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Information about “TURBO C TUTORIAL 1 OF 2 (ALSO 817)”
If you are a programmer in C or other programming languages and have
been thinking about moving up to Turbo C, TURBO C TUTORIAL can help
open the door.
TURBO C TUTORIAL will teach you the basic structure of the C
language. Turbo C is not really a beginner's language and the author
assumes you are familiar with some other programming languages but know
nothing about C. The tutorial starts by explaining the C language and
by the time you have completed the lessons you should be very familiar
with Turbo C.
Included are over 60 C source code files, which are to be used
with the tutorial.
CHAP1.TXT
Chapter 1 - Getting Started
WHAT IS AN IDENTIFIER?
Before you can do anything in any language, you must at
least know how you 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 Turbo C, 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 is permitted by the Turbo
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 is not the same as using
"index" and neither of them is the same as using
"InDeX" for a variable. All three refer to different
variables.
2. As Turbo C is defined, up to 32 significant characters
can be used and will be considered significant. If
more than 32 are used, they will be ignored by the
compiler. You can reduce the number used to anything
less than 32 if you desire as a compiler option. You
should not do this for the duration of your study of
this tutorial as you could get some odd compiler
diagnostics.
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. It adds greatly to the
readability of a program to use descriptive names for
variables and it would be to your 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.
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
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Chapter 1 - Getting Started
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.
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 Turbo 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.
At this point, you should load and run FIRSTEX.C if you
have not yet done so, to see that the Turbo C compiler is
properly loaded and operating. If you have any problems,
see the COMPILER.DOC file for help in properly setting up
your computer to compile and run Turbo C programs.
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. Type LIST FIRSTEX.C
now for an example. The C source code is given later in
Chapter 14 along with a brief description of its operation.
After you have completed your study of Turbo C, you will
have the ability to read and understand the source code for
this program.
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Chapter 1 - Getting Started
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 PRINTALL and watch as your printer fills about
150 sheets of paper with C programs.
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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. 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
contains the requirement for a pointer to 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 the first line
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 type.
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, that is good because
we 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 cases available in Turbo
C to allow more flexible I/O. These are defined on page 95
of the Turbo C Reference Guide. 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
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
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Chapter 10 - File Input/Output
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.
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 "type" it. 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.
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Chapter 10 - File Input/Output
OUTPUTTING A SINGLE CHARACTER AT A TIME
Load the next example file, CHAROUT.C, and display it
on your monitor. This program 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. We have
called the file pointer "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 in 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 "others" is
detected to be a zero.
THE "putc" FUNCTION
The part of the program we are interested in is the
"putc" function. 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. 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, "type" 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
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Chapter 10 - File Input/Output
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 to read 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 it. 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, 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 Turbo 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 in Turbo C. 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 an "char" type variable
for use in returning an EOF.
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
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Chapter 10 - File Input/Output
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 time.
This program is nearly identical as 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 "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.
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 referred to when we looked at READCHAR.C, but I
chose not to expound on it there because 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 program is very similar to
those we have been studying except that we read a complete
line.
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,
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Chapter 10 - File Input/Output
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 "c" will be assigned the
value of NULL. NULL is defined as zero in your "stdio.h"
file.
When we find that "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 file ANYFILE.C for an example of
reading from any file. This 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.
HOW DO WE PRINT?
Load the last example file in this chapter, the one
named PRINTDAT.C for an 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. Turbo C uses the reserved "filename" of PRN that
instructs the compiler to send the output to the printer.
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.
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Chapter 10 - File Input/Output
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.
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CHAP11.TXT
Chapter 11 - Structures and Unions
WHAT IS A STRUCTURE?
A structure is a user defined data type. You have the
ability to 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. An easy
to understand 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
key word "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
variables listed, namely "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
"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 "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. The only problem we have is defining how to use the
simple variable "age" which is a part of the compound
variable "boy". 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.
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
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Chapter 11 - Structures and Unions
actually a "char" type variable, because it was assigned
that in the structure, so it must be assigned a character of
data. Notice that "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 structure definition as
before but this time we define an array of 12 variables
named "kids". This program therefore 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 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
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Chapter 11 - Structures and Unions
used to accomplish this. It should be self explanatory, so
no additional comments will be given.
A RECENT UPGRADE TO THE C LANGUAGE
Turbo C allows you to copy an entire structure with one
statement. This is a fairly recent addition to the C
language and will be a part of the ANSI standard when it is
published, so you should feel free to use it with your Turbo
C compiler. Line 22 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.
