jump out of a loop depending on the
value of some results calculated in the loop. In this case,
when xx reaches 8, the loop is terminated and the last value
printed will be the previous value, namely 7.
The next for loop starting in line 12, contains a continue
statement which does not cause termination of the loop but
jumps out of the present iteration. When the value of xx
reaches 8 in this case, the program will jump to the end of
the loop and continue executing the loop, effectively
eliminating the printf() statement during the pass through the
loop when xx is eight. Compile and run this program.
THE SWITCH STATEMENT
____________________________________________________________
Load and display the file SWITCH.C for an ==============
example of the biggest construct yet in the SWITCH.C
C language, the switch. The switch is not ==============
difficult, so don't let it intimidate you.
It begins with the keyword switch followed by
a variable in parentheses which is the switching variable, in
this case truck. As many cases as desired are then enclosed
within a pair of braces. The reserved word case is used to
begin each case, followed by the value of the variable, then
a colon, and the statements to be executed.
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Chapter 3 - Program Control
In this example, if the variable named truck contains the
value 3 during this pass of the switch statement, the printf()
in line 9 will cause "The value is three" to be displayed, and
the break statement will cause us to jump out of the switch.
Once an entry point is found, statements will be executed
until a break is found or until the program drops through the
bottom of the switch braces. If the variable has the value
5, the statements will begin executing at line 13 where "case
5 :" is found, but the first statements found are where the
case 8 statements are. These are executed and the break
statement in line 17 will direct the execution out the bottom
of the switch. The various case values can be in any order
and if a value is not found, the default portion of the switch
will be executed.
It should be clear that any of the above constructs can be
nested within each other or placed in succession, depending
on the needs of the particular programming project at hand.
Note that the switch is not used as frequently as the loop and
the if statements, in fact the switch is used infrequently.
Be sure to compile and run SWITCH.C and examine the results.
THE EVIL GOTO STATEMENT
____________________________________________________________
Load and display the file GOTOEX.C for an ==============
example of a file with some goto statements GOTOEX.C
in it. To use a goto statement, you simply ==============
use the reserved word goto followed by the
symbolic name to which you wish to jump. The
name is then placed anywhere in the program followed by a
colon. You can jump nearly anywhere within a function, but
you are not permitted to jump into a loop, although you are
allowed to jump out of a loop.
This particular program is really a mess but it is a good
example of why software writers are trying to eliminate the
use of the goto statement as much as possible. The only place
in this program where it is reasonable to use the goto is the
one in line 18 where the program jumps out of the three nested
loops in one jump. In this case it would be rather messy to
set up a variable and jump successively out of all three loops
but one goto statement gets you out of all three in a very
concise manner.
Some persons say the goto statement should never be used under
any circumstances, but this is rather narrow minded thinking.
If there is a place where a goto will clearly do a neater
control flow than some other construct, feel free to use it.
Page 3-5
Chapter 3 - Program Control
It should not be abused however, as it is in the rest of the
program on your monitor.
Entire books are written on "gotoless" programming, better
known as Structured Programming. These will be left to your
study. One point of reference is the Visual Calculator
described in Chapter 14 of this tutorial. This program is
contained in four separately compiled files and is a rather
large complex program. If you spend some time studying the
source code, you will find that there is not a single goto
statement anywhere in it.
Compile and run GOTOEX.C and study its output. It would be
a good exercise to rewrite it and see how much more readable
it is when the statements are listed in order.
FINALLY, A MEANINGFUL PROGRAM
____________________________________________________________
Load the file named TEMPCONV.C for an example ==============
of a useful, even though somewhat limited TEMPCONV.C
program. This is a program that generates a ==============
list of centigrade and fahrenheit
temperatures and prints a message out at the
freezing point of water and another at the boiling point of
water.
Of particular importance is the formatting. The header is
simply several lines of comments describing what the program
does in a manner that catches the readers attention and is
still pleasing to the eye. You will eventually develop your
own formatting style, but this is a good way to start. Also
if you observe the for loop, you will notice that all of the
contents of the compound statement are indented 3 spaces to
the right of the for keyword, and the closing brace is lined
up under the "f" in for. This makes debugging a bit easier
because the construction becomes very obvious. You will also
notice that the printf() statements that are in the if
statements within the big for loop are indented three
additional spaces because they are part of another construct.
This is the first program in which we used more than one
variable. The three variables are simply defined on three
different lines and are used in the same manner as a single
variable was used in previous programs. By defining them on
different lines, we have an opportunity to define each with
a comment.
Be sure to compile and execute this program.
Page 3-6
Chapter 3 - Program Control
ANOTHER POOR PROGRAMMING EXAMPLE
____________________________________________________________
Recalling UGLYFORM.C from the last chapter, ==============
you saw a very poorly formatted program. If DUMBCONV.C
you load and display DUMBCONV.C you will have ==============
an example of poor formatting which is much
closer to what you will find in practice.
This is the same program as TEMPCONV.C with the comments
removed and the variable names changed to remove the
descriptive aspect of the names. Although this program does
exactly the same as the last one, it is much more difficult
to read and understand. You should begin to develop good
programming practices now.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program that writes your name on the monitor ten
times. Write this program three times, once with each
looping method.
2. Write a program that counts from one to ten, prints the
values on a separate line for each, and includes a
message of your choice when the count is 3 and a
different message when the count is 7.
Page 3-7
CHAP04.TXT
Chapter 4
ASSIGNMENT & LOGICAL COMPARES
Throughout this chapter, references are given to various
ranges of variables. Your compiler may use a different range
for some of the variables since the ANSI standard does not
define specific limits for all data types. Consult the
documentation for your compiler for the exact range for each
of the variable types.
INTEGER ASSIGNMENT STATEMENTS
____________________________________________________________
Load the file INTASIGN.C and display it for ==============
an example of assignment statements. Three INTASIGN.C
variables are defined for use in the program ==============
and the rest of the program is merely a
series of illustrations of various
assignments. The first two lines of the assignment statements
assign numerical values to the variables named a and b, and
the next five lines illustrate the five basic arithmetic
functions and how to use them. The fifth is the modulo
operator and gives the remainder if the two variables were
divided. It can only be applied to int or char type
variables. Following these, there are two lines illustrating
how to combine some of the variables in some complex math
expressions. All of the above examples should require no
comment except to say that none of the equations are meant to
be particularly useful except as illustrations. The char type
variable will be defined in the description of the next
example program.
The expressions in lines 17 and 18 are perfectly acceptable
as given, but we will see later in this chapter that there is
another way to write these for more compact code.
VERY STRANGE LOOKING CODE
____________________________________________________________
This leaves us with the last two lines which may appear to you
as being very strange. The C compiler scans the assignment
statement from right to left, (which may seem a bit odd since
we do not read that way), resulting in a very useful
construct, namely the one given here. The compiler finds the
value 20, assigns it to c, then continues to the left finding
that the latest result of a calculation should be assigned to
b. Thinking that the latest calculation resulted in a 20, it
assigns it to b also, and continues the leftward scan
assigning the value 20 to a also. This is a very useful
construct when you are initializing a group of variables. The
last statement illustrates that it is possible to actually do
Page 4-1
Chapter 4 - Assignments and Logical Compares
some calculations to arrive at the value which will be
assigned to all three variables. In fact, the rightmost
expression can contain variables, even a, b, & c.
The program has no output, so compiling and executing this
program will be very uninteresting. Since you have already
learned how to display some integer results using the printf()
function, it would be to your advantage to add some output
statements to this program to see if the various statements
do what you think they should do.
DEFINITIONS FIRST THEN EXECUTABLE STATEMENTS
____________________________________________________________
This would be a good time for a preliminary definition of a
rule to be followed in C. The variable definitions are always
given before any executable statements in any program block.
This is why the variables are defined first in this program
and in every C program. If you try to define a new variable
after executing some statements, your compiler will issue an
error.
ADDITIONAL DATA TYPES
____________________________________________________________
Loading and editing MORTYPES.C will ===============
illustrate how some additional data types can MORETYPES.C
be used. Once again we have defined a few ===============
integer type variables which you should be
fairly familiar with by now, but we have
added two new types, the char, and the float.
The char type of data is nearly the same as the integer except
that it can only be assigned numerical values between -128 to
127 on most implementations of C, since it is stored in only
one byte of memory. The char type of data is usually used for
ASCII data, more commonly known as text. The text you are
reading was originally written on a computer with a word
processor that stored the words in the computer one character
per byte. In contrast, the integer data type is stored in two
bytes of computer memory on nearly all microcomputers.
DATA TYPE MIXING
____________________________________________________________
It would be profitable at this time to discuss the way C
handles the two types char and int. Most functions in C that
are designed to operate with integer type variables will work
equally well with character type variables because they are
a form of an integer variable. Those functions, when called
on to use a char type variable, will actually promote the char
Page 4-2
Chapter 4 - Assignments and Logical Compares
data into integer data before using it. For this reason, it
is possible to mix char and int type variables in nearly any
way you desire. The compiler will not get confused, but you
might. It is good not to rely on this too much, but to
carefully use only the proper types of data where they should
be used.
The second new data type is the float type of data, commonly
called floating point data. This is a data type which usually
has a very large range, a large number of significant digits,
and a large number of computer words are required to store it.
The float data type has a decimal point associated with it and
has an allowable range of from 3.4E-38 to 3.4E+38 when using
most C compilers on microcomputers, and is composed of about
7 significant digits.
HOW TO USE THE NEW DATA TYPES
____________________________________________________________
The first three lines of the program assign values to all nine
of the defined variables so we can manipulate some of the data
between the different types.
Since, as mentioned above, a char data type is in reality an
integer data type, no special considerations need be taken to
promote a char to an int, and a char type data field can be
assigned to an int variable. When going the other way, there
is no standard, so you may simply get garbage if the value of
the integer variable is outside the range of the char type
variable. Most C compilers simply truncate the most
significant bits and use the 8 least significant bits. It
will translate correctly if the value is within the range of
-128 to 127.
The third line illustrates the simplicity of translating an
integer into a float, simply assign it the new value and the
system will do the proper conversion. When going the other
way however, there is an added complication. Since there may
be a fractional part of the floating point number, the system
must decide what to do with it. By definition, it will
truncate it.
This program produces no output, and we haven't covered a way
to print out char and float type variables, so you can't
really get in to this program and play with the results, but
the next program will cover this for you.
Be sure to compile and run this program after you are sure you
understand it completely.
Page 4-3
Chapter 4 - Assignments and Logical Compares
LOTS OF VARIABLE TYPES
____________________________________________________________
Load the file LOTTYPES.C and display it on ==============
your screen. This file contains nearly every LOTTYPES.C
standard simple data type available in the ==============
programming language C. There are other
types, but they are the compound types (ie -
arrays and structures) that we will cover in due time.
Observe the file. First we define a simple int, followed by
a long int which has a range of -2147483648 to 2147483647 with
most C compilers, and a short int which has a range that is
identical to that for the int variable, namely -32768 to
32767. The unsigned is next and is defined as the same size
as the int but with no sign. The unsigned then will cover a
range of 0 to 65535. It should be pointed out that when the
long, short, or unsigned is desired, the int is optional and
is left out by most experienced programmers. We have already
covered the char and the float, which leaves only the double.
The double is a floating point number but covers a greater
range than the float and has more significant digits for more
precise calculations. It also requires more memory to store
a value than the simple float. The double in most C compilers
covers a range of 1.7E-308 to 1.7E+308.
Note that other compounding of types can be done such as long
unsigned int, unsigned char, etc. Check your documentation
for a complete list of variable types.
Another diversion is in order at this point. Your compiler
may have no provision for floating point math, only double
floating point math. It will promote a float to a double
before doing calculations and therefore only one math library
will be needed. Of course, this is transparent to you, so you
don't need to worry about it. Because of this, you may think
that it would be best to simply define every floating point
variable as double, since they are promoted before use in any
calculations, but that may not be a good idea. A float
variable requires 4 bytes of storage and a double requires 8
bytes of storage, so if you have a large volume of floating
point data to store, the double will obviously require much
more memory.
After defining the data types in the program under
consideration, a numerical value is assigned to each of the
defined variables in order to demonstrate the means of
outputting each to the monitor.
Page 4-4
Chapter 4 - Assignments and Logical Compares
SOME LATE ADDITIONS
____________________________________________________________
As any programming language evolves, additional constructs are
added to fill some previously overlooked need. Two new
keywords have been added to C recently, and since they may not
be a part of your compiler, they are not illustrated in
example programs, but they will be discussed here. The two
new keywords are const and volatile and are used to tell the
compiler that variables of these types will need special
consideration. A constant is declared with the const keyword
and declares a value that will never be changed by either the
program or the system itself. If volatile is used, it
declares a value that may be changed by the program but it may
also be changed by some outside influence such as a clock
update pulse incrementing the stored value. Since constants
can never have a value assigned to them in the executable part
of the program, they should always be initialized.
Examples of use in declaring constants of these two types are
given as;
const int index1 = 2;
const index2 = 6;
volatile const int index3 = 12;
volatile index4;
As mentioned earlier in this chapter, the keyword int is
optional when used with these constant definitions
THE CONVERSION CHARACTERS
____________________________________________________________
Following is a list of some of the conversion characters and
the way they are used in the printf() statement. A complete
list of all of the conversion characters should be included
with the documentation for your compiler.
d decimal notation
i decimal notation (new ANSI standard extension)
o octal notation
x hexadecimal notation
u unsigned notation
c character notation
s string notation
f floating point notation
Page 4-5
Chapter 4 - Assignments and Logical Compares
Each of these is used following a percent sign to indicate the
type of output conversion, and between those two characters,
the following fields may be added.
- left justification in its field
(n) a number specifying minimum field width
. to separate n from m
(m) significant fractional digits for a float
l to indicate a long
These are all used in the examples which are included in the
program LOTTYPES.C, with the exception of the string notation
which will be covered later in this tutorial. Note especially
the variable field width specification demonstrated in lines
33 to 36. This is not part of the original definition of C,
but it is included in the ANSI standard and has become part
of the C language. Compile and run this program to see what
effect the various fields have on the output.
You now have the ability to display any of the data fields in
the previous programs and it would be to your advantage to go
back and see if you can display some of the fields anyway you
desire.
COMBINING THE VARIOUS TYPES
____________________________________________________________
Examine the file named COMBINE.C for =============
examples of combining variables of the COMBINE.C
various types in a program. Many times it =============
is necessary to multiply an int type
variable times a float type variable and C
allows this by giving a strict set of rules it will follow in
order to do such combinations.
Five variables of three different types are declared in lines
4 through 6, and three of them are initialized so we have some
data to work with. Line 8 gives an example of adding an int
variable to a float variable and assigning the result to a
char type variable. The cast is used to control the type
addition and is indicated by putting the desired type in front
of the variable as shown. This forces each of the two
variables to the character type prior to doing the addition.
In some cases, when the cast is used, the actual bit patterns
must be modified internally in order to do the type coercion.
Lines 9 through 11 perform the same operation by using
different kinds of type casting to achieve the final result.
Lines 13 through 15 illustrate the use of the cast to multiply
two float variables. In two of the cases the intermediate
results are cast to the int type, with the result being cast
back to the float type. The observant student will notice
Page 4-6
Chapter 4 - Assignments and Logical Compares
that all three lines will not necessarily produce the same
result.
Be sure to compile and execute this program.
LOGICAL COMPARES
____________________________________________________________
Load and view the file named COMPARES.C for ==============
many examples of compare statements in C. We COMPARES.C
begin by defining and initializing nine ==============
variables to use in the following compare
statements. This initialization is new to
you and can be used to initialize variables when they are
defined.
The first group of compare statements represents the simplest
kinds of compares since they simply compare two variables.
Either variable could be replaced with a constant and still
be a valid compare, but two variables is the general case.
The first compare checks to see if the value of x is equal to
the value of y and it uses the double equal sign for the
comparison. A single equal sign could be used here but it
would have a different meaning as we will see shortly. The
second comparison checks to see if the value of x is greater
than the value of z.
The third compare introduces the not operator, the
exclamation, which can be used to invert the result of any
logical compare. The fourth checks for the value of b less
than or equal to the value of c, and the last checks for the
value of r not equal to the value of s. As we learned in the
last chapter, if the result of the compare is true, the
statement following the if clause will be executed and the
results are given in the comments.
Note that "less than" and "greater than or equal to" are also
available, but are not illustrated here.
It would be well to mention the different format used for the
if statement in this example program. A carriage return is
not required as a statement separator and by putting the
conditional clause on the same line as the if, it adds to the
readability of the overall program in this case.
MORE COMPARES
____________________________________________________________
The compares in the second group are a bit more involved.
Starting with the first compare, we find a rather strange
looking set of conditions in the parentheses. To understand
this we must understand just what a true or false is in the
Page 4-7
Chapter 4 - Assignments and Logical Compares
C language. A false is defined as a value of zero, and true
is defined as a non-zero value. Any integer or character type
of variable can be used for the result of a true/false test,
or the result can be an implied integer or character.
Look at the first compare of the second group of compare
statements. The expression "r != s" will evaluate as a true
since the value of r was set to 0.0 in line 13, so the result
will be a non-zero value. With most C compilers, it would
always be set to a 1, but you could get in trouble if you
wrote a program that depended on it being 1 in all cases.
Good programming practice would be to not use the resulting
1 in any calculations. Even though the two variables that are
compared are float variables, the result will be of type
integer. There is no explicit variable to which it will be
assigned so the result of the compare is an implied integer.
Finally the resulting number, probably 1 in this case, is
assigned to the integer variable x. If double equal signs
were used, the phantom value, namely 1, would be compared to
the value of x, but since the single equal sign is used, the
value 1 is simply assigned to the variable named x, as though
the statement were not in parentheses. Finally, since the
result of the assignment in the parentheses was non-zero, the
entire expression is evaluated as true, and z is assigned the
value of 1000. Thus we accomplished two things in this
statement, we assigned x a new value, probably 1, and we
assigned z the value of 1000. We covered a lot in this
statement so you may wish to review it before going on. The
important things to remember are the values that define true
and false, and the fact that several things can be assigned
in a conditional statement. The value assigned to the
variable x was probably a 1, but remember that the only
requirement is that it is nonzero.
The example in line 20 should help clear up some of the above
in your mind. In this example, x is assigned the value of y,
and since the result is 11, the condition is non-zero, which
is true, and the variable z is assigned 222.
The third example of the second group in line 21, compares the
value of x to zero. If the result is true, meaning that if
x is not zero, then z is assigned the value of 333, which it
will be. The last example in this group illustrates the same
concept, since the result will be true if x is non-zero. The
compare to zero in line 21 is not actually needed and the
result of the compare is true. The third and fourth examples
of this group are therefore identical.
ADDITIONAL COMPARE CONCEPTS
____________________________________________________________
The third group of compares will introduce some additional
concepts, namely the logical and and the logical or operators.
Page 4-8
Chapter 4 - Assignments and Logical Compares
We assign the value of 77 to the three integer variables
simply to get started again with some defined values. The
first compare of the third group contains the new control &&,
which is the logical and which results in a true if both sides
of the and are true. The entire statement reads, if x equals
y and if x equals 77 then the result is true. Since this is
true, the variable z is set equal to 333.
The next compare in this group introduces the || operator
which is the logical or operator which results in a true if
either side of the or is true. The statement reads, if x is
greater than y or if z is greater than 12 then the result is
true. Since z is greater than 12, it doesn't matter if x is
greater than y or not, because only one of the two conditions
must be true for the result to be true. The result is true,
so therefore z will be assigned the value of 22.
LOGICAL EVALUATION (SHORT CIRCUIT)
____________________________________________________________
When a compound expression is evaluated, the evaluation
proceeds from left to right and as soon as the result of the
outcome is assured, evaluation stops. Namely, in the case of
an "and" evaluation, when one of the terms evaluates to false,
evaluation is discontinued because additional true terms
cannot make the result ever become true. In the case of an
"or" evaluation, if any of the terms is found to be true,
evaluation stops because it will be impossible for additional
terms to cause the result to be false. In the case of
additionally nested terms, the above rules will be applied to
each of the nested levels. This is called short-circuit
evaluation since the remaining terms are not evaluated.
PRECEDENCE OF OPERATORS
____________________________________________________________
The question will come up concerning the precedence of
operators. Which operators are evaluated first and which
last? There are many rules about this topic, but I would
suggest that you don't worry about it at this point. Instead,
use lots of parentheses to group variables, constants, and
operators in a way meaningful to you. Parentheses always have
the highest priority and will remove any question of which
operations will be done first in any particular statements.
Going on to the next example in group three in line 29, we
find three simple variables used in the conditional part of
the compare. Since all three are non-zero, all three are
true, and therefore the and of the three variables are true,
leading to the result being true, and z being assigned the
value of 11. Note that since the variables, r, s, and t are
float type variables, they could not be used this way, but
Page 4-9
Chapter 4 - Assignments and Logical Compares
they could each be compared to zero and the same type of
expression could be used.
Continuing on to line 30 we find three assignment statements
in the compare part of the if statement. If you understood
the above discussion, you should have no difficulty
understanding that the three variables are assigned their
respective new values, and the result of all three are
non-zero, leading to a resulting value of true.