USING POINTERS AND STRUCTURES TOGETHER
Load and display the file named STRUCT3.C for an
example of using pointers with structures. 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. Since "kids" is
a pointer variable that points to the 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
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Chapter 11 - Structures and Unions
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 29 and 30 are two additional examples of
structure assignment for your benefit.
Compile and run this program.
NESTED AND NAMED STRUCTURES
Load and display the file named NESTED.C for an example
of a nested structure. The 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
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Chapter 11 - Structures and Unions
advantage not to define all of the elements at one pass but
rather to use a hierarchical structure of 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 reserved word "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[25]", 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 "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 16 to be variables of the type
"alldat", so that each of these two variables contain 5
fields which can store data.
NOW TO USE SOME OF THE FIELDS
In the next five lines 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 name "age". The teachers status is
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Chapter 11 - Structures and Unions
handled in exactly the same manner as her age, but the last
two fields are assigned strings using the string copy
"strcpy" function 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.
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 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
"STRUCT.DEF".
WHAT ARE UNIONS?
Load the file named UNION1.C for an example of a union.
Simply stated, a union allows you a way to look at the same
data with different types, or to use the same data with
different names. Examine the program on your monitor.
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Chapter 11 - Structures and Unions
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 the Turbo C compiler, 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 problem will come up in a few
of the later files also.
Compile and run this program and observe that the data
is read out as an "int" and as two "char" variables. The
"char" variables are reversed in order because of the way an
"int" variable is stored internally in your computer. Don't
worry about this. 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 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
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Chapter 11 - Structures and Unions
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 definition.
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
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.
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Chapter 11 - Structures and Unions
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 "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 "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.
WHAT IS A BITFIELD?
Load and display the program named BITFIELD.C for an
example of how to define and use a bitfield, a relatively
new addition to the C programming language. In this
program, we have a union made up of a single "int" type
variable in line 5 and the structure defined in lines 6
through 10. 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 the next two.
Compile and run the program and you will see that as
the variable "index" is incremented by 1 each time through
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Chapter 11 - Structures and Unions
the loop, you will see the bitfields of the union counting
due to their respective locations within the "int"
definition.
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 separate 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 effective use of this feature of C.
PROGRAMMING EXERCISES
1. Define a named structure containing a string field 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.
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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 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 load and display 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 first program statement, 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".
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 the Turbo C compiler 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
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Chapter 12 - Dynamic Allocation
special calls to use data outside of a single segment. In
order to keep the program small and efficient, these calls
are not used, and the memory space is limited but still
adequate for most programs.
In this model Turbo C defines two heaps, the first
being called a "heap", and the second being called the "far
heap". The "heap" is an area within the 64K boundary that
can store dynamically allocated date and the "far 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.
The data and variables are put on the "far heap" by
utilizing calls to "farmalloc", "farcalloc", etc. and
removed through use of the function "farfree". Study your
Turbo C Reference Guide for details of how to use these
features.
MORE ABOUT SEGMENTS
Turbo C gives 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. With Turbo C this is only
possible by using the tiny memory model. 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.
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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" is a
new function, new to us at least, that 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 of type "char" by
default. Many times, if not most, you do not want a pointer
to a "char" type variable, but 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.
USING THE DYNAMICALLY ALLOCATED MEMORY BLOCK
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
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Chapter 12 - Dynamic Allocation
the structure. In the next three lines 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.
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.
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 would be very inefficient. This is only
mentioned to point out that there is nothing magic about a
data structure that would allow it to be dynamically
allocated while simple types could not.
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 38. 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 purposely do in a program. It
is only done here for illustration.
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Chapter 12 - Dynamic Allocation
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. 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 nearly four 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 four 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 files 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 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 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
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Chapter 12 - Dynamic Allocation
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 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 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.
In the program DYNLINK.C, we will be doing the same
thing as your sister forced you to do. We will however, do
it much faster and we will leave a little pile of data at
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Chapter 12 - Dynamic Allocation
each of the intermediate points along the way. We will also
have the capability to return to the beginning and
retraverse 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 11, 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 "start" 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.
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 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",
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Chapter 12 - Dynamic Allocation
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.
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. "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 that points to NULL and
can be used to detect the end.
start->struct1 This diagram should aid in
name understanding the structure of
breed the data at this point.
age
point->struct2
name
breed
age
point->struct3
name
breed
age
point-> . . . . struct7
name
breed
age
point->NULL
It should be clear to you, if you understand the above
structure, that it is not possible to simply jump into the
middle of the structure 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 structure
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
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Chapter 12 - Dynamic Allocation
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 then used to go from record to record reading and
displaying each record one at a time. Printing is
terminated when the NULL on 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, and it is not trivial, to add
elements into the middle of a linked lists. 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.
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Chapter 12 - Dynamic Allocation
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.
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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 character manipulation. More
specifically, it changes the case of alphabetic characters
around. 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(); Is the character upper case?
islower(); Is the character lower case?
toupper(); Make the character upper case.
tolower(); Make the character lower case.
Many more Classification and Conversion routines are
listed on pages 10 and 11 of your Turbo C Reference Guide.
CLASSIFICATION OF CHARACTERS
Load and display the next program, CHARCLAS.C for an
example of character counting. We 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)
A complete list of escape sequences available with your
Turbo C compiler are listed on page 201 of your Turbo C
Reference Manual.
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 someone's name, and as an end-of-line, the
other characters can be used as parts of text or for their
particular functions.
The program on your screen uses the functions that can
determine the class of a character, and counts the
characters in each class. The number of each class is
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Chapter 13 - Character and Bit Manipulation
displayed along with the line itself. The three functions
are as follows;
isalpha(); Is the character alphabetic?
isdigit(); Is the character a numeral?
isspace(); Is the character any of, \n, \t,
or blank?
As noted above, many more Classification Routines are
listed on page 10 of your Turbo C Reference Guide.
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 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.
Run the program and observe the output.
THE SHIFT INSTRUCTIONS
The last two operations to be covered in this chapter
are the left shift and the right shift instructions. Load
the example program SHIFTER.C for an example using these two
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Chapter 13 - Character and Bit Manipulation
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.
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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.
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 source code for the 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
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.
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Chapter 14 - Example Programs
Display the program on your monitor or print it out for
reference. It is merely a loop 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 Nortons book, "Programmers
Guide to the IBM PC" is recommended as a good reference
manual for these calls and many other programming
techniques. Note that Turbo C has a function named
"gettime" that does the same thing. You should spend some
time studying the Turbo C Reference Guide to learn of the
availability of such functions.
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 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.
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Chapter 14 - Example Programs
WHATNEXT.C - The Batch File Interrogator
This is an example of how to read the data on the
command line following the function call. 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
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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 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
Turbo C compiler.
A LIST.PRJ file is included on the SOURCE disk of the
tutorial as an aid to you in compiling and linking this
program. Read pages 62 and 63 of the Turbo C Users Guide
for instructions on how to do it.
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 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
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Chapter 14 - Example Programs
several dozen steps for you to execute, with each step
illustrating some aspect 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 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 char 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 "monitor()" function. 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.
The file VC.PRJ is included as an aid for you to
compile and link this program. The Visual Calculator is
supplied to you as VC.EXE already compiled and linked for
you. Feel free, after understanding this code, to modify it
in any way you desire for your own use.
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CHAP2.TXT
Chapter 2 - Getting started in Turbo C
YOUR FIRST C PROGRAM
The best way to get started with C is to actually look
at a program, so load the file named TRIVIAL.C with the
Integrated Environment for display 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 by
hitting Alt-R if in the Integrated Environment, 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 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 Turbo 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, it 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
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.
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Chapter 2 - Getting started in Turbo C
ANOTHER PROGRAM WITH MORE OUTPUT
Load the program WRTMORE.C and display it on your
monitor for an example of more output 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.
Compile and run this program to see if it does what you
expect it to do. 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.
LETS PRINT SOME NUMBERS
Load the file named ONEINT.C and display it on the
monitor for our first example of how 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 the line containing "int index;", which is used
to define an integer variable named "index". The "int" is a
reserved word in C, and can therefore not be used for
anything else. It defines a variable that can have a value
from -32768 to 32767 in Turbo C, and in most other C
compilers for microcomputers. The variable name, "index",
can be any name that follows the rules for an identifier and
Page 8
Chapter 2 - Getting started in Turbo C
is not one of the reserved words for Turbo C. The Turbo C
User's Guide has a list of reserved words on page 199. The
final character on the line, the semi-colon, is the
statement terminator used in C.