THIS IS A TRICK, BE CAREFUL
____________________________________________________________
The last example of the third group contains a bit of a trick,
but since we have covered it above, it is nothing new to you.
Notice that the first part of the compare evaluates to false.
The remaining parts of the compare are not evaluated, because
it is a logical and so it will definitely be resolved as a
false because the first term is false. If the program was
dependent on the value of y being set to 3 in the next part
of the compare, it will fail because evaluation will cease
following the false found in the first term. Likewise, the
variable named z will not be set to 4, and the variable r will
not be changed.
POTENTIAL PROBLEM AREAS
____________________________________________________________
The last group of compares illustrate three possibilities for
getting into a bit of trouble. All three have the common
result that the variable z will not get set to the desired
value, but for different reasons. In line 37, the compare
evaluates as true, but the semicolon following the second
parentheses terminates the if clause, and the assignment
statement involving z is always executed as the next
statement. The if therefore has no effect because of the
misplaced semicolon. This is actually a null statement and
is legal in C.
The statement in line 38 is much more straightforward because
the variable x will always be equal to itself, therefore the
inequality will never be true, and the entire statement will
never do a thing, but is wasted effort. The statement in line
39 will always assign 0 to x and the compare will therefore
always be false, never executing the conditional part of the
if statement.
The conditional statement is extremely important and must be
thoroughly understood to write efficient C programs. If any
part of this discussion is unclear in your mind, restudy it
until you are confident that you understand it thoroughly
Page 4-10
Chapter 4 - Assignments and Logical Compares
before proceeding onward. Compile and run this program. Add
some printout to see the results of some of the operations.
THE CRYPTIC PART OF C
____________________________________________________________
There are three constructs used in C that =============
make no sense at all when first encountered CRYPTIC.C
because they are not intuitive, but they =============
greatly increase the efficiency of the
compiled code and are used extensively by
experienced C programmers. You should therefore be exposed
to them and learn to use them because they will appear in
most, if not all, of the programs you see in the publications.
Load and examine the file named CRYPTIC.C for examples of the
three new constructs.
In this program, some variables are defined and initialized
in the same statements for use later. The statement in line
8 simply adds 1 to the value of x, and should come as no
surprise to you. The next two statements also add one to the
value of x, but it is not intuitive that this is what happens.
It is simply by definition that this is true. Therefore, by
definition of the C language, a double plus sign either before
or after a variable increments that variable by 1.
Additionally, if the plus signs are before the variable, the
variable is incremented before it is used, and if the plus
signs are after the variable, the variable is used, then
incremented. In line 11, the value of y is assigned to the
variable z, then y is incremented because the plus signs are
after the variable y. In the last statement of the
incrementing group of example statements, line 12, the value
of y is incremented then its value is assigned to the variable
z.
The next group of statements illustrate decrementing a
variable by one. The definition works exactly the same way
for decrementing as it does for incrementing. If the minus
signs are before the variable, the variable is decremented,
then used, and if the minus signs are after the variable, the
variable is used, then decremented.
THE CRYPTIC ARITHMETIC OPERATOR
____________________________________________________________
Another useful but cryptic operator is the arithmetic
operator. This operator is used to modify any variable by
some constant value. The statement in line 23 simply adds 12
to the value of the variable a. The statement in line 24 does
the same, but once again, it is not intuitive that they are
the same. Any of the four basic functions of arithmetic, +,
-, *, or /, can be handled in this way, by putting the
Page 4-11
Chapter 4 - Assignments and Logical Compares
function desired in front of the equal sign and eliminating
the second reference to the variable name. It should be noted
that the expression on the right side of the arithmetic
operator can be any valid expression, the examples are kept
simple for your introduction to this new operator.
Just like the incrementing and decrementing operators, the
arithmetic operator is used extensively by experienced C
programmers and it would pay you well to understand it.
THE CONDITIONAL EXPRESSION
____________________________________________________________
The conditional expression is just as cryptic as the last two,
but once again it can be very useful so it would pay you to
understand it. It consists of three expressions within
parentheses separated by a question mark and a colon. The
expression prior to the question mark is evaluated to
determine if it is true or false. If it is true, the
expression between the question mark and the colon is
evaluated, and if it is not true, the expression following the
colon is evaluated. The result of the evaluation is used for
the assignment as illustrated in line 30. The final result
is identical to that of an if statement with an else clause.
This is illustrated by the example in lines 32 through 35 of
this group of statements. The conditional expression has the
added advantage of more compact code that will compile to
fewer machine instructions in the final program.
Lines 37 and 38 of this example program are given to
illustrate a very compact way to assign the greater of the two
variables a or b to c, and to assign the lessor of the same
two variables to c. Notice how efficient the code is in these
two examples.
TO BE CRYPTIC OR NOT TO BE CRYPTIC
____________________________________________________________
Several students of C have stated that they didn't like these
three cryptic constructs and that they would simply never use
them. This will be fine if they never have to read anybody
else's program, or use any other programs within their own.
I have found many functions that I wished to use within a
program but needed a small modification to use it, requiring
me to understand another person's code. It would therefore
be to your advantage to learn these new constructs, and use
them. They will be used in the remainder of this tutorial, so
you will be constantly exposed to them.
This has been a long chapter but it contained important
material to get you started in using C. In the next chapter,
we will go on to the building blocks of C, the functions. At
Page 4-12
Chapter 4 - Assignments and Logical Compares
that point, you will have enough of the basic materials to
allow you to begin writing meaningful programs.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program that will count from 1 to 12 and print
the count, and its square, for each count.
1 1
2 4
3 9 etc.
2. Write a program that counts from 1 to 12 and prints the
count and its inversion to 5 decimal places for each
count. This will require a floating point number.
1 1.00000
2 .50000
3 .33333
4 .25000
etc.
3. Write a program that will count from 1 to 100 and print
only those values between 32 and 39, one to a line.
Page 4-13
CHAP05.TXT
Chapter 5
FUNCTIONS, VARIABLES, AND PROTOTYPES
OUR FIRST USER DEFINED FUNCTION
____________________________________________________________
Load and examine the file SUMSQRES.C for an ==============
example of a C program with functions. SUMSQRES.C
Actually this is not the first function we ==============
have encountered because the main program we
have been using all along is technically a
function, as is the printf() function. The printf() function
is a library function that was supplied with your compiler.
Notice the executable part of this program which begins in
line 8 with a line that simply says "header();", which is the
way to call any function. The parentheses are required
because the C compiler uses them to determine that it is a
function call and not simply a misplaced variable. When the
program comes to this line of code, the function named
header() is called, its statements are executed, and control
returns to the statement following this call. Continuing on
we come to a for loop which will be executed 7 times and which
calls another function named square() each time through the
loop, and finally a function named ending() will be called and
executed. For the moment ignore the variable name index in
the parentheses of the call to square. We have seen that this
program calls a header, 7 square calls, and an ending. Now
we need to define the functions.
DEFINING THE FUNCTIONS
____________________________________________________________
Following the main program you will see another program
beginning in line 14 that follows all of the rules set forth
so far for a main program except that it is named header().
This is the function which is called from line 8 of the main
program. Each of these statements are executed, and when they
are all complete, control returns to the main program.
The first statement sets the variable named sum equal to zero
because we plan to use it to accumulate a sum of squares.
Since the variable named sum is defined prior to the main
program, it is available to be used in any of the following
functions. It is called a global variable, and its scope is
the entire program and all functions. It is also sometimes
referred to as a file variable because it is available
throughout the file. More will be said about the scope of
variables near the end of this chapter. The statement in line
17 outputs a header message to the monitor. Program control
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then returns to the main program since there are no additional
statements to execute in this function. Essentially, we drop
out of the bottom of the function and return to the caller.
It should be clear to you that the two executable lines from
this function could be moved to the main program, replacing
the header call, and the program would do exactly the same
thing that it does as it is now written. This does not
minimize the value of functions, it merely illustrates the
operation of this simple function in a simple way. You will
find functions to be very valuable in C programming.
PASSING A VALUE TO A FUNCTION (CLASSIC METHOD)
____________________________________________________________
Going back to the main program, and the for loop specifically,
we find the new construct from the end of the last lesson used
in the last part of the for loop, namely the index++ used in
line 9. You should get familiar with this construct, as you
will see it in a lot in C programs.
In the call to the function named square(), we have an added
feature, namely the variable name index within the
parentheses. This is an indication to the compiler that when
you go to the function, you wish to take along the value of
index to use in the execution of that function. Looking ahead
at the function named square(), we find that another variable
name is enclosed in its parentheses, the variable number.
This is the name we prefer to call the variable passed to the
function when we are in the function. We can call it anything
we wish as long as it follows the rules of naming an
identifier. Since the function must know what type the
variable is, it is defined following the function name but
before the opening brace of the function itself. Thus, line
21 containing "int number;" tells the function that the value
passed to it will be an integer type variable. With all of
that out of the way, we now have the value of index from the
main program passed to the function square(), but renamed
number, and available for use within the function. This is
the classic style of defining function variables and has been
in use since C was originally defined. A newer method is
gaining in popularity due to its many benefits and will be
discussed later in this chapter.
Following the opening brace of the function, we define another
variable named numsq for use only within the function itself,
(more about that later) and proceed with the required
calculations. We set the variable named numsq equal to the
square of the value of number, then add numsq to the current
total stored in the variable named sum. Remember that "sum
+= numsq;" has the identical meaning as "sum = sum + numsq;"
from the last lesson. We print the number and its square in
line 27, and return to the main program.
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MORE ABOUT PASSING A VALUE TO A FUNCTION
____________________________________________________________
When we passed the value of the variable named index to the
function, a little more happened than meets the eye. We did
not actually pass the value of index to the function, we
actually passed a copy of the value. In this way the original
value is protected from accidental corruption by a called
function. We could have modified the variable named number
in any way we wished in the function named square(), and when
we returned to the main program, the variable named index
would not have been modified. We thus protect the value of
a variable in the main program from being accidentally
corrupted, but we cannot return a value to the main program
from a function using this technique. We will find a well
defined method of returning values to the main program or to
any calling function when we get to arrays and another method
when we get to pointers. Until then the only way you will be
able to communicate back to the calling function will be with
global variables. We have already hinted at global variables
above, and will discuss them in detail later in this chapter.
Continuing in the main program, we come to the last function
call, the call to the function named ending() in line 11.
This line calls the last function which has no local variables
defined. It prints out a message with the value of the
variable sum contained in it to end the program. The program
ends by returning to the main program and finding nothing else
to do. Compile and run this program and observe the output.
NOW TO CONFESS A LITTLE LIE
____________________________________________________________
I told you a short time ago that the only way =============
to get a value back to the main program was SQUARES.C
through use of a global variable, but there =============
is another way which we will discuss after
you load and display the file named
SQUARES.C. In this file we will see that it is simple to
return a single value from a called function to the calling
function. But once again, it is true that to return more than
one value, we will need to study either arrays or pointers.
In the main program, we define two integers and begin a for
loop in line 6 which will be executed 8 times. The first
statement of the for loop is "y = squ(x);", which is a new and
rather strange looking construct. From past experience, we
should have no trouble understanding that the squ(x) portion
of the statement is a call to the function named squ() taking
along the value of x as a variable. Looking ahead to line 15
of the function itself, we find that the function prefers to
call the input variable in, and it proceeds to square the
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value of in and call the result square. Finally, a new kind
of a statement appears in line 21, the return statement. The
value within the parentheses is assigned to the function
itself and is returned as a usable value in the main program.
Thus, the function call "squ(x)" is assigned the value of the
square and returned to the main program such that the variable
named y is then set equal to that value. If the variable
named x were therefore assigned the value 4 prior to this
call, y would then be set to 16 as a result of this line of
code.
Another way to think of this is to consider the grouping of
characters squ(x) as another variable with a value that is the
square of x, and this new variable can be used any place it
is legal to use a variable of its type. The values of the
variables x and y are then printed out.
To illustrate that the grouping of squ(x) can be thought of
as just another variable, another for loop is introduced in
line 11 in which the function call is placed in the printf()
statement rather than assigning it to a new variable.
One last point must be made, the type of variable returned
must be defined in order to make sense of the data, but the
compiler will default the type to integer if none is
specified. If any other type is desired, it must be
explicitly defined. How to do this will be demonstrated in
the next example program.
Be sure to compile and run this program which also uses the
classic method of defining function variables.
FLOATING POINT FUNCTIONS
____________________________________________________________
Load the program FLOATSQ.C for an example of =============
a function with a floating point type of FLOATSQ.C
return. It begins by defining a global =============
floating point variable named z which we will
use later. Then in the main part of the
program, an integer is defined, followed by two floating point
variables, and then by two strange looking definitions. The
expressions sqr() and glsqr() look like function calls and
they are. This is the proper way in C to define that a
function will return a value that is not of type int, but of
some other type, in this case float. This tells the compiler
that when a value is returned from either of these two
functions, it will be of type float. This is, once again, the
classic method of defining functions.
Now refer to the function named sqr() starting in line 22 and
you will see that the function name is preceded by the keyword
float. This is an indication to the compiler that this
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function will return a value of type float to any program that
calls it. The function is now compatible with the call to it.
The line following the function name contains float inval;,
which indicates to the compiler that the variable passed to
this function from the calling program will be of type float.
The function named glsqr() beginning in line 31, will also
return a float type variable, but it uses a global variable
for input. It does the squaring right within the return
statement and therefore has no need to define a separate
variable to store the product.
The overall structure of this program should pose no problem
and will not be discussed in any further detail. As is
customary with all example programs, compile and run this
program.
THE CLASSIC STYLE
____________________________________________________________
The three programs we have studied in this chapter so far use
the classic style of function definition. Although this style
was the first defined for C, it is rapidly being replaced with
the modern method of function definition because it does so
much for you in detecting and flagging errors. As you read
articles on C, you will see programs written in the classic
style, so you need to be capable of reading them. It would
be highly recommended, however, that you learn and use the
modern method which will be covered shortly in this tutorial.
The remainder of this tutorial will use the modern method as
recommended and defined by the ANSI standard. If you have an
older compiler, it may not work on some of these files and it
will be up to you to modify the programs as needed to conform
to the classic style.
SCOPE OF VARIABLES
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Load the next program, SCOPE.C, and display =============
it for a discussion of the scope of variables SCOPE.C
in a program. You can ignore the 4 =============
statements in lines 2 through 5 of this
program for a few moments. We will discuss
them later.
The first variable defined is a global variable named count
which is available to any function in the program since it is
defined before any of the functions. It is always available
because it does not come and go as the program is executed.
(That will make sense shortly.) Farther down in the program,
another global variable named counter is defined in line 25
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which is also global but is not available to the main program
since it is defined following the main program. A global
variable is any variable that is defined outside of any
function. Note that both of these variables are sometimes
referred to as external variables because they are external
to any functions, and they are sometimes called file
variables.
Return to the main program and you will see the variable named
index defined as an integer in line 11. Ignore the word
register for the moment. This variable is only available
within the main program because that is where it is defined.
In addition, it is an automatic variable, which means that it
only comes into existence when the function in which it is
contained is invoked, and ceases to exist when the function
is finished. This really means nothing here because the main
program is always in operation, even when it gives control to
another function. Another integer is defined within the for
braces named stuff. Any pairing of braces can contain a
variable definition which will be valid and available only
while the program is executing statements within those braces.
The variable will be an automatic variable and will cease to
exist when execution leaves the braces. This is convenient
to use for a loop counter or some other very localized
variable.
MORE ON AUTOMATIC VARIABLES
____________________________________________________________
Observe the function named head1() in line 26 which looks a
little funny because of void being used twice. The purpose
of the uses of the word void will be explained shortly. The
function contains a variable named index, which has nothing
to do with the variable named index in line 11 of the main
program, except that both are automatic variables. When the
program is not actually executing statements in this function,
this variable named index does not even exist. When head1()
is called, the variable is generated, and when head1()
completes its task, the variable in head1() named index is
eliminated completely from existence. (The automatic variable
is stored on the stack. This topic will be covered later.)
Keep in mind however that this does not affect the variable
of the same name in the main program, since it is a completely
separate entity.
Automatic variables therefore, are automatically generated and
disposed of when needed. The important thing to remember is
that from one call of a function to the next call, the value
of an automatic variable is not preserved and must therefore
be reinitialized.
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WHAT ARE STATIC VARIABLES?
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An additional variable type must be mentioned at this point,
the static variable. By putting the keyword static in front
of a variable definition within a function, the variable or
variables in that definition are static variables and will
stay in existence from call to call of the particular
function.
By putting the same keyword in front of an external variable,
one outside of any function, it makes the variable private and
not accessible to use in any other file. (This is a
completely different use of the same keyword.) This implies
that it is possible to refer to external variables in other
separately compiled files, and that is true. Examples of this
usage will be given in chapter 14 of this tutorial.
USING THE SAME NAME AGAIN
____________________________________________________________
Refer to the function named head2(). It contains another
definition of the variable named count. Even though count has
already been defined as a global variable, it is perfectly all
right to reuse the name in this function. It is a completely
new variable that has nothing to do with the global variable
of the same name, and causes the global variable to be
unavailable in this function. This allows you to write
programs using existing functions without worrying about what
names were used for variables in the functions because there
can be no conflict. You only need to worry about the
variables that interface with the functions.
WHAT IS A REGISTER VARIABLE?
____________________________________________________________
Now to fulfill a promise made earlier about what a register
variable is. A computer can keep data in a register or in
memory. A register is much faster in operation than memory
but there are very few registers available for the programmer
to use. If there are certain variables that are used
extensively in a program, you can designate that those
variables are to be stored in a register in order to speed up
the execution of the program. The method of doing this is
illustrated in line 11. Your compiler probably allows you to
use one or more register variables and will ignore additional
requests if you request more than are available. The
documentation for your compiler will list how many registers
are available with your compiler. It will also inform you of
what types of variables can be stored in a register. If your
compiler does not allow the use of register variables, the
register request will simply be ignored.
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Chapter 5 - Functions, Variables, & Prototypes
WHERE DO I DEFINE VARIABLES?
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Now for a refinement on a general rule stated earlier. When
you have variables brought to a function as arguments to the
function, and you are using the classic style of programming,
they are defined immediately after the function name and prior
to the opening brace for the executable statements. Other
variables used in the function are defined at the beginning
of the function, immediately following the opening brace of
the function, and before any executable statements.
WHAT IS PROTOTYPING?
____________________________________________________________
A prototype is a model of a real thing and when programming
with a good up-to-date C compiler, you have the ability to
define a model of each function for the compiler. The
compiler can then use the model to check each of your calls
to the function and determine if you have used the correct
number of arguments in the function call and if they are of
the correct type. By using prototypes, you let the compiler
do some additional error checking for you. The ANSI standard
for C contains prototyping as part of its recommended
standard. Every good C compiler will have prototyping
available, so you should learn to use it. Much more will be
said about prototyping throughout the remainder of this
tutorial.
Returning to lines 3, 4, and 5 in SCOPE.C, we have the
prototypes for the three functions contained within the
program. The first void in each line tells the compiler that
these particular functions do not return a value, so that the
compiler would flag the statement index = head1(); as an error
because nothing is returned to assign to the variable named
index. The word void within the parentheses tells the
compiler that this function requires no parameters and if a
variable were included, it would be an error and the compiler
would issue a warning message. If you wrote the statement
head1(index);, it would be a error. This allows you to use
type checking when programming in C in much the same manner
that it is used in Pascal, Modula-2, or Ada, although the type
checking in C is very weak.
Note the addition of the word void in line 9. This is an
indication to the system that we do not plan to return a value
to the operating system when we terminate operation of this
program. The main program also can return a value in the same
manner as any other function.
You should enable prototype checking at this time, if it is
available with your compiler. Check your documentation for
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Chapter 5 - Functions, Variables, & Prototypes
the details of how to do it. Prototyping will be used
throughout the remainder of this tutorial. If your compiler
does not support prototyping and the modern method of function
definition, you will have to modify the remaining example
programs. A much better solution would be to purchase a
better compiler.
Line 2 of SCOPE.C tells the system to go to the include files
and get the file named STDIO.H which contains the prototypes
for the standard input and output functions so they can be
checked for proper variable types. Don't worry about the
include yet, it will be covered in detail later in this
tutorial. Be sure to compile and execute this program.
STANDARD FUNCTION LIBRARIES
____________________________________________________________
Every compiler comes with some standard predefined functions
which are available for your use. These are mostly
input/output functions, character and string manipulation
functions, and math functions. We will cover most of these
in subsequent chapters. Prototypes are defined for you by the
writer of your compiler for all of the functions that are
included with your compiler. A few minutes spent studying
your reference guide will give you an insight in where the
prototypes are defined for each of the functions.
Most compilers have additional functions predefined that are
not standard but allow the programmer to get the most out of
his particular computer. In the case of the IBM-PC and
compatibles, most of these functions allow the programmer to
use the BIOS services available in the operating system, or
to write directly to the video monitor or to any place in
memory. These will not be covered in any detail as you will
be able to study these unique aspects of your compiler on your
own. Many of these kinds of functions are used in the example
programs in chapter 14.
WHAT IS RECURSION?
____________________________________________________________
Recursion is another of those programming ==============
techniques that seem very intimidating the RECURSON.C
first time you come across it, but if you ==============
will load and display the example program
named RECURSON.C, we will take all of the
mystery out of it. This is probably the simplest recursive
program that it is possible to write and it is therefore a
stupid program in actual practice, but for purposes of
illustration, it is excellent.
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Chapter 5 - Functions, Variables, & Prototypes
Recursion is nothing more than a function that calls itself.
It is therefore in a loop which must have a way of
terminating. In the program on your monitor, the variable
named index is set to 8 in line 9, and is used as the argument
to the function named count_dn(). The function simply
decrements the variable, prints it out in a message, and if
the variable is not zero, it calls itself, where it decrements
the variable again, prints it, etc. etc. etc. Finally, the
variable will reach zero, and the function will not call
itself again. Instead, it will return to the prior time it
called itself, and return again, until finally it will return
to the main program and from there return to DOS.
For purposes of understanding you can think of it as having
8 copies of the function named count_dn() available and it
simply called all of them one at a time, keeping track of
which copy it was in at any given time. That is not what
actually happened, but it is a reasonable illustration for you
to begin understanding what it was really doing.
WHAT DID IT DO?
____________________________________________________________
A better explanation of what actually happened is in order.
When you called the function from itself, it stored all of the
variables and all of the internal flags it needs to complete
the function in a block somewhere. The next time it called
itself, it did the same thing, creating and storing another
block of everything it needed to complete that function call.
It continued making these blocks and storing them away until
it reached the last function when it started retrieving the
blocks of data, and using them to complete each function call.
The blocks were stored on an internal part of the computer
called the stack. This is a part of memory carefully
organized to store data just as described above. It is beyond
the scope of this tutorial to describe the stack in detail,
but it would be good for your programming experience to read
some material describing the stack. A stack is used in nearly
all modern computers for internal housekeeping chores.
In using recursion, you may desire to write a program with
indirect recursion as opposed to the direct recursion
described above. Indirect recursion would be when a function
A calls the function B, which in turn calls A, etc. This is
entirely permissible, the system will take care of putting the
necessary things on the stack and retrieving them when needed
again. There is no reason why you could not have three
functions calling each other in a circle, or four, or five,
etc. The C compiler will take care of all of the details for
you.
The thing you must remember about recursion is that at some
point, something must go to zero, or reach some predefined
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Chapter 5 - Functions, Variables, & Prototypes
point to terminate the loop. If not, you will have an
infinite loop, and the stack will fill up and overflow, giving
you an error and stopping the program rather abruptly.
Be sure to compile and run this program.
ANOTHER EXAMPLE OF RECURSION
____________________________________________________________
The program named BACKWARD.C is another ==============
example of recursion, so load it and display BACKWARD.C
it on your screen. This program is similar ==============
to the last one except that it uses a
character array. Each successive call to the
function named forward_and_backwards() causes one character
of the message to be printed. Additionally, each time the
function ends, one of the characters is printed again, this
time backwards as the string of recursive function calls is
retraced.
This program uses the modern method of function definition and
includes full prototype definitions. The modern method of
function definition moves the types of the variables into the
parentheses along with the variable names themselves. The
final result is that the line containing the function
definition looks more like the corresponding line in Pascal,
Modula-2, or Ada. The prototype in line 5 is simply a copy
of the function header in line 18 followed by a semicolon.
The designers of C even allow you to include a variable name
along with each type. The name is ignored by the compiler but
including the name in the prototype could give you a good idea
of how the variable is used, acting like a comment.
Don't worry about the character array defined in line 9 or the
other new material presented here. After you complete chapter
7 of this tutorial, this program will make sense. It was felt
that introducing a second example of recursion was important
so this file is included here.
Compile and run this program with prototype checking enabled
and observe the results.
IF YOU CAN'T USE PROTOTYPING WITH YOUR COMPILER
____________________________________________________________
If your compiler does not support prototyping remove line 5
from BACKWARD.C and replace line 18 with the following lines;
forward_and_backwards(line_of_char, index)
char line_of_char[];
int index;
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Load and display the program named FLOATSQ2.C =============
which is an exact copy of the program FLOATSQ2.C
FLOATSQ.C which we considered earlier with =============
prototyping added. The use of prototyping is
a good practice for all C programmers to get
into.
Several things should be mentioned about this program. First,
the word float at the beginning of lines 27 and 35 indicate
to the compiler that these functions are functions that return
float type values. Also, since the prototypes are given
before main, the functions are not required to be identified
in line 12 as they were in line 7 of FLOATSQ.C earlier in this
chapter.
Notice also that the type of the variable named inval is
included within the parentheses in line 27. It would be very
educational for you to modify this program so that you
included a call to sqr() with a variable of type int within
the parentheses to see what kind of a warning you would get.
Do the same thing in the program without prototype checking,
FLOATSQ.C.
PROGRAMMING EXERCISES
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1. Rewrite TEMPCONV.C, from an earlier chapter, and move
the temperature calculation to a function.
2. Write a program that writes your name on the monitor 10
times by calling a function to do the writing. Move the
called function ahead of the main function to see if your
C compiler will allow it.
3. Add prototyping to the programs named SUMSQRES.C and
SQUARES.C, and change the function definitions to the
modern method.
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CHAP06.TXT
Chapter 6
DEFINES AND MACROS
AIDS TO CLEAR PROGRAMMING
____________________________________________________________
Load and display the file named DEFINE.C for ==============
your first look at some defines and macros. DEFINE.C
Notice lines 4 through 7 of the program, each ==============
starting with #define. This is the way all
defines and macros are defined. Before the
actual compilation starts, the compiler goes through a
preprocessor pass to resolve all of the defines. In the
present case, it will find every place in the program where
the combination START is found and it will replace it with the
0 since that is the definition. The compiler itself will
never see the word START, so as far as the compiler is
concerned, the zeros were always there. Note that if the
string is found in a string constant or in a comment, it will
not be changed.
It should be clear to you that putting the word START in your
program instead of the numeral 0 is only a convenience to you
and actually acts like a comment since the word START helps
you to understand what the zero is used for.
In the case of a very small program, such as that before you,
it doesn't really matter what you use. If, however, you had
a 2000 line program before you with 27 references to START,
it would be a completely different matter. If you wanted to
change all of the STARTs in the program to a new number, it
would be simple to change the one #define. If this technique
were not used, it would be difficult to find and change all
of the references to it manually, and possibly disastrous if
you missed one or two of the references.
In the same manner, the preprocessor will find all occurrences
of the word ENDING and change them to 9, then the compiler
will operate on the changed file with no knowledge that ENDING
ever existed.
It is a fairly common practice in C programming to use all
capital letters for a symbolic constant such as START and
ENDING and use all lower case letters for variable names. You
can use any method you choose since it is mostly a matter of
personal taste.
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Chapter 6 - Defines and Macros
IS THIS REALLY USEFUL?
____________________________________________________________
When we get to the chapters discussing input and output, we
will need an indicator to tell us when we reach the
end-of-file of an input file. Since different compilers use
different numerical values for this, although most use either
a zero or a minus 1, we will write the program with a #define
to define the EOF used by our particular compiler. If at some
later date, we change to a new compiler, it is a simple matter
to change this one #define to fix the entire program. In most
C compilers, the EOF is defined in the STDIO.H file. You can
observe this for yourself by listing the STDIO.H file that was
supplied with your compiler.
WHAT IS A MACRO?
____________________________________________________________
A macro is nothing more than another define, but since it is
capable of at least appearing to perform some logical
decisions or some math functions, it has a unique name.
Consider line 6 of the program on your screen for an example
of a macro. In this case, anytime the preprocessor finds the
word MAX followed by a group in parentheses, it expects to
find two terms in the parentheses and will do a replacement
of the terms into the second part of the definition. Thus the
first term will replace every A in the second part of the
definition and the second term will replace every B in the
second part of the definition. When line 15 of the program
is reached, index will be substituted for every A, and count
will be substituted for every B. Once again, it must be
stated that string constants and comments will not be
affected. Remembering the cryptic construct we studied a
couple of chapters ago will reveal that mx will receive the
maximum value of index or count. In like manner, the MIN
macro will result in mn receiving the minimum value of index
or count.
When defining a macro, it is imperative that there is no space
between the macro name and the opening parenthesis. If there
is a space, the compiler cannot determine that it is a macro,
but will handle it like a simple substitution define
statement. The results are then printed out in line 17.
There are a lot of seemingly extra parentheses in the macro
definition but they are not extra, they are essential. We
will discuss the extra parentheses in our next example
program. Be sure to compile and execute DEFINE.C before going
on to the next program.
Page 6-2
Chapter 6 - Defines and Macros
LET'S LOOK AT A WRONG MACRO
____________________________________________________________
Load the file named MACRO.C and display it on =============
your screen for a better look at a macro and MACRO.C
its use. Line 4 defines a macro named WRONG =============
that appears to get the cube of A, and indeed
it does in some cases, but it fails miserably
in others. The second macro named CUBE actually does get the
cube in all cases.
Consider the program itself where the CUBE of i+offset is
calculated. If i is 1, which it is the first time through,
then we will be looking for the cube of 1+5 = 6, which will
result in 216. When using CUBE, we group the values like this,
(1+5)*(1+5)*(1+5) = 6*6*6 = 216. However, when we use WRONG,
we group them as 1+5*1+5*1+5 = 1+5+5+5 = 16 which is a wrong
answer. The parentheses are therefore required to properly
group the variables together. It should be clear to you that
either CUBE or WRONG would arrive at a correct answer for a
single term replacement such as we did in the last program.
The correct values of the cube and the square of the numbers
are printed out as well as the wrong values for your
inspection.
In line 7 we define the macro ADD_WRONG according to the above
rules but we still have a problem when we try to use the macro
in line 25 and 26. In line 26 when we say we want the program
to calculate 5*ADD_WRONG(i) with i = 1, we get the result 5*1
+ 1 which evaluates to 5 + 1 or 6, and this is most assuredly
not what we had in mind. We really wanted the result to be
5*(1 + 1) = 5*2 = 10 which is the answer we get when we use
the macro named ADD_RIGHT, because of the extra parentheses
in the definition given in line 8. A little time spent
studying the program and the result will be worth your effort
in understanding how to use macros.
In order to prevent the above problems, most experienced C
programmers include parentheses around each variable in a
macro and additional parentheses around the entire expression.
The remainder of the program is simple and will be left to
your inspection and understanding.
WHAT IS AN ENUMERATION VARIABLE?
____________________________________________________________
Load and display the program named ENUM.C for ============
an example of how to use the enum type ENUM.C
variable. Line 6 contains the first enum ============
type variable named result which is a
variable which can take on any of the values
contained within the parentheses. Actually the variable
Page 6-3
Chapter 6 - Defines and Macros
result is an int type variable but can be assigned any of the
values defined for it. The names within the parentheses are
int type constants and can be used anywhere it is legal to use
an int type constant. The constant win is assigned the value
of 0, tie the value 1, bye the value 2, etc.
In use, the variable named result is used just like any int
variable would be used as can be seen by its use in the
program. The enum type of variable is intended to be used by
you, the programmer, as a coding aid since you can use a
constant named mon for control structures rather than the
meaningless (at least to you) value of 1. Notice that days
is assigned the values of days of the week in the remainder
of the program. If you were to use a switch statement, it
would be much more meaningful to use the labels sun, mon, etc,
rather than the more awkward 0, 1, 2, etc.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program to count from 7 to -5 by counting down.
Use #define statements to define the limits. (Hint, you
will need to use a decrementing variable in the third
part of the for loop control.
Page 6-4
CHAP07.TXT
Chapter 7
STRINGS AND ARRAYS
WHAT IS A STRING?
____________________________________________________________
A string is a group of characters, usually letters of the
alphabet. In order to format your printout in such a way that
it looks nice, has meaningful names and titles, and is
aesthetically pleasing to you and the people using the output
of your program, you need the ability to output text data.
Actually you have already been using strings, because the
second program in this tutorial, way back in Chapter 2, output
a message that was handled internally as a string. A complete
definition of a string is a series of char type data
terminated by a NULL character.
When C is going to use a string of data in some way, either
to compare it with another string, output it, copy it to
another string, or whatever, the functions are set up to do
what they are called to do until a NULL, which is a zero, is
detected. Such a string is often called an ASCIIZ string.
We will use a few ASCIIZ strings in this chapter.
WHAT IS AN ARRAY?
____________________________________________________________
An array is a series of homogeneous pieces of data that are
all identical in type, but the type can be quite complex as
we will see when we get to the chapter of this tutorial
discussing structures. A string is simply a special case of
an array, a series of char type data.
The best way to see these principles is by ===============
use of an example, so load the program CHRSTRG.C
CHRSTRG.C and display it on your monitor. ===============
The first thing new is in line 6 which
defines a char type of data entity. The
square brackets define an array subscript in C, and in the
case of the data definition statement, the 5 in the brackets
defines 5 data fields of type char all defined as part of the
variable. In the C language, all subscripts start at 0 and
increase by 1 each step up to the maximum which in this case
is 4. We therefore have 5 char type variables named, name[0],
name[1], name[2], name[3], and name[4]. You must keep in mind
that in C, the subscripts actually go from 0 to one less than
the number defined in the definition statement. This is due
to the original definition of C and these limits cannot be
changed or redefined by the programmer.
Page 7-1
Chapter 7 - Strings and Arrays
HOW DO WE USE THE STRING?
____________________________________________________________
The variable name is therefore a string which can hold up to
5 characters, but since we need room for the NULL terminating
character, there are actually only four useful characters.
To load something useful into the string, we have 5
statements, each of which assigns one alphabetical character
to one of the string characters. Finally, the last place in
the string is filled with the numeral 0 as the end indicator
and the string is complete. (A define would allow us to use
NULL instead of a zero, and this would add greatly to the
clarity of the program. It would be very obvious that this was
a NULL and not simply a zero for some other purpose.) Now
that we have the string, we will print it out with some other
string data in the output statement, line 14.
The %s is the output definition to output a string and the
system will output characters starting with the first one in
the string name until it comes to the NULL character, and it
will quit. Notice that in the printf() statement, only the
variable name which happens to be name needs to be given, with
no subscript since we are interested in starting at the
beginning. (There is actually another reason that only the
variable name is given without brackets. The discussion of
that topic will be given in the next chapter.)
OUTPUTTING PART OF A STRING
____________________________________________________________
The printf() in line 15 illustrates that we can output any
single character of the string by using the %c and naming the
particular character of the variable name we want by including
the subscript. The last printf() illustrates how we can
output part of the string by stating the starting point by
using a subscript. The & specifies the address of name[1].
We will study this in the next chapter but I thought you would
benefit from a little glimpse ahead, so don't worry about this
construct yet.
This example may make you feel that strings are rather
cumbersome to use since you have to set up each character one
at a time. The next example program will illustrate that
strings are very easy to use. Be sure to compile and run this
program.
SOME STRING FUNCTIONS
____________________________________________________________
Load the example program STRINGS.C for an example of some ways
to use strings. First we define four strings. Next we come
to a new function that you will find very useful, the strcpy()
Page 7-2
Chapter 7 - Strings and Arrays
function, or string copy. It copies from one
string to another until it comes to the NULL =============
character in the source string. Remember STRINGS.C
that the NULL is actually a zero and is added =============
to the character string by the system. It is
easy to remember which one gets copied to which if you think
of them like an assignment statement. Thus if you were to
say, for example, x = 23;, the data is copied from the right
entity to the left one. In the strcpy() function, the data
is also copied from the right entity to the left, so that
after execution of the first statement, the string variable
name1 will contain the string "Rosalinda", but without the
double quotes, they are the compiler's way of knowing that you
are defining a string.
Likewise, the string "Zeke" is copied into name2 in line 11,
then the title string is copied into the string named title.
The title and both names are then printed out. Note that it
is not necessary for the defined string to be exactly the same
size as the string it will be called upon to store, only that
it is at least as long as the string plus one more character
for the NULL.
ALPHABETICAL SORTING OF STRINGS
____________________________________________________________
The next function we will look at is the strcmp() or the
string compare function illustrated in line 18. It will
return a 1 if the first string is larger than the second, zero
if they are the same length and have the same characters, and
-1 if the first string is smaller than the second. One of the
strings, depending on the result of the compare is copied into
the string variable mixed, and the largest name alphabetically
is printed out. It should come as no surprise to you that
Zeke wins because it is alphabetically larger, length doesn't
matter, only the alphabet. It might be wise to mention that
the result would also depend on whether the letters were upper
or lower case. There are functions available with your C
compiler to change the case of a string to all upper or all
lower case if you desire. These will be used in an example
program later in this tutorial.
COMBINING STRINGS
____________________________________________________________
Lines 25 through 28 illustrate another new feature, the
strcat(), or string concatenation function. This function
simply adds the characters from one string onto the end of
another string taking care to adjust the NULL so everything
is still all right. In this case, name1 is copied into mixed,
then two blanks are concatenated to mixed, and finally name2
Page 7-3
Chapter 7 - Strings and Arrays
is concatenated to the combination. The result is printed out
with both names in the one string variable mixed.
Strings are not difficult and are extremely useful. You
should spend some time getting familiar with them before
proceeding on to the next topic.
Compile and run this program and observe the results for
compliance with this definition.
AN ARRAY OF INTEGERS
____________________________________________________________
Load the file INTARRAY.C and display it on ==============
your monitor for an example of an array of INTARRAY.C
integers. Notice that the array is defined ==============
in much the same way we defined an array of
char in order to do the string manipulations
in the last example program. We have 12 integer variables to
work with not counting the one named index. The names of the
variables are values[0], values[1], ... , and values[11]. In
lines 9 and 10 we have a loop to assign nonsense, but well
defined, data to each of the 12 variables, then print all 12
out in lines 12 and 13. Note carefully that each element of
the array is simply an int type variable capable of storing
an integer value. The only difference between the variables
index and values[2], for example, is in the way you address
them. You should have no trouble following this program, but
be sure you understand it. Compile and run it to see if it
does what you expect it to do.
AN ARRAY OF FLOATING POINT DATA
____________________________________________________________
Load and display the program named BIGARRAY.C ==============
for an example of a program with an array of BIGARRAY.C
float type data. This program has an extra ==============
feature to illustrate how strings can be
initialized. Line 4 of the program
illustrates how to initialize a string of characters. Notice
that the square brackets are empty leaving it up to the
compiler to count the characters and allocate enough space for
our string including the terminating NULL. Another string is
initialized in line 11 of the body of the program but it must
be declared static here. This prevents it from being
allocated as an automatic variable and allows it to retain the
string once the program is started. There is nothing else new
here, the variables are assigned nonsense data and the results
of all the nonsense are printed out along with a header. This
program should also be easy for you to follow, so study it
until you are sure of what it is doing before going on to the
next topic.
Page 7-4
Chapter 7 - Strings and Arrays
GETTING DATA BACK FROM A FUNCTION
____________________________________________________________
Back in chapter 5 when we studied functions, ==============
I hinted to you that there was a way to get PASSBACK.C
data back from a function by using an array, ==============
and that is true. Examine the program
PASSBACK.C for an example of doing that. In
this program, we define an array of 20 variables named matrix
in line 8, then assign some nonsense data to the variables,
and print out the first five. In line 16 we call the function
dosome() taking along the entire array by putting the name of
the array in the parentheses.
The function dosome() beginning in line 22 has a name in its
parentheses also but it prefers to call the array list. The
function needs to be told that it is really getting an array
passed to it and that the array is of type int. Line 22 does
that by defining list as an integer type variable and
including the square brackets to indicate an array. It is not
necessary to tell the function how many elements are in the
array, but you could if you so desired. Generally a function
works with an array until some end-of-data marker is found,
such as a NULL for a string, or some other previously defined
data or pattern. Many times, another piece of data is passed
to the function with a count of how many elements to work
with. In our present illustration, we will use a fixed number
of elements to keep it simple.
So far nothing is different from the previous functions we
have called except that we have passed more data points to the
function this time than we ever have before, having passed 20
integer values. We print out the first 5 again in lines 26
and 27 to see if they did indeed get passed here. In lines
29 and 30 we add ten to each of the elements and print out the
new values. Finally we return to the main program and print
out the same 5 data points. We find that we have indeed
modified the data stored in the calling program from within
the function, and when we returned to the main program, we
brought the changes back. Compile and run this program to
verify this conclusion.
ARRAYS PASS DATA BOTH WAYS
____________________________________________________________
We stated during our study of functions that when we passed
data to a function, the system made a copy to use in the
function which was thrown away when we returned. This is not
the case with arrays. The actual array is passed to the
function and the function can modify it any way it wishes to.
The result of the modifications will be available back in the
calling program. This may seem strange to you that arrays are
Page 7-5
Chapter 7 - Strings and Arrays
handled differently from single point data, but they are. It
really does make sense, but you will have to wait until we get
to pointers to understand it.
A HINT AT A FUTURE LESSON
____________________________________________________________
Another way of getting data back from a function to the
calling program is by using pointers which we will cover in
the next chapter. When we get there we will find that an
array is in reality a pointer to a list of values. Don't let
that worry you now, it will make sense when we get there. In
the meantime concentrate on arrays and understand the basics
of them because when we get to the study of structures we will
be able to define some pretty elaborate arrays.