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 first one
assigns the value of 13 to "index", and its value is printed
out. (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. 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 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 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
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Chapter 2 - Getting started in Turbo C
statement. Keep in mind however, that it is important 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 COMMENTS.C and observe it on your monitor
for an example of how 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 and continues for four lines. This is perfectly
legal because a comment can continue for as many lines as
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 printed out.
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Chapter 2 - Getting started in Turbo C
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.
The Turbo C compiler will allow you to "nest" comments
which can be very handy if you need to "comment out" a
section of code during debugging. In order to nest
comments, you must make a change in the default by selecting
the "Options" menu, followed by the "Compiler" menu, and
finally the "Source" menu. The default for nested comments
is off, but changing it will not affect any of the files in
this tutorial.
GOOD FORMATTING STYLE
Load the file GOODFORM.C and observe it on your
monitor. It is an example of a well 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 concerning how you format 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 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
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.
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Chapter 2 - Getting started in Turbo C
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.
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CHAP3.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 comment and the program
name, 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 executed. In this case, since the variable
count is incremented by one every time the statements are
executed, it will eventually reach 6, 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 braces but can
stand alone.
Compile and run this program.
THE DO-WHILE LOOP
A variation of the while loop is illustrated in the
program DOWHILE.C, which you should load and display. This
program is nearly identical to the last one except that the
loop begins with the reserved word "do", followed by a
compound statement in braces, then the reserved word
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Chapter 3 - Program Control
"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 this
statement. 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 "i" were not changed within the
loop, the loop would never terminate, and hence the program
would never terminate. Finally, just like for 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. Load and edit the
file named FORLOOP.C for an example of a program with a
"for" loop. The "for" loop consists of the reserved word
"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.
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 executed 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
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Chapter 3 - Program Control
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 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 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 reserved word, the "else" following the
first printf statement. 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
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Chapter 3 - Program Control
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 in the first "for", 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, 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 the program to see if it does what
you expect.
THE SWITCH STATEMENT
Load and display the file SWITCH.C for an example of
the biggest construct yet in the 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 entered followed by the value of the variable,
then a colon, and the statements to be executed.
In this example, if the variable "truck" contains the
value 3 during this pass of the switch statement, the printf
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 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 the "case 8" portion will direct the
execution out the bottom of the switch. The various case
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Chapter 3 - Program Control
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.
Compile and run SWITCH.C to see if it does what you
expect it to after this discussion.
Load and display the file GOTOEX.C for an example of a
file with some "goto" statements 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 are not allowed to jump into any loop, but you are
allowed to jump out of a loop. Also, you are not allowed to
jump out of any function into another. These attempts will
be flagged by your Turbo C compiler as an error if you
attempt any of them.
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.
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. 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 programs
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.
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Chapter 3 - Program Control
FINALLY, A MEANINGFUL PROGRAM
Load the file named TEMPCONV.C for an example of a
useful, even though somewhat limited 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" reserved word,
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.
ANOTHER POOR PROGRAMMING EXAMPLE
Recalling UGLYFORM.C from the last chapter, you saw a
very poorly formatted program. If 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.
Compile and run this program to see that it does
exactly what the last one did.
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Chapter 3 - Program Control
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.
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CHAP4.TXT
Chapter 4 - Assignment & Logical compares
INTEGER ASSIGNMENT STATEMENTS
Load the file INTASIGN.C and display it for an example
of assignment statements. Three 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 "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, and of course "int"
extensions such as "long", "short", etc. 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.
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 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.
This would be a good time for a preliminary definition
of a rule to be followed in C. The data definitions are
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Chapter 4 - Assignment & Logical compares
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, the Turbo C
compiler, and every other C compiler will issue an error.
ADDITIONAL DATA TYPES
Loading and editing MORTYPES.C will illustrate how some
additional data types can 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 values between
-128 to 127, 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 microcomputers using Turbo C.
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" 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, in Turbo C, has an allowable
range of from 3.4E-38 to 3.4E+38, 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
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Chapter 4 - Assignment & Logical compares
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. 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 definitions , 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.
Compile and run this program, then go back to the
Edit/Message screen (by pressing any key) and observe the
message window. Notice that there are several warning
messages telling you that 6 variables were assigned a value
and never used. These warning messages are for your
benefit, so it would pay you to keep a close eye on the
message window when writing and debugging a program.
LOTS OF VARIABLE TYPES
Load the file LOTTYPES.C and display it on your screen.