MULTI-DIMENSIONAL ARRAYS
____________________________________________________________
Load and display the file named MULTIARY.C ==============
for an example of a program with doubly MULTIARY.C
dimensioned arrays. The variable big is an ==============
8 by 8 array that contains 8 times 8 or 64
elements total. The first element is
big[0][0], and the last is big[7][7]. Another array named
large is also defined which is not square to illustrate that
the array need not be square. Both are filled up with data,
one representing a multiplication table and the other being
formed into an addition table.
To illustrate that individual elements can be modified at
will, one of the elements of big is assigned the value from
one of the elements of large after being multiplied by 22 in
line 17. Next big[2][2] is assigned the arbitrary value of
5, and this value is used for the subscripts of the assignment
statement in line 19. The assignment statement in line 19 is
in reality big[5][5] = 177; because each of the subscripts
contain the value 5. This is only done to illustrate that any
valid expression can be used for a subscript. It must only
meet two conditions, it must be an integer (although a char
will work just as well), and it must be within the range of
the subscript it is being used for.
The entire matrix variable big is printed out in a square form
in lines 21 through 25 so you can check the values to see if
they did get set the way you expected them to.
Page 7-6
Chapter 7 - Strings and Arrays
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program with three short strings, about 6
characters each, and use strcpy to copy the string
literals "one", "two", and "three" into them. Concatenate
the three strings into one string and print the result
out 10 times.
2. Define two integer arrays, each 10 elements long, called
array1 and array2. Using a loop, put some kind of
nonsense data in each and add them term for term into
another 10 element array named arrays. Finally, print
all results in a table with an index number.
1 2 + 10 = 12
2 4 + 20 = 24
3 6 + 30 = 36 etc.
Hint; The print statement will be similar to;
printf("%4d %4d + %4d = %4d\n",index,array1[index],
array2[index],arrays[index]);
Page 7-7
CHAP08.TXT
Chapter 8
POINTERS
WHAT IS A POINTER?
____________________________________________________________
Simply stated, a pointer is an address. =============
Instead of being a variable, it is a pointer POINTER.C
to a variable stored somewhere in the address =============
space of the program. It is always best to
use an example so load the file named
POINTER.C and display it on your monitor for an example of a
program with some pointers in it.
For the moment, ignore the data declaration statement where
we define index and two other fields beginning with a star.
It is properly called an asterisk, but for reasons we will see
later, let's agree to call it a star. If you observe the
first statement, it should be clear that we assign the value
of 39 to the variable named index. This is no surprise, we
have been doing it for several programs now. The statement
in line 9 however, says to assign to pt1 a strange looking
value, namely the variable index with an ampersand in front
of it. In this example, pt1 and pt2 are pointers, and the
variable named index is a simple variable. Now we have a
problem. We need to learn how to use pointers in a program,
but to do so requires that first we define the means of using
the pointers in the program.
The following two rules will be somewhat confusing to you at
first but we need to state the definitions before we can use
them. Take your time, and the whole thing will clear up very
quickly.
TWO VERY IMPORTANT RULES
____________________________________________________________
The following two rules are very important when using pointers
and must be thoroughly understood.
1. A variable name with an ampersand in front of it defines
the address of the variable and therefore points to the
variable. You can therefore read line nine as "pt1 is
assigned the value of the address of index".
2. A pointer with a star in front of it refers to the value
of the variable pointed to by the pointer. Line twelve
of the program can be read as "The stored (starred) value
to which the pointer pt1 points is assigned the value
Page 8-1
Chapter 8 - Pointers
13". Now you can see why it is convenient to think of
the asterisk as a star, it sort of sounds like the word
store.
MEMORY AIDS
____________________________________________________________
1. Think of & as an address.
2. Think of * as a star referring to stored.
Assume for the moment that pt1 and pt2 are pointers (we will
see how to define them shortly). As pointers, they do not
contain a variable value but an address of a variable and can
be used to point to a variable. Figure 8-1 is a graphical
representation of the data space as it is configured at this
time. A box represents a variable, and a box with a dot in
it represents a pointer. At this time the pointers are not
pointing at anything, so they have no arrows emanating from
the boxes.
Continuing execution of the program, we come to line 9 which
assigns the pointer pt1 to point to the variable we have
already defined as index because we have assigned the address
of index to pt1. Since we have a pointer to index, we can
manipulate the value of index by using either the variable
name itself, or the pointer. Figure 8-2 depicts the condition
of the data space after executing line 9.
Line 12 modifies the value by using the pointer. Since the
pointer pt1 points to the variable named index, then putting
a star in front of the pointer name refers to the memory
location to which it is pointing. Line 12 therefore assigns
to index the value of 13. Anyplace in the program where it
is permissible to use the variable name index, it is also
permissible to use the name *pt1 since they are identical in
meaning until the pointer is reassigned to some other
variable.
ANOTHER POINTER
____________________________________________________________
Just to add a little intrigue to the system, we have another
pointer defined in this program, pt2. Since pt2 has not been
assigned a value prior to statement 10, it doesn't point to
anything, it contains garbage. Of course, that is also true
of any variable until a value is assigned to it. The
statement in line 10 assigns pt2 the same address as pt1, so
that now pt2 also points to the variable named index. So to
continue the definition from the last paragraph, anyplace in
the program where it is permissible to use the variable index,
it is also permissible to use the name *pt2 because they are
Page 8-2
Chapter 8 - Pointers
identical in meaning. This fact is illustrated in the
printf() statement in line 11 since this statement uses the
three means of identifying the same variable to print out the
same variable three times. Refer to figure 8-3 for the
representation of the data space at this time.
THERE IS ONLY ONE VARIABLE
____________________________________________________________
Note carefully that, even though it appears that there are
three variables, there is really only one variable. The two
pointers point to the single variable. This is illustrated
in the next statement which assigns the value of 13 to the
variable index, because that is where the pointer pt1 is
pointing. The printf() statement in line 13 causes the new
value of 13 to be printed out three times. Keep in mind that
there is really only one variable to be changed, not three.
Figure 8-4 is the graphical representation of the data space
at this point.
This is admittedly a very difficult concept, but since it is
used extensively in all but the most trivial C programs, it
is well worth your time to stay with this material until you
understand it thoroughly.
HOW DO YOU DECLARE A POINTER?
____________________________________________________________
Now to keep a promise and tell you how to define a pointer.
Refer to line 6 of the program and you will see our old
familiar way of defining the variable index, followed by two
more definitions. The second definition can be read as "the
storage location to which pt1 points will be an int type
variable". Therefore, pt1 is a pointer to an int type
variable. Likewise, pt2 is another pointer to an int type
variable, because it has a star (asterisk) in front of it.
A pointer must be defined to point to a specific type of
variable. Following a proper definition, it cannot be used
to point to any other type of variable or it will result in
a type incompatibility error. In the same manner that a float
type of variable cannot be added to an int type variable, a
pointer to a float variable cannot be used to point to an
integer variable without a little extra trouble.
Compile and run this program and observe that there is only
one variable and the single statement in line 12 changes the
one variable which is displayed three times. This material
is so important that you should review it carefully if you do
not fully understand it at this time.
Page 8-3
Chapter 8 - Pointers
THE SECOND PROGRAM WITH POINTERS
____________________________________________________________
In these few pages so far on pointers, we ==============
have covered a lot of territory, but it is POINTER2.C
important territory. We still have a lot of ==============
material to cover so stay in tune as we
continue this important aspect of C. Load
the next file named POINTER2.C and display it on your monitor
so we can continue our study.
In this program we have defined several variables and two
pointers. The first pointer named there is a pointer to a
char type variable and the second named pt points to an int
type variable. Notice also that we have defined two array
variables named strg and list. We will use them to show the
correspondence between pointers and array names.
Figure 8-5 depicts the data space at this time. There are
three variables, two pointers, and two strings. Each string
is composed of the string itself and a pointer which points
to the beginning of the string. This will be completely
defined in the next paragraph. The string itself is composed
of a number of identical elements of which only a few at the
beginning and a few at the end are depicted graphically.
A STRING VARIABLE IS ACTUALLY A POINTER
____________________________________________________________
In the programming language C, a string variable is defined
to be simply a pointer to the beginning of a string. This
will take some explaining. Refer to the example program on
your monitor. You will notice that in line 10 we assign a
string constant to the string variable named strg so we will
have some data to work with. Next, we assign the value of the
first element to the variable one, a simple char variable.
Next, since the string name is a pointer by definition of the
C language, we can assign the same value to two by using the
star and the string name. The result of the two assignments
are such that one now has the same value as two, and both
contain the character T, the first character in the string.
Note that it would be incorrect to write line 10 as two =
*strg[0]; because the star takes the place of the square
brackets.
For all practical purposes, strg is a pointer. It does,
however, have one restriction that a true pointer does not
have. It cannot be changed like a variable, but must always
contain the initial value and therefore always points to its
string. It could be thought of as a pointer constant, and in
some applications you may desire a pointer that cannot be
corrupted in any way. Even though it cannot be changed, it
can be used to refer to other values than the one it is
Page 8-4
Chapter 8 - Pointers
defined to point to, as we will see in the next section of the
program.
Moving ahead to line 16, the variable one is assigned the
value of the ninth variable (since the indexing starts at
zero) and two is assigned the same value because we are
allowed to index a pointer to get to values farther ahead in
the string. Both variables now contain the character a.
POINTER INDEXING
____________________________________________________________
The C programming language takes care of indexing for us
automatically by adjusting the indexing for the type of
variable the pointer is pointing to. In this case, the index
of 8 is simply added to the pointer value before looking up
the desired result because a char type variable is one byte
long. If we were using a pointer to an int type variable, the
index would be doubled and added to the pointer before looking
up the value because an int type variable uses two bytes per
value stored on most microcomputers. When we get to the
chapter on structures, we will see that a variable can have
many, even into the hundreds or thousands, of bytes per
variable, but the indexing will be handled automatically for
us by the system.
The data space is now in the condition defined graphically in
figure 8-6. The string named strg has been filled and the two
variables named one and two have the letter a stored in them.
Since there is already a pointer, it can be assigned the
address of the 11th element of strg by the statement in line
20 of this program. Remember that since there is a pointer
to type char, it can be assigned any value as long as that
value represents a char type of address. It should be clear
that the pointers must be typed in order to allow the pointer
arithmetic described in the last paragraph to be done
properly. The third and fourth outputs will be the same,
namely the letter c.
POINTER ARITHMETIC
____________________________________________________________
Not all forms of arithmetic are permissible on a pointer.
Only those things that make sense, considering that a pointer
is an address somewhere in the computer. It would make sense
to add a constant to an address, thereby moving it ahead in
memory that number of places. Likewise, subtraction is
permissible, moving it back some number of locations. Adding
two pointers together would not make sense because absolute
memory addresses are not additive. Pointer multiplication is
Page 8-5
Chapter 8 - Pointers
also not allowed, as that would be a funny number. If you
think about what you are actually doing, it will make sense
to you what is allowed, and what is not.
NOW FOR AN INTEGER POINTER
____________________________________________________________
The array named list is assigned a series of values from 100
to 199 in order to have some data to work with in lines 24 and
25. Next we assign the pointer pt the address of the 28th
element of the list and print out the same value both ways to
illustrate that the system truly will adjust the index for the
int type variable. You should spend some time in this program
until you feel you fairly well understand these lessons on
pointers.
Compile and execute POINTER2.C and study the output. At the
termination of execution, the data space will be as depicted
in figure 8-7.
FUNCTION DATA RETURN WITH A POINTER
____________________________________________________________
You may recall that back in the lesson on func- ============
tions we mentioned that there were two ways to TWOWAY.C
get variable data back from a function. One way ============
is through use of the array, and you should be
right on the verge of guessing the other way. If
your guess is through use of a pointer, you are correct. Load
and display the example program named TWOWAY.C for an example
of this.
In TWOWAY.C, there are two variables defined in the main
program, pecans and apples. Notice that neither of these is
defined as a pointer. We assign values to both of these and
print them out, then call the function named fixup() taking
with us both of these values. The variable pecans is simply
sent to the function, but the address of the variable apples
is sent to the function. Now we have a problem. The two
arguments are not the same, the second is a pointer to a
variable. We must somehow alert the function to the fact that
it is supposed to receive an integer variable and a pointer
to an integer variable. This turns out to be very simple.
Notice that the parameter definitions in line 20 defines nuts
as an integer, and fruit as a pointer to an integer. The call
in the main program therefore is now in agreement with the
function heading and the program interface will work just
fine.
In the body of the function, we print the two values sent to
the function, then modify them and print the new values out.
This should be perfectly clear to you by now. The surprise
Page 8-6
Chapter 8 - Pointers
occurs when we return to the main program and print out the
two values again. We will find that the value of pecans will
be restored to its value before the function call because the
C language makes a copy of the item in question and takes the
copy to the called function, leaving the original intact. In
the case of the variable apples, we made a copy of a pointer
to the variable and took the copy of the pointer to the
function. Since we had a pointer to the original variable,
even though the pointer was a local copy, we had access to the
original variable and could change it in the function. When
we returned to the main program, we found a changed value in
apples when we printed it out.
This is illustrated graphically in figure 8-8. The state of
the system is illustrated following execution of line 25 of
the program.
By using a pointer in a function call, we can have access to
the data in the function and change it in such a way that when
we return to the calling program, we have a changed value of
the original variable. It must be pointed out however, that
if you modify the value of the pointer itself in the function,
you will have a restored pointer when you return because the
pointer you use in the function is a copy of the original.
In this example, there was no pointer in the main program
because we simply sent the address to the function, but in
many programs you will use pointers in function calls. One
of the places you will find need for pointers in function
calls will be when you request data input using standard
input/output routines. These will be covered in the next two
chapters. Compile and run TWOWAY.C and observe the output.
POINTERS ARE VALUABLE
____________________________________________________________
Even though you are probably somewhat intimidated at this
point by the use of pointers, you will find that after you
gain experience, you will use them profusely in many ways.
You will also use pointers in every program you write other
than the most trivial because they are so useful. You should
probably go over this material carefully several times until
you feel comfortable with it because it is very important in
the area of input/output which is next on the agenda.
A POINTER TO A FUNCTION
____________________________________________________________
Examine the example program named FUNCPNT.C =============
for the most unusual pointer yet. This FUNCPNT.C
program contains a pointer to a function, and =============
illustrates how to use it.
Page 8-7
Chapter 8 - Pointers
Line 8 of this program defines function_pointer as a pointer
to a function and not to just any function, it points to a
function with a single formal parameter of type float. The
function must also return nothing because of the void before
the pointer definition. The parentheses are required around
the pointer name as illustrated or the system will think it
is a prototype definition for a function that returns a
pointer to void.
You will note the prototypes given in lines 4 through 6 that
declare three functions that use the same parameter and return
type as the pointer. Since they are the same as the pointer,
the pointer can be used to refer to them as is illustrated in
the executable part of the program. Line 15 contains a call
to the print_stuff() function, and line 16 assigns the value
of print_stuff to function_pointer. Because the name of a
function is defined as a pointer to that function, its name
can be assigned to a function pointer variable. You will
recall that the name of an array is actually a pointer
constant to the first element of the array. In like manner,
a function name is actually a pointer constant which is
pointing to the function itself.
A function pointer can be passed to another function as a
parameter and can be used within the function to call the
function which is pointed to. You are not permitted to
increment or add a constant to a function pointer, it can only
be assigned the value of a function with the same parameters
and return with which it was initially declared.
If you continue your study of this program, you will see that
the function pointer is used to refer to all three functions
in various ways.
PROGRAMMING EXERCISES
____________________________________________________________
1. Define a character array and use strcpy() to copy a
string into it. Print the string out by using a loop
with a pointer to print out one character at a time.
Initialize the pointer to the first element and use the
double plus sign to increment the pointer. Use a separate
integer variable to count the characters to print.
2. Modify the program from programming exercise 1 to print
out the string backwards by pointing to the end and using
a decrementing pointer.
Page 8-8
CHAP09.TXT
Chapter 9
STANDARD INPUT/OUTPUT
THE STDIO.H HEADER FILE
____________________________________________________________
Load the file SIMPLEIO.C for our first look ==============
at a file with standard I/O. Standard I/O SIMPLEIO.C
refers to the most usual places where data is ==============
either read from, the keyboard, or written
to, the video monitor. Since they are used
so much, they are used as the default I/O devices and do not
need to be named in the Input/Output instructions. This will
make more sense when we actually start to use them so lets
look at the file in front of you.
The first thing you should take notice of is the second line
of the file, the line with #include "stdio.h". This is very
much like the #define we have already studied, except that
instead of a simple substitution, an entire file is read in
at this point. The system will find the file named STDIO.H
and read its entire contents in, replacing this statement.
Obviously then, the file named STDIO.H must contain valid C
source statements that can be compiled as part of a program.
This particular file is composed of several standard #defines
to define some of the standard I/O operations. The file is
called a header file and you will find several different
header files on the source disks that came with your C
compiler. Each of the header files has a specific purpose and
any or all of them can be included in any program.
Your C compiler uses the double quote marks to indicate that
the search for the include file will begin in the current
directory, and if it not found there, the search will continue
in the include directory as set up in the environment. It
also uses the "less than" and "greater than" signs to indicate
that the file search should begin in the directory specified
in the environment. Most of the programs in this tutorial
have the double quotes in the include statements. The next
program uses the "<" and ">" to illustrate the usage. Note
that this will result is a slightly faster (but probably
unnoticeable) compilation because the system will not bother
to search the current directory first.
INPUT/OUTPUT OPERATIONS IN C
____________________________________________________________
Actually the C programming language has no input or output
operations defined as part of the language, they must be user
defined. Since everybody does not want to reinvent his own
input and output operations, the compiler writers have done
Page 9-1
Chapter 9 - Standard Input/Output
a lot of this for us and supplied us with several input
functions and several output functions to aid in our program
development. The functions have become a standard, and you
will find the same functions available in nearly every
compiler. In fact, the industry standard of the C language
definition has become the book written by Kernigan and
Ritchie, and they have included these functions in their
definition. You will often, when reading literature about C,
find an author refer to K & R. This refers to the book, "The
C Programming Language", written by Kernigan and Ritchie. You
would be advised to purchase a copy for reference. As of this
writing, a second edition of this book is available and is the
preferred edition.
You should print out the file named STDIO.H and spend some
time studying it. There will be a lot that you will not
understand about it, but parts of it will look familiar. The
name STDIO.H is sort of cryptic for "standard input/output
header", because that is exactly what it is. It defines the
standard input and output functions in the form of #defines
and macros. Don't worry too much about the details of this
now. You can always return to this topic later for more study
if it interests you, but you will really have no need to
completely understand the STDIO.H file. You will have a
tremendous need to use it however, so these comments on its
use and purpose are necessary.
OTHER INCLUDE FILES
____________________________________________________________
When you begin writing larger programs and splitting them up
into separately compiled portions, you will have occasion to
use some statements common to each of the portions. It would
be to your advantage to make a separate file containing the
statements and use the #include to insert it into each of the
files. If you want to change any of the common statements,
you will only need to change one file and you will be assured
of having all of the common statements agree. This is getting
a little ahead of ourselves but you now have an idea how the
#include directive can be used.
BACK TO THE FILE NAMED SIMPLEIO.C
____________________________________________________________
Lets continue our tour of the file in question. The one
variable named c is defined and a message is printed out with
the familiar printf() function. We then find ourselves in a
continuous loop as long as the value of c is not equal to
capital X. If there is any question in your mind about the
loop control, you should review chapter 3 before continuing.
The two new functions within the loop are of paramount
interest in this program since they are the new functions.
Page 9-2
Chapter 9 - Standard Input/Output
These are functions to read a character from the keyboard and
display it on the monitor one character at a time.
The function getchar() reads a single character from the
standard input device, the keyboard being assumed because that
is the standard input device, and assigns it to the variable
named c. The next function putchar(), uses the standard
output device, the video monitor, and outputs the character
contained in the variable named c. The character is output
at the current cursor location and the cursor is advanced one
space for the next character. The system is therefore taking
care of a lot of the overhead for us. The loop continues
reading and displaying characters until we type a capital X
which terminates the loop.
Compile and run this program for a few surprises. When you
type on the keyboard, you will notice that what you type is
displayed faithfully on the screen, and when you hit the
return key, the entire line is repeated. We only told it to
output each character once but it seems to be saving the
characters up and redisplaying them. A short explanation is
in order.
DOS IS HELPING US OUT
____________________________________________________________
We need to understand a little bit about how DOS works to
understand what is happening here. When data is read from the
keyboard, under DOS control, the characters are stored in a
buffer until a carriage return is entered at which time the
entire string of characters is given to the program. When the
characters are being typed, however, the characters are
displayed one at a time on the monitor. This is called echo,
and happens in many of the applications you run.
With the above paragraph in mind, it should be clear that when
you are typing a line of data into SIMPLEIO, the characters
are being echoed by DOS, and when you return the carriage by
hitting return or enter, the characters are given to the
program. As each character is given to the program, it
displays it on the screen resulting in a repeat of the line
typed in. To better illustrate this, type a line with a
capital X somewhere in the middle of the line. You can type
as many characters as you like following the X and they will
all display because the characters are being read in under
DOS, echoed to the monitor, and placed in the DOS input
buffer. DOS doesn't think there is anything special about a
capital X. When the string is given to the program, however,
the characters are accepted by the program one at a time and
sent to the monitor one at a time, until a capital X is
encountered. After the capital X is displayed, the loop is
terminated, and the program is terminated. The characters on
the input line following the capital X are not displayed
Page 9-3
Chapter 9 - Standard Input/Output
because the capital X signalled program termination. Compile
and run SIMPLEIO.C. After running the program several times
and feeling confident that you understand the above
explanation, we will go on to another program.
Don't get discouraged by the above seemingly weird behavior
of the I/O system. It is strange, but there are other ways
to get data into the computer. You will actually find the
above method useful for many applications, and you will
probably find some of the following useful also.
ANOTHER STRANGE I/O METHOD
____________________________________________________________
Load the file named SINGLEIO.C and display it ==============
on your monitor for another method of SINGLEIO.C
character I/O. Once again, we start with the ==============
standard I/O header file using the "<" and
">" method of defining it. Then we define a
variable named c, and finally we print a welcoming message.
Like the last program, we are in a loop that will continue to
execute until we type a capital X, but the action is a little
different here.
The function named getch() is a get character function. It
differs from the function named getchar() in that it does not
get tied up in DOS. It reads the character in without echo,
and puts it directly into the program where it is operated on
immediately. This function therefore reads a character,
immediately displays it on the screen, and continues the
operation until a capital X is typed. Note that although
getch() is available with most popular microcomputer C
compilers, it is not included in the ANSI standard and may not
be available with all C compilers. It's use may therefore
make a program nonportable.
When you compile and run this program, you will find that
there is no repeat of the lines when you hit a carriage
return, and when you hit the capital X, the program terminates
immediately. No carriage return is needed to get it to accept
the line with the X in it. We do have another problem here,
however, there is no linefeed with the carriage return.
NOW WE NEED A LINE FEED
____________________________________________________________
It is not apparent to you in most application ==============
programs but when you hit the enter key, the BETTERIN.C
program supplies a linefeed to go with the ==============
carriage return. You need to return to the
left side of the monitor and you also need to
drop down a line. The linefeed is not automatic. We need to
Page 9-4
Chapter 9 - Standard Input/Output
improve our program to do this also. If you will load and
display the program named BETTERIN.C, you will find a change
to incorporate this feature.
In BETTERIN.C, we have two additional statements at the
beginning that will define the character codes for the
linefeed (LF), and the carriage return (CR). If you look at
any ASCII table you will find that the codes 10 and 13 are
exactly as defined here. In the main program, after
outputting the character in line 15, we compare it to CR, and
if it is equal to CR, we also output a linefeed which is the
LF. We could have completely omitted the two #define
statements and used the statement if (c == 13) putchar(10);
but it would not be very descriptive of what we are doing
here. The method used in the program represents better
programming practice.
Compile and run BETTERIN.C to see if it does what we have said
it should do. It should display exactly what you type in,
including a linefeed with each carriage return, and should
stop immediately when you type a capital X.
WHICH METHOD IS BEST?
____________________________________________________________
We have examined two methods of reading characters into a C
program, and are faced with a choice of which one we should
use. It really depends on the application because each method
has advantages and disadvantages.
When using the first method, DOS is actually doing all of the
work for us by storing the characters in an input buffer and
signalling us when a full line has been entered. We could
write a program that, for example, did a lot of calculations,
then went to get some input. While we were doing the
calculations, DOS would be accumulating a line of characters
for us, and they would be there when we were ready for them.
However, we could not read in single keystrokes because DOS
would not report a buffer of characters to us until it
recognized a carriage return.
The second method, used in BETTERIN.C, allows us to get a
single character, and act on it immediately. We do not have
to wait until DOS decides we can have a line of characters.
We cannot do anything else while we are waiting for a
character because we are waiting for the input keystroke and
tying up the entire machine. This method is useful for highly
interactive types of program interfaces. It is up to you as
the programmer to decide which is best for your needs.
I should mention at this point that there is also an ungetch()
function that works with the getch() function. If you getch
a character and find that you have gone one too far, you can
Page 9-5
Chapter 9 - Standard Input/Output
ungetch it back to the input device. This simplifies some
programs because you don't know that you don't want the
character until you get it. You can only ungetch() one
character back to the input device, but that is sufficient to
accomplish the task this function was designed for. It is
difficult to demonstrate this function in a simple program so
its use will be up to you to study when you need it. Another
function that may be available with your compiler, but is not
part of the ANSI standard, is the getche() function which is
identical to the getch() function except that it echoes the
character to the monitor for you.
The discussion so far in this chapter, should be a good
indication that, while the C programming language is very
flexible, it does put a lot of responsibility on you as the
programmer to keep many details in mind.
NOW TO READ IN SOME INTEGERS
____________________________________________________________
Load and display the file named INTIN.C for =============
an example of reading in some formatted data. INTIN.C
The structure of this program is very similar =============
to the last three except that we define an
int type variable and loop until the variable
somehow acquires the value of 100.
Instead of reading in a character at a time, as we have in the
last three example programs, we read in an entire integer
value with one call using the function named scanf(). This
function is very similar to the printf() that you have been
using for quite some time by now except that it is used for
input instead of output. Examine the line with the scanf()
and you will notice that it does not ask for the variable
valin directly, but gives the address of the variable since
it expects to have a value returned from the function. Recall
that a function must have the address of a variable in order
to return a value to that variable in the calling program.
Failing to supply a pointer in the scanf() function is the
most common problem encountered in using this function.
The function scanf() scans the input line until it finds the
first data field. It ignores leading blanks and in this case,
it reads integer characters until it finds a blank or an
invalid decimal character, at which time it stops reading and
returns the value.
Remembering our discussion above about the way the DOS input
buffer works, it should be clear that nothing is actually
acted on until a complete line is entered and it is terminated
by a carriage return. At this time, the buffer is input, and
our program will search across the line reading all integer
values it can find until the line is completely scanned. This
Page 9-6
Chapter 9 - Standard Input/Output
is because we are in a loop and we tell it to find a value,
print it, find another, print it, etc. If you enter several
values on one line, it will read each one in succession and
display the values. Entering the value of 100 will cause the
program to terminate, and entering the value 100 with other
values following, will cause termination before the following
values are considered.
IT MAKES WRONG ANSWERS SOMETIMES
____________________________________________________________
If you enter a number up to and including 32767, it will
display correctly, but if you enter a larger number, it will
appear to make an error unless your system uses a much larger
range for an int type variable. For example, if you enter the
value 32768, it will display the value of -32768, entering the
value 65536 will display as a zero. These are not errors but
are caused by the way an integer is defined. The most
significant bit of the 16 bit pattern available for the
integer variable is the sign bit, so there are only 15 bits
left for the value. The variable can therefore only have the
values from -32768 to 32767, any other values are outside the
range of integer variables. This is up to you to take care
of in your programs. It is another example of the increased
responsibility you must assume using C rather than another
high level language such as Pascal, Modula-2, etc.
The above paragraph is true for most MS-DOS C compilers.
There is a very small possibility that your compiler uses an
integer value stored in a field size other than 16 bits. If
that is the case, the same principles will be true but with
different limits than those given above.
Compile and run this program, entering several numbers on a
line to see the results, and with varying numbers of blanks
between the numbers. Try entering numbers that are too big
to see what happens, and finally enter some invalid characters
to see what the system does with nondecimal characters.
CHARACTER STRING INPUT
____________________________________________________________
Load and display the file named STRINGIN.C ==============
for an example of reading a string variable. STRINGIN.C
This program is identical to the last one ==============
except that instead of an integer variable,
we have defined a string variable with an
upper limit of 24 characters (remember that a string variable
must have a null character at the end). The variable in the
scanf() does not need an & because big is an array variable
Page 9-7
Chapter 9 - Standard Input/Output
and by definition it is already a pointer. This program
should require no additional explanation. Compile and run it
to see if it works the way you expect.
You probably got a surprise when you ran it because it
separated your sentence into separate words. When used in the
string mode of input, scanf() reads characters into the string
until it comes to either the end of a line or a blank
character. Therefore, it reads a word, finds the blank
following it, and displays the result. Since we are in a
loop, this program continues to read words until it exhausts
the DOS input buffer. We have written this program to stop
whenever it finds a capital X in column 1, but since the
sentence is split up into individual words, it will stop
anytime a word begins with capital X. Try entering a 5 word
sentence with a capital X as the first character in the third
word. You should get the first three words displayed, and the
last two simply ignored when the program stops.
Try entering more than 24 characters to see what the program
does. In an actual program, it is your responsibility to
count characters and stop when the input buffer is full. You
may be getting the feeling that a lot of responsibility is
placed on you when writing in C. It is, but you also get a
lot of flexibility in the bargain too.
INPUT/OUTPUT PROGRAMMING IN C
____________________________________________________________
C was not designed to be used as a language for lots of input
and output, but as a systems language where a lot of internal
operations are required. You would do well to use another
language for I/O intensive programming, but C could be used
if you desire. The keyboard input is very flexible, allowing
you to get at the data in a very low level way, but very
little help is given you. It is therefore up to you to take
care of all of the bookkeeping chores associated with your
required I/O operations. This may seem like a real pain in
the neck, but in any given program, you only need to define
your input routines once and then use them as needed. Don't
let this worry you. As you gain experience with C, you will
easily handle your I/O requirements.
One final point must be made about these I/O functions. It
is perfectly permissible to intermix scanf() and getchar()
functions during read operations. In the same manner, it is
also fine to intermix the output functions, printf() and
putchar().
Page 9-8
Chapter 9 - Standard Input/Output
IN MEMORY I/O
____________________________________________________________
The next operation may seem a little strange =============
at first, but you will probably see lots of INMEM.C
uses for it as you gain experience. Load the =============
file named INMEM.C and display it for another
type of I/O, one that never accesses the
outside world, but stays in the computer.
In INMEM.C, we define a few variables, then assign some values
to the ones named numbers for illustrative purposes and then
use an sprintf() function. The function acts just like a
normal printf() function except that instead of printing the
line of output to a device, it prints the line of formatted
output to a character string in memory. In this case the
string goes to the string variable named line, because that
is the string name we inserted as the first argument in the
sprintf() function. The spaces after the 2nd %d were put
there to illustrate that the next function will search
properly across the line. We print the resulting string and
find that the output is identical to what it would have been
by using a printf() instead of the sprintf() in the first
place. You will see that when you compile and run the program
shortly.
Since the generated string is still in memory, we can now read
it with the function sscanf(). We tell the function in its
first argument that line is the string to use for its input,
and the remaining parts of the line are exactly what we would
use if we were going to use the scanf() function and read data
from outside the computer. Note that it is essential that we
use pointers to the data because we want to return data from
a function. Just to illustrate that there are many ways to
declare a pointer several methods are used, but all are
pointers. The first two simply declare the address of the
elements of the array, while the last three use the fact that
result, without the accompanying subscript, is a pointer.
Just to keep it interesting, the values are read back in
reverse order. Finally the values are displayed on the
monitor.
IS THAT REALLY USEFUL?
____________________________________________________________
It seems sort of silly to read input data from within the
computer but it does have a real purpose. It is possible to
read data from an input device using any of the standard
functions and then do a format conversion in memory. You
could read in a line of data, look at a few significant
characters, then use these formatted input routines to reduce
the line of data to internal representation. That would sure
beat writing your own data formatting routines.
Page 9-9
Chapter 9 - Standard Input/Output
STANDARD ERROR OUTPUT
____________________________________________________________
Sometimes it is desirable to redirect the =============
output from the standard output device to a SPECIAL.C
file. However, you may still want the error =============
messages to go to the standard output device,
in our case the monitor. This next function
allows you to do that. Load and display SPECIAL.C for an
example of this new function.
The program consists of a loop with two messages output, one
to the standard output device and the other to the standard
error device. The message to the standard error device is
output with the function fprintf() and includes the device
name stderr as the first argument. Other than those two small
changes, it is the same as our standard printf function. (You
will see more of the fprintf() function in the next chapter,
but its operation fit in better as a part of this chapter.)
Ignore the line with the exit for the moment, we will return
to it.
Compile and run this program, and you will find 12 lines of
output on the monitor. To see the difference, run the program
again with redirected output to a file named STUFF by entering
the following line at the DOS prompt;
A> special >stuff
This time you will only get the 6 lines output to the standard
error device, and if you look in your directory, you will find
the file named STUFF containing the other 6 lines, those to
the standard output device. You can use I/O redirection with
any of the programs we have run so far, and as you may guess,
you can also read from a file using I/O redirection but we
will study a better way to read from a file in the next
chapter. More information about I/O redirection can be found
in your DOS manual.
WHAT ABOUT THE exit(4) STATEMENT?
____________________________________________________________
Now to keep our promise about the exit(4) statement. Redisplay
the file named SPECIAL.C on your monitor. The last statement
simply exits the program and returns the value of 4 to DOS.
Any number from 0 to 9 can be used in the parentheses for DOS
communication. If you are operating in a BATCH file, this
number can be tested with the ERRORLEVEL command.
Most compilers that operate in several passes return a 1 with
this mechanism to indicate that a fatal error has been
Page 9-10
Chapter 9 - Standard Input/Output
detected and it would be a waste of time to go on to another
pass resulting in even more errors.
It is therefore wise to use a batch file for compiling
programs and testing the returned value for errors. A check
of the documentation for my Compaq, resulted in a minimal and
confusing documentation of the ERRORLEVEL command, so a brief
description of it is given in this file in case your
documentation does not include enough information to allow you
to use it.
One additional feature must be mentioned here. Since we wish
to return an int value to the operating system, we must define
the main program entry point as returning an int rather than
a void as we have used in most of the example programs to this
point. Refer to line 5 for an example of this extension.
PROGRAMMING EXERCISE
____________________________________________________________
1. Write a program to read in a character using a loop, and
display the character in its normal char form. Also
display it as a decimal number. Check for a dollar sign
to use as the stop character. Use the getch() form of
input so it will print immediately. Hit some of the
special keys, such as function keys, when you run the
program for some surprises. You will get two inputs from
the special keys, the first being a zero which is the
indication to the system that a special key was hit.
Page 9-11
CHAP10.TXT
Chapter 10
FILE INPUT/OUTPUT
OUTPUT TO A FILE
____________________________________________________________
Load and display the file named FORMOUT.C for =============
your first example of writing data to a file. FORMOUT.C
We begin as before with the include statement =============
for STDIO.H, then define some variables for
use in the example including a rather strange
looking new type.
The type FILE is used for a file variable and is defined in
the STDIO.H file. It is used to define a file pointer for use
in file operations. The definition of C requires a pointer
to a FILE type to access a file, and as usual, the name can
be any valid variable name.
OPENING A FILE
____________________________________________________________
Before we can write to a file, we must open it. What this
really means is that we must tell the system that we want to
write to a file and what the filename is. We do this with the
fopen() function illustrated in line 11 of the program. The
file pointer, fp in our case, points to the file and two
arguments are required in the parentheses, the filename first,
followed by the file attribute. The filename is any valid DOS
filename, and can be expressed in upper or lower case letters,
or even mixed if you so desire. It is enclosed in double
quotes. For this example we have chosen the name
TENLINES.TXT. This file should not exist on your disk at this
time. If you have a file with this name, you should change
its name or move it because when we execute this program, its
contents will be erased. If you don't have a file by this
name, this program will create one and put some data into it.
READING ("r")
____________________________________________________________
The second parameter is the file attribute and can be any of
three letters, "r", "w", or "a", and must be lower case.
There are actually additional attributes available in C to
allow more flexible I/O. When an "r" is used, the file is
opened for reading, a "w" is used to indicate a file to be
used for writing, and an "a" indicates that you desire to
append additional data to the data already in an existing
file. Opening a file for reading requires that the file
Page 10-1
Chapter 10 - File Input/Output
already exist. If it does not exist, the file pointer will
be set to NULL and can be checked by the program.
WRITING ("w")
____________________________________________________________
When a file is opened for writing, it will be created if it
does not already exist and it will be reset if it does
resulting in deletion of any data already there.
APPENDING ("a")
____________________________________________________________
When a file is opened for appending, it will be created if it
does not already exist and it will be initially empty. If it
does exist, the data input point will be the end of the
present data so that any new data will be added to any data
that already exists in the file.
OUTPUTTING TO THE FILE
____________________________________________________________
The job of actually outputting to the file is nearly identical
to the outputting we have already done to the standard output
device. The only real differences are the new function names
and the addition of the file pointer as one of the function
arguments. In the example program, fprintf() replaces our
familiar printf() function name, and the file pointer defined
earlier is the first argument within the parentheses. The
remainder of the statement looks like, and in fact is
identical to, the printf() statement.
CLOSING A FILE
____________________________________________________________
To close a file, you simply use the function fclose() with the
file pointer in the parentheses. Actually, in this simple
program, it is not necessary to close the file because the
system will close all open files before returning to DOS. It
would be good programming practice for you to get in the habit
of closing all files in spite of the fact that they will be
closed automatically, because that would act as a reminder to
you of what files are open at the end of each program.
You can open a file for writing, close it, and reopen it for
reading, then close it, and open it again for appending, etc.
Each time you open it, you could use the same file pointer,
or you could use a different one. The file pointer is simply
a tool that you use to point to a file and you decide what
file it will point to.
Page 10-2
Chapter 10 - File Input/Output
Compile and run this program. When you run it, you will not
get any output to the monitor because it doesn't generate any.
After running it, look at your directory for a file named
TENLINES.TXT and examine it's contents. That is where your
output will be. Compare the output with that specified in the
program. It should agree.
Do not erase the file named TENLINES.TXT yet. We will use it
in some of the other examples in this chapter.
OUTPUTTING A SINGLE CHARACTER AT A TIME
____________________________________________________________
Load the next example file, CHAROUT.C, and =============
display it on your monitor. This program CHAROUT.C
will illustrate how to output a single =============
character at a time.
The program begins with the include statement, then defines
some variables including a file pointer. The file pointer is
named point this time, but we could have used any other valid
variable name. We then define a string of characters to use
in the output function using a strcpy() function. We are
ready to open the file for appending and we do so with the
fopen() function, except this time we use the lower cases for
the filename. This is done simply to illustrate that DOS
doesn't care about the case of the filename. Notice that the
file will be opened for appending so we will add to the lines
inserted during the last program.
The program is actually two nested for loops. The outer loop
is simply a count to ten so that we will go through the inner
loop ten times. The inner loop calls the function putc()
repeatedly until a character in the string named others is
detected to be a zero.
THE putc() FUNCTION
____________________________________________________________
The part of the program we are interested in is the putc()
function in line 16. It outputs one character at a time, the
character being the first argument in the parentheses and the
file pointer being the second and last argument. Why the
designer of C made the pointer first in the fprintf()
function, and last in the putc() function is a good question
for which there may be no answer. It seems like this would
have been a good place to have used some consistency.
When the textline others is exhausted, a newline is needed
because a newline was not included in the definition above.
Page 10-3
Chapter 10 - File Input/Output
A single putc() is then executed which outputs the \n
character to return the carriage and do a linefeed.
When the outer loop has been executed ten times, the program
closes the file and terminates. Compile and run this program
but once again there will be no output to the monitor.
Following execution of the program, examine the contents of
the file named TENLINES.TXT and you will see that the 10 new
lines were added to the end of the 10 that already existed.
If you run it again, yet another 10 lines will be added. Once
again, do not erase this file because we are still not
finished with it.
READING A FILE
____________________________________________________________
Load the file named READCHAR.C and display it ==============
on your monitor. This is our first program READCHAR.C
which can read from a file. ==============
This program begins with the familiar
include, some data definitions, and the file opening statement
which should require no explanation except for the fact that
an "r" is used here because we want to read from this file.
In this program, we check to see that the file exists, and if
it does, we execute the main body of the program. If it
doesn't exist, we print a message and quit. If the file does
not exist, the system will set the pointer equal to NULL which
we can test.
The main body of the program is one do while loop in which a
single character is read from the file and output to the
monitor until an EOF (end of file) is detected from the input
file. The file is then closed and the program is terminated.
CAUTION CAUTION CAUTION
____________________________________________________________
At this point, we have the potential for one of the most
common and most perplexing problems of programming in C. The
variable returned from the getc() function is a character, so
we can use a char variable for this purpose. There is a
problem that could develop here if we happened to use an
unsigned char however, because C returns a minus one for an
EOF which an unsigned char type variable is not capable of
containing. An unsigned char type variable can only have the
values of zero to 255, so it will return a 255 for a minus
one. This is a very frustrating problem to try to find. The
program can never find the EOF and will therefore never
terminate the loop. This is easy to prevent, always use a
char type variable for use in returning an EOF.
Page 10-4
Chapter 10 - File Input/Output
There is another problem with this program but we will worry
about it when we get to the next program and solve it with the
one following that.
After you compile and run this program and are satisfied with
the results, it would be a good exercise to change the name
of TENLINES.TXT and run the program again to see that the NULL
test actually works as stated. Be sure to change the name
back because we are still not finished with TENLINES.TXT.
READING A WORD AT A TIME
____________________________________________________________
Load and display the file named READTEXT.C ==============
for an example of how to read a word at a READTEXT.C
time. ==============
This program is nearly identical to the last
except that this program uses the fscanf() function to read
in a string at a time. Because the fscanf() function stops
reading when it finds a space or a newline character, it will
read a word at a time, and display the results one word to a
line. You will see this when you compile and run it, but
first we must examine a programming problem.
THIS IS A PROBLEM
____________________________________________________________
Inspection of the program will reveal that when we read data
in and detect the EOF, we print out something before we check
for the EOF resulting in an extra line of printout. What we
usually print out is the same thing printed on the prior pass
through the loop because it is still in the buffer named
oneword. We therefore must check for EOF before we execute
the printf() function. This has been done in READGOOD.C,
which you will shortly examine, compile, and execute.
Compile and execute the original program we have been
studying, READTEXT.C and observe the output. If you haven't
changed TENLINES.TXT you will end up with "Additional" and
"lines." on two separate lines with an extra "lines."
displayed because of the printf() before checking for EOF.
Note that some compilers apparently clear the buffer after
printing so you may get an extra blank line instead of two
lines with "lines." on them.
Compile and execute READGOOD.C and observe that the extra
"lines." does not get displayed because of the extra check for
the EOF in the middle of the loop. This was also the problem
Page 10-5
Chapter 10 - File Input/Output
referred to when we looked at READCHAR.C, but ==============
I chose not to expound on it there because READGOOD.C
the error in the output was not so obvious. ==============
FINALLY, WE READ A FULL LINE
____________________________________________________________
Load and display the file READLINE.C for an ==============
example of reading a complete line. This READLINE.C
program is very similar to those we have been ==============
studying except that we read a complete line
in this example program.
We are using fgets() which reads in an entire line, including
the newline character, into a buffer. The buffer to be read
into is the first argument in the function call, and the
maximum number of characters to read is the second argument,
followed by the file pointer. This function will read
characters into the input buffer until it either finds a
newline character, or it reads the maximum number of
characters allowed minus one. It leaves one character for the
end of string NULL character. In addition, if it finds an
EOF, it will return a value of NULL. In our example, when the
EOF is found, the pointer named c will be assigned the value
of NULL. NULL is defined as zero in your STDIO.H file.
When we find that the pointer named c has been assigned the
value of NULL, we can stop processing data, but we must check
before we print just like in the last program. Last of
course, we close the file.
HOW TO USE A VARIABLE FILENAME
____________________________________________________________
Load and display the program ANYFILE.C for an =============
example of reading from any file. This ANYFILE.C
program asks the user for the filename =============
desired, reads in the filename and opens that
file for reading. The entire file is then
read and displayed on the monitor. It should pose no problems
to your understanding so no additional comments will be made.
Compile and run this program. When it requests a filename,
enter the name and extension of any text file available, even
one of the example C programs.
Page 10-6
Chapter 10 - File Input/Output
HOW DO WE PRINT?
____________________________________________________________
Load the last example program in this ==============
chapter, the one named PRINTDAT.C for an PRINTDAT.C
example of how to print. This program should ==============
not present any surprises to you so we will
move very quickly through it.
Once again, we open TENLINES.TXT for reading and we open PRN
for writing. Printing is identical to writing data to a disk
file except that we use a standard name for the filename.
Most C compilers use the reserved filename of PRN that
instructs the compiler to send the output to the printer.
There are other names that are used occasionally such as LPT,
LPT1, or LPT2. Check the documentation for your particular
compiler.
Some of the newest compilers use a predefined file pointer
such as stdprn for the print file. Once again, check your
documentation.
The program is simply a loop in which a character is read, and
if it is not the EOF, it is displayed and printed. When the
EOF is found, the input file and the printer output files are
both closed. Note that good programming practice would
include checking both file pointers to assure that the files
were opened properly.
You can now erase TENLINES.TXT from your disk. We will not
be using it in any of the later chapters.
PROGRAMMING EXERCISES
____________________________________________________________
1. Write a program that will prompt for a filename for a
read file, prompt for a filename for a write file, and
open both plus a file to the printer. Enter a loop that
will read a character, and output it to the file, the
printer, and the monitor. Stop at EOF.
2. Prompt for a filename to read. Read the file a line at
a time and display it on the monitor with line numbers.
3. Modify ANYFILE.C to test if the file exists and print a
message if it doesn't. Use a method similar to that used
in READCHAR.C.
Page 10-7
CHAP11.TXT
Chapter 11
STRUCTURES AND UNIONS
WHAT IS A STRUCTURE?
____________________________________________________________
A structure is a user defined data type. ===============
Using a structure you have the ability to STRUCT1.C
define a new type of data considerably more ===============
complex than the types we have been using.
A structure is a combination of several
different previously defined data types, including other
structures we have defined. A simple definition is, "a
structure is a grouping of related data in a way convenient
to the programmer or user of the program." The best way to
understand a structure is to look at an example, so if you
will load and display STRUCT1.C, we will do just that.
The program begins with a structure definition. The keyword
struct is followed by some simple variables between the
braces, which are the components of the structure. After the
closing brace, you will find two variable names listed, boy,
and girl. According to the definition of a structure, boy is
now a variable composed of three elements, initial, age, and
grade. Each of the three fields are associated with boy, and
each can store a variable of its respective type. The
variable named girl is also a variable containing three fields
with the same names as those of boy but are actually different
variables. We have therefore defined 6 simple variables.
A SINGLE COMPOUND VARIABLE
____________________________________________________________
Lets examine the variable named boy more closely. As stated
above, each of the three elements of boy are simple variables
and can be used anywhere in a C program where a variable of
their type can be used. For example, the age element is an
integer variable and can therefore be used anywhere in a C
program where it is legal to use an integer variable, in
calculations, as a counter, in I/O operations, etc. We now
have the problem of defining how to use the simple variable
named age which is a part of the compound variable named boy.
To do so we use both names with a decimal point between them
with the major name first. Thus boy.age is the complete
variable name for the age field of boy. This construct can
be used anywhere in a C program that it is desired to refer
to this field. In fact, it is illegal to use the name boy or
age alone because they are only partial definitions of the
complete field. Alone, the names refer to nothing. (Actually
the name boy alone does have meaning when used with some of
the newest C compilers. We will discuss this later.)
Page 11-1
Chapter 11 - Structures and Unions
ASSIGNING VALUES TO THE VARIABLES
____________________________________________________________
Using the above definition, we can assign a value to each of
the three fields of boy and each of the three fields of girl.
Note carefully that boy.initial is actually a char type
variable, because it was assigned that in the structure, so
it must be assigned a character of data. In line 13,
boy.initial is assigned the character R in agreement with the
above rules. The remaining two fields of boy are assigned
values in accordance with their respective types. Finally the
three fields of girl are assigned values but in a different
order to illustrate that the order of assignment is not
critical.
HOW DO WE USE THE RESULTING DATA?
____________________________________________________________
Now that we have assigned values to the six simple variables,
we can do anything we desire with them. In order to keep this
first example simple, we will simply print out the values to
see if they really do exist as assigned. If you carefully
inspect the printf() statements, you will see that there is
nothing special about them. The compound name of each
variable is specified because that is the only valid name by
which we can refer to these variables.
Structures are a very useful method of grouping data together
in order to make a program easier to write and understand.
This first example is too simple to give you even a hint of
the value of using structures, but continue on through these
lessons and eventually you will see the value of using
structures. Compile and run STRUCT1.C and observe the output.
AN ARRAY OF STRUCTURES
____________________________________________________________
Load and display the next program named ===============
STRUCT2.C. This program contains the same STRUCT2.C
structure definition as before but this time ===============
we define an array of 12 variables named
kids. It should be clear that this program
contains 12 times 3 = 36 simple variables, each of which can
store one item of data provided that it is of the correct
type. We also define a simple variable named index for use
in the for loops.
In order to assign each of the fields a value, we use a for
loop and each pass through the loop results in assigning a
value to three of the fields. One pass through the loop
assigns all of the values for one of the kids. This would not
Page 11-2
Chapter 11 - Structures and Unions
be a very useful way to assign data in a real situation, but
a loop could read the data in from a file and store it in the
correct fields. You might consider this the crude beginning
of a data base, which it is.
In the next few instructions of the program we assign new
values to some of the fields to illustrate the method used to
accomplish this. It should be self explanatory, so no
additional comments will be given.
A RECENT UPGRADE TO THE C LANGUAGE
____________________________________________________________
Most modern C compilers will allow you to copy an entire
structure with one statement. This is a fairly recent
addition to the C language and is a part of the ANSI standard,
so you should feel free to use it with your C compiler if it
is available. Line 24 is an example of using a structure
assignment. In this statement, all 3 fields of kids[4] are
copied into their respective fields of kids[10].
WE FINALLY DISPLAY ALL OF THE RESULTS
____________________________________________________________
The last few statements contain a for loop in which all of the
generated values are displayed in a formatted list. Compile
and run the program to see if it does what you expect it to
do. You will need to remove line 24 if your compiler does not
support structure assignments.
USING POINTERS AND STRUCTURES TOGETHER
____________________________________________________________
Examine the file named STRUCT3.C for an =============
example of using pointers with structures. STRUCT3.C
This program is identical to the last program =============
except that it uses pointers for some of the
operations.
The first difference shows up in the definition of variables
following the structure definition. In this program we define
a pointer named point which is defined as a pointer that
points to the structure. It would be illegal to try to use
this pointer to point to any other variable type. There is
a very definite reason for this restriction in C as we have
alluded to earlier and will review in the next few paragraphs.
The next difference is in the for loop where we use the
pointer for accessing the data fields. Recall from chapter
8 of this tutorial that we said that the name of an array is
actually a pointer to the first element of the array. Since
Page 11-3
Chapter 11 - Structures and Unions
kids is a pointer variable that points to the first element
of the array which is a structure, we can define point in
terms of kids. The variable kids is a constant so it cannot
be changed in value, but point is a pointer variable and can
be assigned any value consistent with its being required to
point to the structure. If we assign the value of kids to
point then it should be clear that it will point to the first
element of the array, a structure containing three fields.
POINTER ARITHMETIC
____________________________________________________________
Adding 1 to point will now cause it to point to the second
field of the array because of the way pointers are handled in
C. The system knows that the structure contains three
variables and it knows how many memory elements are required
to store the complete structure. Therefore if we tell it to
add one to the pointer, it will actually add the number of
memory elements required to get to the next element of the
array. If, for example, we were to add 4 to the pointer, it
would advance the value of the pointer 4 times the size of the
structure, resulting in it pointing 4 elements farther along
the array. This is the reason a pointer cannot be used to
point to any data type other than the one for which it was
defined.
Now to return to the program displayed on your monitor. It
should be clear from the previous discussion that as we go
through the loop, the pointer will point to the beginning of
one of the array elements each time. We can therefore use the
pointer to reference the various elements of the structure.
Referring to the elements of a structure with a pointer occurs
so often in C that a special method of doing that was devised.
Using point->initial is the same as using (*point).initial
which is really the way we did it in the last two programs.
Remember that *point is the stored data to which the pointer
points and the construct should be clear. The "->" is made
up of the minus sign and the greater than sign.
Since the pointer points to the structure, we must once again
define which of the elements we wish to refer to each time we
use one of the elements of the structure. There are, as we
have seen, several different methods of referring to the
members of the structure, and in the for loop used for output
at the end of the program, we use three different methods.
This would be considered very poor programming practice, but
is done this way here to illustrate to you that they all lead
to the same result. This program will probably require some
study on your part to fully understand, but it will be worth
your time and effort to grasp these principles.
Lines 31 and 32 are two additional examples of structure
assignment for your benefit. Compile and run this program,
Page 11-4
Chapter 11 - Structures and Unions
and once again, if your compiler does not support structure
assignment, you will need to remove lines 31 and 32.
NESTED AND NAMED STRUCTURES
____________________________________________________________
Examine the file named NESTED.C for an ==============
example of a nested structure. The NESTED.C
structures we have seen so far have been very ==============
simple, although useful. It is possible to
define structures containing dozens and even
hundreds or thousands of elements but it would be to the
programmers advantage not to define all of the elements at one
pass but rather to use a hierarchical structure definition.
This will be illustrated with the program on your monitor.
The first structure contains three elements but is followed
by no variable name. We therefore have not defined any
variables, only a structure, but since we have included a name
at the beginning of the structure, the structure is named
person. The name person can be used to refer to the structure
but not to any variable of this structure type. It is
therefore a new type that we have defined, and we can use the
new type in nearly the same way we use int, char, or any other
types that exist in C. The only restriction is that this new
name must always be associated with the keyword struct.
The next structure definition contains three fields with the
middle field being the previously defined structure which we
named person. The variable which has the type of person is
named descrip. So the new structure contains two simple
variables, grade and a string named lunch, and the structure
named descrip. Since descrip contains three variables, the
new structure actually contains 5 variables. This structure
is also given a name alldat, which is another type definition.
Finally we define an array of 53 variables each with the
structure defined by the type alldat, and each with the name
student. If that is clear, you will see that we have defined
a total of 53 times 5 variables, each of which is capable of
storing a value.
TWO MORE VARIABLES
____________________________________________________________
Since we have a new type definition we can use it to define
two more variables. The variables teacher and sub are defined
in line 19 to be variables of the type alldat, so that each
of these two variables contain 5 fields in which we can store
data.
Page 11-5
Chapter 11 - Structures and Unions
NOW TO USE SOME OF THE FIELDS
____________________________________________________________
In lines 21 through 25 of the program, we will assign values
to each of the fields of teacher. The first field is the
grade field and is handled just like the other structures we
have studied because it is not part of the nested structure.
Next we wish to assign a value to her age which is part of the
nested structure. To address this field we start with the
variable name teacher to which we append the name of the group
descrip, and then we must define which field of the nested
structure we are interested in, so we append the variable name
age. The teachers status is handled in exactly the same
manner as her age, but the last two fields are assigned
strings using the string copy function strcpy() which must be
used for string assignment. Notice that the variable names
in the strcpy() function are still variable names even though
they are made up of several parts each.
The variable sub is assigned nonsense values in much the same
way, but in a different order since they do not have to occur
in any required order. Finally, a few of the student
variables are assigned values for illustrative purposes and
the program ends. None of the values are printed for
illustration since several were printed in the last examples.
Compile and run this program, but when you run it, you may get
a stack overflow error. C uses its own internal stack to
store the automatic variables on, but some C compilers use
only a 2048 byte stack as a default. This program requires
more than that for the defined structures so it will be
necessary for you to increase the stack size. Consult your
compiler documentation for details concerning the method of
increasing the stack size. There is no standard way to do
this. There is another way around this problem, and that is
to move the structure and variable definitions outside of the
program where they will be external variables and therefore
static. The result is that they will not be kept on the
internal stack and the stack will not overflow. It would be
good for you to try both methods of fixing this problem.
MORE ABOUT STRUCTURES
____________________________________________________________
It is possible to continue nesting structures until you get
totally confused. If you define them properly, the computer
will not get confused because there is no stated limit as to
how many levels of nesting are allowed. There is probably a
practical limit of three beyond which you will get confused,
but the language has no limit. In addition to nesting, you
can include as many structures as you desire in any level of
structures, such as defining another structure prior to alldat
and using it in alldat in addition to using person. The
Page 11-6
Chapter 11 - Structures and Unions
structure named person could be included in alldat two or more
times if desired, as could pointers to it.
Structures can contain arrays of other structures which in
turn can contain arrays of simple types or other structures.
It can go on and on until you lose all reason to continue.
I am only trying to illustrate to you that structures are very
valuable and you will find them great aids to programming if
you use them wisely. Be conservative at first, and get bolder
as you gain experience.
More complex structures will not be illustrated here, but you
will find examples of additional structures in the example
programs included in the last chapter of this tutorial. For
example, see the include file named STRUCT.DEF on the
distribution disk.
WHAT ARE UNIONS?
____________________________________________________________
Examine the file named UNION1.C for an ==============
example of a union. Simply stated, a union UNION1.C
allows you a way to look at the same data ==============
with different types, or to use the same data
with different names.
In this example we have two elements to the union, the first
part being the integer named value, which is stored as a two
byte variable somewhere in the computers memory. The second
element is made up of two character variables named first and
second. These two variables are stored in the same storage
locations that value is stored in, because that is what a
union does. A union allows you to store different types of
data in the same physical storage locations. In this case,
you could put an integer number in value, then retrieve it in
its two halves by getting each half using the two names first
and second. This technique is often used to pack data bytes
together when you are, for example, combining bytes to be used
in the registers of the microprocessor.
Accessing the fields of the union are very similar to
accessing the fields of a structure and will be left to you
to determine by studying the example.
One additional note must be given here about the program.
When it is run using some C compilers, the data will be
displayed with two leading f's due to the hexadecimal output
promoting the char type variables to int and extending the
sign bit to the left. Converting the char type data fields
to int type fields prior to display should remove the leading
f's from your display. This will involve defining two new int
type variables and assigning the char type variables to them.
This will be left as an exercise for you. Note that the same
Page 11-7
Chapter 11 - Structures and Unions
problem will come up in a few of the later files in this
tutorial.
Compile and run this program and observe that the data is read
out as an int and as two char variables. The char variables
may be reversed in order because of the way an int variable
is stored internally in your computer. If your system
reverses these variables, don't worry about it. It is not a
problem but it can be a very interesting area of study if you
are so inclined.
ANOTHER UNION EXAMPLE
____________________________________________________________
Load and display the file named UNION2.C for ==============
another example of a union, one which is much UNION2.C
more common. Suppose you wished to build a ==============
large database including information on many
types of vehicles. It would be silly to
include the number of propellers on a car, or the number of
tires on a boat. In order to keep all pertinent data,
however, you would need those data points for their proper
types of vehicles. In order to build an efficient data base,
you would need several different types of data for each
vehicle, some of which would be common, and some of which
would be different. That is exactly what we are doing in the
example program on your monitor.
In this program, we will define a complete structure, then
decide which of the various types can go into it. We will
start at the top and work our way down. First, we define a
few constants with the #defines, and begin the program itself.
We define a structure named automobile containing several
fields which you should have no trouble recognizing, but we
define no variables at this time.
A NEW CONCEPT, THE TYPEDEF
____________________________________________________________
Next we define a new type of data with a typedef. This
defines a complete new type that can be used in the same way
that int or char can be used. Notice that the structure has
no name, but at the end where there would normally be a
variable name there is the name BOATDEF. We now have a new
type, BOATDEF, that can be used to define a structure anyplace
we would like to. Notice that this does not define any
variables, only a new type. Capitalizing the name is a
personal preference only and is not a C standard. It makes
the typedef look different from a variable name.
We finally come to the big structure that defines our data
using the building blocks already defined above. The
Page 11-8
Chapter 11 - Structures and Unions
structure is composed of 5 parts, two simple variables named
vehicle and weight, followed by the union, and finally the
last two simple variables named value and owner. Of course
the union is what we need to look at carefully here, so focus
on it for the moment. You will notice that it is composed of
four parts, the first part being the variable car which is a
structure that we defined previously. The second part is a
variable named boat which is a structure of the type BOATDEF
previously defined. The third part of the union is the
variable airplane which is a structure defined in place in the
union. Finally we come to the last part of the union, the
variable named ship which is another structure of the type
BOATDEF.
I hope it is obvious to you that all four could have been
defined in any of the three ways shown, but the three
different methods were used to show you that any could be
used. In practice, the clearest definition would probably
have occurred by using the typedef for each of the parts.
WHAT DO WE HAVE NOW?
____________________________________________________________
We now have a structure that can be used to store any of four
different kinds of data structures. The size of every record
will be the size of that record containing the largest union.
In this case part 1 is the largest union because it is
composed of three integers, the others being composed of an
integer and a character each. The first member of this union
would therefore determine the size of all structures of this
type. The resulting structure can be used to store any of the
four types of data, but it is up to the programmer to keep
track of what is stored in each variable of this type. The
variable named vehicle was designed into this structure to
keep track of the type of vehicle stored here. The four
defines at the top of the page were designed to be used as
indicators to be stored in the variable named vehicle.
A few examples of how to use the resulting structure are given
in the next few lines of the program. Some of the variables
are defined and a few of them are printed out for illustrative
purposes.
The union is not used too frequently, and almost never by
beginning programmers. You will encounter it occasionally so
it is worth your effort to at least know what it is. You do
not need to know the details of it at this time, so don't
spend too much time studying it. When you do have a need for
a variant structure, a union, you can learn it at that time.
For your own benefit, however, do not slight the structure.
You should use the structure often.
Page 11-9
Chapter 11 - Structures and Unions
WHAT IS A BITFIELD?
____________________________________________________________
Load and display the program named BITFIELD.C ==============
for an example of how to define and use a BITFIELD.C
bitfield, a relatively new addition to the ==============
programming language C. In this program, we
have a union made up of a single int type
variable in line 7 and the structure defined in lines 8
through 12. The structure is composed of three bitfields
named x, y, and z. The variable named x is only one bit wide,
the variable y is two bits wide and adjacent to the variable
x, and the variable z is two bits wide and adjacent to y.
Moreover, because the union causes the bits to be stored in
the same memory location as the variable index, the variable
x is the least significant bit of the variable index, y is the
next two bits, and z is stored in the next two bits of index.
Compile and run the program and you will see that as the
variable index is incremented by 1 each time you will see the
bitfields of the union counting due to their respective
locations within the int definition. Note that your compiler
may not support the bitfield since it is a relatively new
construct to the C programming language.
One thing must be pointed out, the bitfields must be defined
as parts of an unsigned int or your compiler will issue an
error message.
WHAT IS THE BITFIELD GOOD FOR?
____________________________________________________________
The bitfield is very useful if you have a lot of data to
separate into individual bits or groups of bits. Many systems
use some sort of a packed format to get lots of data stored
in a few bytes. Your imagination is your only limitation to
use of this feature of C.
PROGRAMMING EXERCISES
____________________________________________________________
1. Define a named structure containing a string for a name,
an integer for feet, and another for arms. Use the new
type to define an array of about 6 items. Fill the
fields with data and print them out as follows.
A human being has 2 legs and 2 arms.
A dog has 4 legs and 0 arms.
A television set has 4 legs and 0 arms.
A chair has 4 legs and 2 arms.
etc.
2. Rewrite exercise 1 using a pointer to print the data out.
Page 11-10
CHAP12.TXT
Chapter 12
DYNAMIC ALLOCATION
WHAT IS DYNAMIC ALLOCATION?
____________________________________________________________
Dynamic allocation is very intimidating to a =============
person the first time he comes across it, but DYNLIST.C
that need not be. Simply relax and read this =============
chapter carefully and you will have a good
grounding in a very valuable programming
resource. All of the variables in every program up to this
point have been static variables as far as we are concerned.
(Actually, some of them have been automatic and were
dynamically allocated for you by the system, but it was
transparent to you.) In this chapter, we will study some
dynamically allocated variables. They are variables that do
not exist when the program is loaded, but are created
dynamically as they are needed. It is possible, using these
techniques, to create as many variables as needed, use them,
and deallocate their space for use by other variables. As
usual, the best teacher is an example, so examine the program
named DYNLIST.C.
We begin by defining a named structure animal with a few
fields pertaining to dogs. We do not define any variables of
this type, only three pointers. If you search through the
remainder of the program, you will find no variables defined
so we have nothing to store data in. All we have to work with
are three pointers, each of which point to the defined
structure. In order to do anything, we need some variables,
so we will create some dynamically.
DYNAMIC VARIABLE CREATION
____________________________________________________________
The statement in line 14, which assigns something to the
pointer pet1 will create a dynamic structure containing three
variables. The heart of the statement is the malloc()
function buried in the middle of the statement. This is a
memory allocate function that needs the other things to
completely define it. The malloc() function, by default, will
allocate a piece of memory on a heap that is "n" characters
in length and will be of type character. The "n" must be
specified as the only argument to the function. We will
discuss "n" shortly, but first we need to define a heap.
Page 12-1
Chapter 12 - Dynamic Allocation
WHAT IS A HEAP?
____________________________________________________________
Every compiler has a set of limitations on it as to how big
the executable file can be, how many variables can be used,
how long the source file can be, etc. One limitation placed
on users by most MS-DOS C compilers is a limit of 64K for the
executable code if you happen to be in the small memory model.
This is because the IBM-PC uses a microprocessor with a 64K
segment size, and it requires special calls to use data
outside of a single segment. In order to keep the program
small and efficient, these calls are not used in some MS-DOS
implementations of C, and the memory space is limited but
still adequate for most programs.
A heap is an area outside of this 64K boundary which can be
accessed by the program to store data and variables. The data
and variables are put on the heap by the system as calls to
malloc() are made. The system keeps track of where the data
is stored. Data and variables can be deallocated as desired
leading to holes in the heap. The system knows where the
holes are and will use them for additional data storage as
more malloc() calls are made. The structure of the heap is
therefore a very dynamic entity, changing constantly.
MORE ABOUT SEGMENTS
____________________________________________________________
Most C compilers give the user a choice of memory models to
use. The user has a choice of using a model with a 64K
limitation for either program or data leading to a small fast
program or selecting a 640K limitation and requiring longer
address calls leading to less efficient addressing. Using the
larger address space requires inter segment addressing,
resulting in the slightly slower running time. The time is
probably insignificant in most programs, but there are other
considerations.
If a program uses no more than 64K bytes for the total of its
code and memory and if it doesn't use a stack, it can be made
into a .COM file. Since a .COM file is already in a memory
image format, it can be loaded very quickly whereas a file in
an .EXE format must have its addresses relocated as it is
loaded. Therefore a tiny memory model can generate a program
that loads faster than one generated with a larger memory
model. Don't let this worry you, it is a fine point that few
programmers worry about.
Using dynamic allocation, it is possible to store the data on
the heap and that may be enough to allow you to use the small
memory model. Of course, you wouldn't store local variables
such as counters and indexes on the heap, only very large
arrays or structures.
Page 12-2
Chapter 12 - Dynamic Allocation
Even more important than the need to stay within the small
memory model is the need to stay within the computer. If you
had a program that used several large data storage areas, but
not at the same time, you could load one block storing it
dynamically, then get rid of it and reuse the space for the
next large block of data. Dynamically storing each block of
data in succession, and using the same storage for each block
may allow you to run your entire program in the computer
without breaking it up into smaller programs.
BACK TO THE MALLOC FUNCTION
____________________________________________________________
Hopefully the above description of the heap and the overall
plan for dynamic allocation helped you to understand what we
are doing with the malloc() function. It simply asks the
system for a block of memory of the size specified, and gets
the block with the pointer pointing to the first element of
the block. The only argument in the parentheses is the size
of the block desired and in our present case, we desire a
block that will hold one of the structures we defined at the
beginning of the program. The sizeof operator is new, new to
us at least. It returns the size in bytes of the argument
within its parentheses. It therefore, returns the size of the
structure named animal, in bytes, and that number is sent to
the system with the malloc() call. At the completion of that
call, we have a block on the heap allocated to us, with pet1
pointing to the block of data.
WHAT IS A CAST?
____________________________________________________________
We still have a funny looking construct at the beginning of
the malloc() function call. That is called a cast. The
malloc() function returns a block with the pointer pointing
to it being a pointer to type void by default. You really
cannot use a pointer to void, so it must be changed to some
other type. You can define the pointer type with the
construct given on the example line. In this case we want the
pointer to point to a structure of type animal, so we tell the
compiler with this strange looking construct. Even if you
omit the cast, most compilers will return a pointer correctly,
give you a warning, and go on to produce a working program.
It is better programming practice to provide the compiler with
the cast to prevent getting the warning message. The data
space of the computer is depicted graphically by figure 12-1.
Page 12-3
Chapter 12 - Dynamic Allocation
USING THE DYNAMICALLY ALLOCATED STRUCTURE
____________________________________________________________
If you remember our studies of structures and pointers, you
will recall that if we have a structure with a pointer
pointing to it, we can access any of the variables within the
structure. In lines 15 through 17 of the program, we assign
some silly data to the structure for illustration. It should
come as no surprise to you that these assignment statements
look just like assignments to statically defined variables.
Figure 12-2 illustrates the state of the data space at this
point in the program execution.
In the next statement, we assign the value of pet1 to pet2
also. This creates no new data, we simply have two pointers
to the same object. Since pet2 is pointing to the structure
we created above, pet1 can be reused to get another
dynamically allocated structure which is just what we do next.
Keep in mind that pet2 could have just as easily been used for
the new allocation. The new structure is filled with silly
data for illustration.
Finally, we allocate another block on the heap using the
pointer pet3, and fill its block with illustrative data.
Figure 12-3 illustrates the condition of the data space
following execution of line 29 of the program.
Printing the data out should pose no problem to you since
there is nothing new in the three print statements. It is
left for you to study.
Even though it is not illustrated in this tutorial, you can
dynamically allocate and use simple variables such as a single
char type variable. This should be discouraged however since
it is very inefficient.
GETTING RID OF THE DYNAMICALLY ALLOCATED DATA
____________________________________________________________
Another new function is used to get rid of the data and free
up the space on the heap for reuse, the function free(). To
use it, you simply call it with the pointer to the block as
the only argument, and the block is deallocated.
In order to illustrate another aspect of the dynamic
allocation and deallocation of data, an additional step is
included in the program on your monitor. The pointer pet1 is
assigned the value of pet3 in line 42. In doing this, the
block that pet1 was pointing to is effectively lost since
there is no pointer that is now pointing to that block. It
can therefore never again be referred to, changed, or disposed
of. That memory, which is a block on the heap, is wasted from
this point on. This is not something that you would ever
Page 12-4
Chapter 12 - Dynamic Allocation
purposely do in a program. It is only done here for
illustration.
The first free() function call removes the block of data that
pet1 and pet3 were pointing to, and the second free() call
removes the block of data that pet2 was pointing to. We
therefore have lost access to all of our data generated
earlier. There is still one block of data that is on the heap
but there is no pointer to it since we lost the address to it.
Figure 12-4 illustrates the data space as it now appears.
Trying to free the data pointed to by pet1 would result in an
error because it has already been freed by the use of pet3.
There is no need to worry, when we return to DOS, the entire
heap will be disposed of with no regard to what we have put
on it. The point does need to made that, if you lose a
pointer to a block of the heap, it forever removes that block
of data storage from our use and we may need that storage
later.
Compile and run the program to see if it does what you think
it should do based on this discussion.
THAT WAS A LOT OF DISCUSSION
____________________________________________________________
It took six pages to get through the discussion of the last
program but it was time well spent. It should be somewhat
exciting to you to know that there is nothing else to learn
about dynamic allocation, the last six pages covered it all.
Of course, there is a lot to learn about the technique of
using dynamic allocation, and for that reason, there are two
more example programs to study. But the fact remains, there
is nothing more to learn about dynamic allocation than what
was given so far in this chapter.
AN ARRAY OF POINTERS
____________________________________________________________
Load and display the file BIGDYNL.C for =============
another example of dynamic allocation. This BIGDYNL.C
program is very similar to the last one since =============
we use the same structure, but this time we
define an array of pointers to illustrate the
means by which you could build a large database using an array
of pointers rather than a single pointer to each element. To
keep it simple we define 12 elements in the array and another
working pointer named point.
The *pet[12] is new to you so a few words would be in order.
What we have defined is an array of 12 pointers, the first
being pet[0], and the last pet[11]. Actually, since an array
is itself a pointer, the name pet by itself is a pointer to
Page 12-5
Chapter 12 - Dynamic Allocation
a pointer. This is valid in C, and in fact you can go farther
if needed but you will get quickly confused. I know of no
limit as to how many levels of pointing are possible, so a
definition such as int ****pt; is legal as a pointer to a
pointer to a pointer to a pointer to an integer type variable,
if I counted right. Such usage is discouraged until you gain
considerable experience.
Now that we have 12 pointers which can be used like any other
pointer, it is a simple matter to write a loop to allocate a
data block dynamically for each and to fill the respective
fields with any data desirable. In this case, the fields are
filled with simple data for illustrative purposes, but we
could be reading in a database, readings from some test
equipment, or any other source of data.
A few fields are randomly picked in lines 23 through 25 to
receive other data to illustrate that simple assignments can
be used, and the data is printed out to the monitor. The
pointer point is used in the printout loop only to serve as
an illustration, the data could have been easily printed using
the pet[n] means of definition. Finally, all 12 blocks of
data are freed before terminating the program.
Compile and run this program to aid in understanding this
technique. As stated earlier, there was nothing new here
about dynamic allocation, only about an array of pointers.
A LINKED LIST
____________________________________________________________
We finally come to the grandaddy of all =============
programming techniques as far as being DYNLINK.C
intimidating. Load the program DYNLINK.C for =============
an example of a dynamically allocated linked
list. It sounds terrible, but after a little
time spent with it, you will see that it is simply another
programming technique made up of simple components that can
be a powerful tool.
In order to set your mind at ease, consider the linked list
you used when you were a child. Your sister gave you your
birthday present, and when you opened it, you found a note
that said, "Look in the hall closet." You went to the hall
closet, and found another note that said, "Look behind the TV
set." Behind the TV you found another note that said, "Look
under the coffee pot." You continued this search, and finally
you found your pair of socks under the dogs feeding dish.
What you actually did was to execute a linked list, the
starting point being the wrapped present and the ending point
being under the dogs feeding dish. The list ended at the dogs
feeding dish since there were no more notes.
Page 12-6
Chapter 12 - Dynamic Allocation
In the program DYNLINK.C, we will be doing the same thing your
sister forced you to do. However, we will do it much faster
and we will leave a little pile of data at each of the
intermediate points along the way. We will also have the
capability to return to the beginning and traverse the entire
list again and again if we so desire.
THE DATA DEFINITIONS
____________________________________________________________
This program starts similarly to the last two with the
addition of the definition of a constant to be used later.
The structure is nearly the same as that used in the last two
programs except for the addition of another field within the
structure in line 13, the pointer. This pointer is a pointer
to another structure of this same type and will be used to
point to the next structure in order. To continue the above
analogy, this pointer will point to the next note, which in
turn will contain a pointer to the next note after that.
We define three pointers to this structure for use in the
program, and one integer to be used as a counter, and we are
ready to begin using the defined structure for whatever
purpose we desire. In this case, we will once again generate
nonsense data for illustrative purposes.
THE FIRST FIELD
____________________________________________________________
Using the malloc() function, we request a block of storage on
the heap and fill it with data. The additional field in this
example, the pointer, is assigned the value of NULL, which is
only used to indicate that this is the end of the list. We
will leave the pointer named start pointing at this structure,
so that it will always point to the first structure of the
list. We also assign prior the value of start for reasons we
will see soon. Keep in mind that the end points of a linked
list will always have to be handled differently than those in
the middle of a list. We have a single element of our list
now and it is filled with representative data. Figure 12-5
is the graphical representation of the data space following
execution of line 24.
FILLING ADDITIONAL STRUCTURES
____________________________________________________________
The next group of assignments and control statements are
included within a for loop so we can build our list fast once
it is defined. We will go through the loop a number of times
equal to the constant RECORDS defined at the beginning of our
program. Each time through, we allocate memory, fill the
Page 12-7
Chapter 12 - Dynamic Allocation
first three fields with nonsense, and fill the pointers. The
pointer in the last record is given the address of this new
record because the prior pointer is pointing to the prior
record. Thus prior->next is given the address of the new
record we have just filled. The pointer in the new record is
assigned the value NULL, and the pointer prior is given the
address of this new record because the next time we create a
record, this one will be the prior one at that time. That may
sound confusing but it really does make sense if you spend
some time studying it.
Figure 12-6 illustrates the data space following execution of
the loop two times. The list is growing downward by one
element each time we execute the statements in the loop. When
we have gone through the for loop 6 times, we will have a list
of 7 structures including the one we generated prior to the
loop. The list will have the following characteristics.
1. The pointer named start points to the first structure in
the list.
2. Each structure contains a pointer to the next structure.
3. The last structure has a pointer containing the value
NULL and can be used to detect the end.
It should be clear to you, if you understand the overall data
structure, that it is not possible to simply jump into the
middle of the list and change a few values. The only way to
get to the third structure is by starting at the beginning and
working your way down through the list one record at a time.
Although this may seem like a large price to pay for the
convenience of putting so much data outside of the program
area, it is actually a very good way to store some kinds of
data.
A word processor would be a good application for this type of
data structure because you would never need to have random
access to the data. In actual practice, this is the basic
type of storage used for the text in a word processor with
one line of text per record. Actually, a program with any
degree of sophistication would use a doubly linked list. This
would be a list with two pointers per record, one pointing
down to the next record, and the other pointing up to the
record just prior to the one in question. Using this kind of
a record structure would allow traversing the data in either
direction.
PRINTING THE DATA OUT
____________________________________________________________
To print the data out, a similar method is used as that used
to generate the data. The pointers are initialized and are
Page 12-8
Chapter 12 - Dynamic Allocation
then used to go from record to record reading and displaying
each record one at a time. Printing is terminated when the
NULL in the last record is found, so the program doesn't even
need to know how many records are in the list. Finally, the
entire list is deleted to make room in memory for any
additional data that may be needed, in this case, none. Care
must be taken to assure that the last record is not deleted
before the NULL is checked. Once the data is gone, it is
impossible to know if you are finished yet.
MORE ABOUT DYNAMIC ALLOCATION AND LINKED LISTS
____________________________________________________________
It is not difficult, nor is it trivial, to add elements into
the middle of a linked list. It is necessary to create the
new record, fill it with data, and point its pointer to the
record it is desired to precede. If the new record is to be
installed between the 3rd and 4th, for example, it is
necessary for the new record to point to the 4th record, and
the pointer in the 3rd record must point to the new one.
Adding a new record to the beginning or end of a list are each
special cases. Consider what must be done to add a new record
in a doubly linked list.
Entire books are written describing different types of linked
lists and how to use them, so no further detail will be given.
The amount of detail given should be sufficient for a
beginning understanding of C and its capabilities.
ANOTHER NEW FUNCTION - calloc()
____________________________________________________________
One more function must be mentioned, the calloc() function.
This function allocates a block of memory and clears it to all
zeros which may be useful in some circumstances. It is
similar to malloc and will be left as an exercise for you to
read about and use calloc() if you desire.
PROGRAMMING EXERCISES
____________________________________________________________
1. Rewrite the example program STRUCT1.C from chapter 11 to
dynamically allocate the two structures.
2. Rewrite the example program STRUCT2.C from chapter 11 to
dynamically allocate the 12 structures.
Page 12-9
CHAP13.TXT
Chapter 13
CHARACTER AND BIT MANIPULATION
UPPER AND LOWER CASE
____________________________________________________________
Load and display the program UPLOW.C for an =============
example of a program that does lots of UPLOW.C
character manipulation. More specifically, =============
it changes the case of alphabetic characters.
It illustrates the use of four functions that
have to do with case. It should be no problem for you to
study this program on your own and understand how it works.
The four functions on display in this program are all within
the user written function, mix_up_the_chars(). Compile and
run the program with the file of your choice. The four
functions are;
isupper(c); Is the character upper case?
islower(c); Is the character lower case?
toupper(c); Make the character upper case.
tolower(c); Make the character lower case.
Many more classification and conversion routines should be
listed in the reference material for your compiler.
CLASSIFICATION OF CHARACTERS
____________________________________________________________
Load and display the next program, CHARCLAS.C ==============
for an example of character counting. We CHARCLAS.C
have repeatedly used the backslash n ==============
character representing a new line. These
are called escape sequences, and some of the
more commonly used are defined in the following table;
\n Newline
\t Tab
\b Backspace
\" Double quote
\\ Backslash
\0 NULL (zero)
Consult your compiler documentation for a complete list of
escape sequences available with your compiler.
By preceding each of the above characters with the backslash
character, the character can be included in a line of text for
display, or printing. In the same way that it is perfectly
all right to use the letter n in a line of text as a part of
Page 13-1
Chapter 13 - Character and Bit Manipulation
someone's name, and as an end-of-line, the other characters
can be used as parts of text or for their particular
functions.
This example program uses the functions that can determine the
class of a character, and counts the characters in each class.
The number of each class is displayed along with the line
itself. The three functions are as follows;
isalpha(c); Is the character alphabetic?
isdigit(c); Is the character a numeral?
isspace(c); Is the character any of, \n, \t, or blank?
As noted above, many more classification routines are
available with your compiler.
This program should be simple for you to find your way through
so no explanation will be given. It was necessary to give an
example with these functions used. Compile and run this
program with any file you choose.
THE LOGICAL FUNCTIONS
____________________________________________________________
Load and display the program BITOPS.C. The ==============
functions in this group of functions are used BITOPS.C
to do bitwise operations, meaning that the ==============
operations are performed on the bits as
though they were individual bits. No carry
from bit to bit is performed as would be done with a binary
addition. Even though the operations are performed on a
single bit basis, an entire byte or integer variable can be
operated on in one instruction. The operators and the
operations they perform are given in the following table;
& Logical AND, if both bits are 1, the result is 1.
| Logical OR, if either bit is one, the result is 1.
^ Logical XOR, (exclusive OR), if one and only one bit
is 1, the result is 1.
~ Logical invert, if the bit is 1, the result is 0,
and if the bit is 0, the result is 1.
The example program uses several fields that are combined in
each of the ways given above. The data is in hexadecimal
format. It will be assumed that you already know hexadecimal
format if you need to use these operations. If you don't,
you will need to study it on your own. Teaching the
hexadecimal format of numbers is beyond the scope of this
tutorial. Be sure to compile and execute this program and
observe the output.
Page 13-2
Chapter 13 - Character and Bit Manipulation
THE SHIFT INSTRUCTIONS
____________________________________________________________
The last two operations to be covered in this =============
chapter are the left shift and the right SHIFTER.C
shift instructions. Load the example program =============
SHIFTER.C for an example using these two
instructions. The two operations use the
following operators;
<< n Left shift n places.
>> n Right shift n places.
Once again the operations are carried out and displayed using
the hexadecimal format. The program should be simple for you
to understand on your own, there is no tricky code.
Page 13-3
CHAP14.TXT
Chapter 14
EXAMPLE PROGRAMS
WHY THIS CHAPTER?
____________________________________________________________
Although every program in this tutorial has been a complete
program, each one has also been a very small program intended
to teach you some principle of programming in C. It would do
you a disservice to leave you at that point without
introducing you to a few larger programs to illustrate how to
put together the constructs you have learned to create a major
program. This chapter contains four programs of increasing
complexity, each designed to take you into a higher plateau
of programming, and each designed to be useful to you in some
way.
This course was originally written for use on an IBM-PC or
compatible running the PC-DOS operating system. Some of the
example programs in this chapter will not compile and execute
properly if you are using some other computer or operating
system, but the techniques may still be useful to you. It
would be advantageous to you to spend some time studying these
programs regardless of what computer you are using.
DOSEX will illustrate how to make DOS system calls and will
teach you, through self-study, how the system responds to the
keyboard. WHATNEXT reads commands input on the command line
and will aid you in setting up a variable batch file, one that
requests an operator input and responds to the input by
branching to a different part of the batch file.
LIST is the C source code for a program that operates
similarly to the listing program you used to print out the C
source files when you began studying C with the aid of this
tutorial. Finally we come to VC, the Visual Calculator, which
you should find to be a useful program even if you don't study
its source code. VC uses most of the programming techniques
we have studied in this course and a few that we never even
mentioned such as separately compiled subroutines.
We will take a look at the example programs one at a time but
without a complete explanation of any of them because you have
been studying C for some time now and should be able to read
and understand most of these programs on your own.
DOSEX.C - The DOS Example Program
____________________________________________________________
The copy of DOS that you received with your IBM-PC or
compatible has about 80 internal DOS calls that you can use
Page 14-1
Chapter 14 - Example Programs
as a programmer to control your peripheral devices and read
information or status from them. Some of the earlier IBM DOS
manuals, DOS 2.0 and earlier, have these calls listed in the
back of the manual along with how to use them. Most of the
manuals supplied with compatible computers make no mention of
these calls even though they are extremely useful. These
calls can be accessed from nearly any programming language but
they do require some initial study to learn how to use them.
This program is intended to aid you in this study.
Display the program on your monitor or print =============
it out for reference. It is merely a loop DOSEX.C
watching for a keyboard input or a change in =============
the time. If either happens, it reacts
accordingly. In line 32, the function
kbhit() returns a value of 1 if a key has been hit but not yet
read from the input buffer by the program.
Look at the function named get_time() for an example of a DOS
call. An interrupt 21(hex) is called after setting the AH
register to 2C(hex) = 44(decimal). The time is returned in
the CH, CL, and DH registers. Refer to the DOS call
definitions in your copy of DOS. If the definitions are not
included there, Peter Norton's book, "Programmers Guide to the
IBM PC" is recommended as a good reference manual for these
calls and many other programming techniques. Your compiler
may have a built in function to do this. If you read your
documentation, you will probably find many useful functions
available with your compiler that are included as a
convenience for you by your compiler writer.
Another useful function is the pos_cursor() function that
positions the cursor anywhere on the monitor that you desire
by using a DOS interrupt. In this case, the interrupt used
is 10(hex) which is the general monitor interrupt. This
particular service is number 2 of about 10 different monitor
services available. This function is included here as another
example to you.
The next function, service number 6 of interrupt 10(hex) is
the window scroll service. It should be self explanatory.
In this program, the cursor is positioned and some data is
output to the monitor, then the cursor is hidden by moving it
to line 26 which is not displayed. After you compile and run
the program, you will notice that the cursor is not visible
on the monitor. This is possible in any program, but be sure
to put the cursor in view before returning to DOS because DOS
does not like to have a hidden cursor and may do some strange
things.
Some time spent studying this program will be valuable to you
as it will reveal how the keyboard data is input to the
computer. Especially of importance is how the special keys
Page 14-2
Chapter 14 - Example Programs
such as function keys, arrows, etc. are handled. Also note
that this program uses full prototype checking and is a good
example of how to use it. Since it also uses the modern
method of function definitions, it is a good example of that
also.
WHATNEXT.C - The Batch File Interrogator
____________________________________________________________
This is an example of how to read the data on ==============
the command line following the function call. WHATNEXT.C
Notice that there are two variables listed ==============
within the parentheses following the main()
call. The first variable is a count of words
in the entire command line including the command itself and
the second variable is a pointer to an array of pointers
defining the actual words on the command line.
First the question on the command line, made up of some number
of words, is displayed on the monitor and the program waits
for the operator to hit a key. If the key hit is one of those
in the last word of the group of words on the command line,
the number of the character within the group is returned to
the program where it can be tested with the ERRORLEVEL command
in the batch file. You could use this technique to create a
variable AUTOEXEC.BAT file or any other batch file can use
this for a many way branch. Compile and run this file with
TEST.BAT for an example of how it works in practice. You may
find this technique useful in one of your batch files and you
will almost certainly need to read in the command line
parameters someday.
An interesting alternative would be for you to write a program
named WOULD.C that would return a 1 if a Y or y were typed and
a zero if any other key were hit. Then your batch file could
have a line such as;
WOULD YOU LIKE TO USE THE ALTERNATIVE METHOD (Y/N)
Dos would use WOULD as the program name, ignore the rest of
the statement except for displaying it on the screen. You
would then respond to the question on the monitor with a
single keyhit. Your batch file would then respond to the 1
or 0 returned and either run the alternative part of the batch
file or the primary part whatever each part was.
WOULD YOU LIKE PRIMARY (Y/N)
IF ERRORLEVEL 1 GOTO PRIMARY
(secondary commands)
GOTO DONE
:PRIMARY
(primary commands)
:DONE
Page 14-3
Chapter 14 - Example Programs
LIST.C - The Program Lister
____________________________________________________________
This program is actually composed of two ==============
files, LIST.C and LISTF.C that must be LIST.C
separately compiled and linked together with ==============
your linker. There is nothing new here and
you should have no trouble compiling and
linking this program by reading the documentation supplied
with your C compiler.
The only thing that is new in this program is the inclusion
of three extern variables in the LISTF.C listing. The only
purpose for this is to tie these global variables to the main
program and tell the compiler that these are not new
variables. The compiler will therefore not generate any new
storage space for them but simply use their names during the
compile process. At link time, the linker will get their
actual storage locations from the LIST.OBJ file and use those
locations for the variables in the LISTF part of the memory
map also. The variables of those names in both files are
therefore the same identical variables and can be used just
as any other global variables could be used if both parts of
the program were in one file.
There is no reason why the variables couldn't have been
defined in the LISTF.C part of the program and declared as
extern in the LIST.C part. Some of the variables could have
been defined in one and some in the other. It is merely a
matter of personal taste. Carried to an extreme, all of the
variables could have been defined in a third file and named
extern in both of these files. The third file would then be
compiled and included in the linking process.
It would be to your advantage to compile, link, and run this
program to prepare you for the next program which is composed
of 6 separate files which must all work together.
VC.C - The Visual Calculator
____________________________________________________________
This program finally ties nearly everything ==============
together because it uses nearly every concept VC.C
covered in the entire tutorial. It is so big ==============
that I will not even try to cover the finer
points of its operation. Only a few of the
more important points will be discussed.
The first thing you should do is go through the tutorial for
VC included in the file VC.DOC. There are several dozen steps
for you to execute, with each step illustrating some aspect
Page 14-4
Chapter 14 - Example Programs
of the Visual Calculator. You will get a
good feel for what it is capable of doing and ==============
make your study of the source code very VC.DOC
profitable. In addition, you will probably ==============
find many ways to use the Visual Calculator
to solve problems involving calculations where the simplicity
of the problem at hand does not warrant writing a program.
Notice that the structure definitions, used in all of the
separate parts of the program, are defined in the file
STRUCT.DEF. During program development, when it became
necessary to change one of the structures slightly, it was not
necessary to change it in all of the files, only one file
required modification which was then included in the source
files. Notice that the transcript data is stored in a doubly
linked list with the data itself being stored in a separate
dynamically allocated character string. This line is pointed
to by the pointer lineloc.
For ease of development, the similar functions were grouped
together and compiled separately. Thus, all of the functions
involving the monitor were included in the file named VIDEO.C,
and all of the functions involving the data storage were
grouped into the FILE.C collection. Dividing your program in
a way similar to this should simplify debugging and future
modifications.
Of special interest is the function named monitor(). This
function examines the video mode through use of a DOS command
and if it is a 7, it assumes it is a monochrome monitor,
otherwise it assumes a color monitor. The colors of the
various fields are established at this time and used
throughout the program. Most of the data is written directly
to the video memory, but some is written through the standard
BIOS routines.
The file DEFIN.H is a catalogue of the functions to aid in
finding the functions. This file was generated as one of the
first files and was maintained and updated for use during the
entire design and coding lifetime. It also contains all of
the prototype definitions for the functions in all of the
source files, and is included in every source file to do
prototype checking.
Page 14-5
FILE0577.TXT
Disk No: 577
Disk Title: C Tutor 1 of 2 (578 2nd disk in set) (Disk 1 of 2)
PC-SIG Version: S2.2
Program Title: C Tutor
Author Version: 2.4
Author Registration: $15.00 to $39.95.
Special Requirements: None.
C TUTOR is a comprehensive instructional course for the C programming
language which assumes the user has a moderate amount of programming
experience.
The C source code is included for each of several examples discussed
throughout the tutorial. These examples are meant to be studied and
then compiled and run by the student. The examples are short and focus
on specific topics.
All of the points of C language, including properly-structured
programming techniques, are covered at the elementary level. The
description and instruction are applicable to most compilers with some
slight differences.
PC-SIG
1030D East Duane Avenue
Sunnyvale Ca. 94086
(408) 730-9291
(c) Copyright 1989 PC-SIG, Inc.
GO.TXT
╔═════════════════════════════════════════════════════════════════════════╗
║ <<<< Disk #577 C TUTOR (Disk 1 of 2) >>>> ║
╠═════════════════════════════════════════════════════════════════════════╣
║ To print the Tutorial, Type: PRINTEXT (press enter) ║
╚═════════════════════════════════════════════════════════════════════════╝
INTRO.TXT
Introduction to the C Tutorial
C IS USUALLY FIRST
____________________________________________________________
The programming language C was originally developed by Dennis
Ritchie of Bell Laboratories and was designed to run on a
PDP-11 with a UNIX operating system. Although it was
originally intended to run under UNIX, there has been a great
interest in running it under the MS-DOS operating system on
the IBM PC and compatibles. It is an excellent language for
this environment because of the simplicity of expression, the
compactness of the code, and the wide range of applicability.
Also, due to the simplicity and ease of writing a C compiler,
it is usually the first high level language available on any
new computer, including microcomputers, minicomputers, and
mainframes.
C is not a good beginning language because it is somewhat
cryptic in nature. It allows the programmer a wide range of
operations from high level down to a very low level,
approaching the level of assembly language. There seems to
be no limit to the flexibility available. One experienced C
programmer made the statement, "You can program anything in
C", and the statement is well supported by my own experience
with the language. Along with the resulting freedom however,
you take on a great deal of responsibility because it is very
easy to write a program that destroys itself due to the silly
little errors that a good Pascal compiler will flag and call
a fatal error. In C, you are very much on your own as you
will soon find.
I ASSUME YOU KNOW A LITTLE PROGRAMMING
____________________________________________________________
Since C is not a beginners language, I will assume you are not
a beginning programmer, and I will not attempt to bore you by
defining a constant and a variable. You will be expected to
know these basic concepts. You will, however, be expected to
know nothing of the C programming language. I will begin with
the most basic concepts of C and take you up to the highest
level of C programming including the usually intimidating
concepts of pointers, structures, and dynamic allocation. To
fully understand these concepts, it will take a good bit of
time and work on your part because they not particularly easy
to grasp, but they are very powerful tools. Enough said about
that, you will see their power when we get there, just don't
allow yourself to worry about them yet.
Page I-1
Introduction to the C Tutorial
Programming in C is a tremendous asset in those areas where
you may want to use Assembly Language but would rather keep
it a "simple to write" and "easy to maintain" program. It has
been said that a program written in C will pay a premium of
a 20 to 50% increase in runtime because no high level language
is as compact or as fast as Assembly Language. However, the
time saved in coding can be tremendous, making it the most
desirable language for many programming chores. In addition,
since most programs spend 90 percent of their operating time
in only 10 percent or less of the code, it is possible to
write a program in C, then rewrite a small portion of the code
in Assembly Language and approach the execution speed of the
same program if it were written entirely in Assembly Language.
Even though the C language enjoys a good record when programs
are transported from one implementation to another, there are
differences in compilers as you will find anytime you try to
use another compiler. Most of the differences become apparent
when you use nonstandard extensions such as calls to the DOS
BIOS when using MS-DOS, but even these differences can be
minimized by careful choice of programming means.
Throughout this tutorial, every attempt will be made to
indicate to you what constructs are available in every C
compiler because they are part of the accepted standard of
programming practice.
WHAT IS THE ANSI-C STANDARD?
____________________________________________________________
When it became evident that the C programming language was
becoming a very popular programming language available on a
wide range of computers, a group of concerned individuals met
to propose a standard set of rules for the use of the C
programming language. The group represented all sectors of
the software industry and after many meetings, and many
preliminary drafts, they finally wrote an acceptable standard
for the C language. By the time you read this, it should be
accepted by the American National Standards Institute (ANSI),
and by the International Standards Organization (ISO). It is
not forced upon any group or user, but since it is so widely
accepted, it would be economical suicide for any compiler
writer to refuse to conform to the standard.
YOU MAY NEED A LITTLE HELP
____________________________________________________________
Modern C compilers are very capable systems, but due to the
tremendous versatility of a C compiler, it could be very
difficult for you to learn how to use it effectively. If you
Page I-2
Introduction to the C Tutorial
are a complete novice to programming, you will probably find
the installation instructions somewhat confusing. You may be
able to find a colleague or friend that is knowledgeable about
computers to aid you in setting up your compiler for use.
This tutorial cannot cover all aspects of programming in C,
simply because there is too much to cover, but it will
instruct you in all you need for the majority of your
programming in C. You will receive instruction in all of the
programming constructs in C, but what must be omitted are
methods of programming since these can only be learned by
experience. More importantly, it will teach you the
vocabulary of C so that you can go on to more advanced
techniques using the programming language C. A diligent
effort on your part to study the material presented in this
tutorial will result in a solid base of knowledge of the C
programming language. You will then be able to intelligently
read technical articles or other textbooks on C and greatly
expand your knowledge of this modern and very popular
programming language.
HOW TO USE THIS TUTORIAL
____________________________________________________________
This tutorial is written in such a way that the student should
sit before his computer and study each example program by
displaying it on the monitor and reading the text which
corresponds to that program. Following his study of each
program, he should then compile and execute it and observe the
results of execution with his compiler. This enables the
student to gain experience using his compiler while he is
learning the C programming language. It is strongly
recommended that the student study each example program in the
given sequence then write the programs suggested at the end
of each chapter in order to gain experience in writing C
programs.
THIS IS WRITTEN PRIMARILY FOR MS-DOS
____________________________________________________________
This tutorial is written primarily for use on an IBM-PC or
compatible computer but can be used with any ANSI standard
compiler since it conforms so closely to the ANSI standard.
In fact, a computer is not even required to study this
material since the result of execution of each example program
is given in comments at the end of each program.
Page I-3
Introduction to the C Tutorial
RECOMMENDED READING AND REFERENCE MATERIAL
____________________________________________________________
"The C Programming Language - Second Edition",
Brian W. Kernigan & Dennis M. Ritchie, Prentiss-Hall, 1988
This is the definitive text of the C programming language
and is required reading for every serious C programmer.
Although the first edition was terse and difficult to
read, this edition is easier to read and extremely useful
as both a learning resource and a reference guide.
"C Programming Guide - 3rd Edition",
Jack Purdum, Que Corporation, 1988
This book is written for the beginner in C. It is very
clearly written and makes no assumptions of any prior C
experience.
A SPECIAL NOTE FOR THE SHAREWARE VERSION
____________________________________________________________
It is impossible to include the graphics diagrams in chapters
8 and 12 in a pure ASCII text. They are therefore omitted
from this version of the tutorial. If you need these dia-
grams, they can be purchased directly from Coronado Enter-
prises along with your registration. See the READ.ME file on
either diskette for more information.
Page I-4
TABCONT.TXT
CORONADO ENTERPRISES
C TUTORIAL - Version 2.4
This documentation and the accompanying software, including all
of the example C programs and text files, are protected under
United States copyright law to protect them from unauthorized
commercialization. This version of the tutorial is distributed
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 anything 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 lastest list of available tutorials and a list of
the authorized Public Domain libraries that distribute our full
line of programming language tutorials.
Gordon Dodrill - Nov 10, 1989
Copyright (c) 1988, 1989, Coronado Enterprises
Coronado Enterprises
12501 Coronado Ave NE
Albuquerque, New Mexico 87122
C TUTORIAL - TABLE OF CONTENTS
Introduction Page I-1
Chapter 1 - Getting started Page 1-1
FIRSTEX.C The first example program 1-3
Chapter 2 - Program Structure Page 2-1
TRIVIAL.C The minimum program 2-1
WRTSOME.C Write some output 2-1
WRTMORE.C Write more output 2-2
ONEINT.C One integer variable 2-3
COMMENTS.C Comments in C 2-4
GOODFORM.C Good program style 2-5
UGLYFORM.C Bad program style 2-5
Chapter 3 - Program Control Page 3-1
WHILE.C The While loop 3-1
DOWHILE.C The Do-While loop 3-2
FORLOOP.C The For loop 3-2
IFELSE.C The If & If-Else construct 3-3
BREAKCON.C The Break & Continue 3-4
SWITCH.C The Switch construct 3-4
GOTOEX.C The Goto Statement 3-5
TEMPCONV.C The temperature conversion 3-6
DUMBCONV.C Poor program style 3-7
Chapter 4 - Assignment & Logical Compare Page 4-1
INTASIGN.C Integer assignments 4-1
MORTYPES.C More data types 4-2
LOTTYPES.C Lots of data types 4-4
COMBINE.C Combining different types 4-6
COMPARES.C Logical compares 4-7
CRYPTIC.C The cryptic constructs 4-11
Chapter 5 - Functions, variables, & prototyping Page 5-1
SUMSQRES.C First functions 5-1
SQUARES.C Return a value 5-3
FLOATSQ.C Floating returns 5-4
SCOPE.C Scope of variables 5-5
RECURSON.C Simple Recursion Program 5-9
BACKWARD.C Another Recursion Program 5-11
FLOATSQ2.C Floating returns with prototypes 5-12
Chapter 6 - Defines & Macros Page 6-1
DEFINE.C Defines 6-1
MACRO.C Macros 6-3
ENUM.C Enumerated type 6-3
Chapter 7 - Strings and Arrays Page 7-1
CHRSTRG.C Character Strings 7-1
STRINGS.C More Character strings 7-3
INTARRAY.C Integer Array 7-4
BIGARRAY.C Many Arrays 7-4
PASSBACK.C Getting data from Functions 7-5
MULTIARY.C Multidimensional arrays 7-6
Chapter 8 - Pointers Page 8-1
POINTER.C Simple Pointers 8-1
POINTER2.C More pointers 8-4
TWOWAY.C Twoway Function Data 8-6
FUNCPNT.C A pointer to a function 8-7
Chapter 9 - Standard Input/Output Page 9-1
SIMPLEIO.C Simplest standard I/O 9-1
SINGLEIO.C Single character I/O 9-4
BETTERIN.C Better form of single I/O 9-4
INTIN.C Integer input 9-6
STRINGIN.C String input 9-7
INMEM.C In memory I/O conversion 9-9
SPECIAL.C Standard error output 9-10
Chapter 10 - File Input/Output Page 10-1
FORMOUT.C Formatted output 10-1
CHAROUT.C Single character output 10-3
READCHAR.C Read single characters 10-4
READTEXT.C Read single words 10-5
READGOOD.C Better read and display 10-6
READLINE.C Read a full line 10-6
ANYFILE.C Read in any file 10-6
PRINTDAT.C Output to the printer 10-7
Chapter 11 - Structures Page 11-1
STRUCT1.C Minimum structure example 11-1
STRUCT2.C Array of structures 11-2
STRUCT3.C Structures with pointers 11-3
NESTED.C Nested structure 11-5
UNION1.C An example union 11-7
UNION2.C Another Union example 11-8
BITFIELD.C Bitfield example 11-10
Chapter 12 - Dynamic Allocation Page 12-1
DYNLIST.C Simple Dynamic Allocation 12-1
BIGDYNL.C Large Dynamic Allocation 12-5
DYNLINK.C Dynamic Linked List Program 12-6
Chapter 13 - Character and Bit Manipulation Page 13-1
UPLOW.C Upper/Lower Case Text 13-1
CHARCLAS.C Character Classification 13-1
BITOPS.C Logical Bit Operations 13-2
SHIFTER.C Bit Shifting Operations 13-3
Chapter 14 - Example programs Page 14-1
DOSEX.C DOS call examples 14-2
WHATNEXT.C Ask Question in Batch File 14-3
LIST.C Source Code Lister 14-4
VC.C Visual Calculator 14-5
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
currently employed by a large research and development
laboratory where he continues to study, teach, and apply the
newer programming languages.
Directory of PC-SIG Library Disk #0577
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