This file contains nearly every 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 in Turbo C, and a "short int" which has a range
that is identical to that for the "int" variable in Turbo C,
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 in Turbo C.
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
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Chapter 4 - Assignment & Logical compares
the "char" and the "float", which leaves only the "double".
The "double" 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 Turbo C 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. See pages 84
to 86 of the TURBO C version 1.0 User's Guide.
Another diversion is in order at this point. Your
Turbo C compiler, like most other C compilers has no
provisions for floating point math, but 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 totally 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.
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 is given on page 171 to 178 of the Turbo C
Reference Guide.
d decimal notation
o octal notation
x hexadecimal notation
u unsigned notation
c character notation
s string notation
f floating point notation
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.
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Chapter 4 - Assignment & Logical compares
- 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 presently displayed on your monitor, 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 proposed ANSI standard and will 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.
LOGICAL COMPARES
Load and view the file named COMPARES.C for many
examples of compare statements in C. We 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 while 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 "x" is
equal to "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 "x" is greater than "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 "b" less than or
equal to "c", and the last checks for "r" not equal to "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
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Chapter 4 - Assignment & Logical compares
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.
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 C language. A "false" is defined as a
value of zero, and "true" is defined as a non-zero value.
Any integer or char type of variable can be used for the
result of a true/false test, or the result can be an implied
integer or char.
Look at the first compare of the second group of
compare statements. The expression "r != s" will evaluate
as a "true" since "r" was set to 0.0 above, so the result
will be a non-zero value. In Turbo C, it seems to always be
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 "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 "x" was probably a 1, but remember
that the only requirement is that it is nonzero.
The next example 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 therefore assigned
222.
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Chapter 4 - Assignment & Logical compares
The third example, in the second group, compares "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 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". 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".
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 33.
The next compare in this group introduces the "||"
operator which is the "OR". 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
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.
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,
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Chapter 4 - Assignment & Logical compares
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, 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 they could each be compared to zero
and the same type of expression could be used.
Continuing on to the fourth example of the third group
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 an "AND" and 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, "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 "z" will not get set to the
desired value, but for different reasons. In the case of
the first one, 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. The
second statement is much more straightforward because "x"
will always be equal to itself, therefore the inequality
will never be true, and the entire statement will never do a
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Chapter 4 - Assignment & Logical compares
thing, but is wasted effort. The last statement 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 before proceeding onward.
Compile and run this program and once again, return to
the Edit/Message screen to observe the warning messages.
The Turbo C compiler is warning us that we have a "possibly
incorrect assignment in function main" because we have used
an assignment statement within the conditional part of the
"if" statements. Once again, the warnings are for our aid,
and should be carefully attended to. In this case, the
assignments were purposely inserted for instructional
purposes but assignments are not generally used in compare
statements and Turbo C is warning you that you may have
intended to use the double equal sign.
Compile and run this program, then add some printout to
see the results of some of the comparisons.
THE CRYPTIC PART OF C
There are three constructs used in C that make no sense
at all when first encountered 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 below. The first
executable statement 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 the
next statement, the value of "y" is assigned to the variable
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Chapter 4 - Assignment & Logical compares
"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, 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 first statement of the "arithmetic
operator" group of statements simply adds 12 to the value of
the variable "a". The second statement 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 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. The final result is identical to
that of an "if" statement with an "else" clause. This is
illustrated by the second example in this group. The
conditional expression has the added advantage of more
compact code that will compile to fewer machine instructions
in the final program.
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Chapter 4 - Assignment & Logical compares
The final two lines of this example program are given
to illustrate a very compact way to assign the greater of
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.
Compile and run this program and notice that once again
we get a few warning to take heed to. Since we are only
demonstrating how to do these things, they are not actually
a problem but in a real program, these assignments would be
wasted code.
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 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.
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CHAP5.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. 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. It begins 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 "index" in the parentheses of the call to
"square". We have seen that this program therefore 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
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 within 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 "sum" equal to
zero because we will use it to accumulate a sum of squares.
Since the variable "sum" is defined as an integer type
variable prior to the main program, it is available to be
used in any of the following functions. It is called a
"global" variable, and it's scope is the entire program and
all functions. More will be said about the scope of
variables near the end of this chapter. The next statement
outputs a header message to the monitor. Program control
then returns to the main program since there are no
additional statements to execute in this function.
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
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Chapter 5 - Functions, variables, and prototypes
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
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++". You should get used to seeing this, as you
will see it a lot in C programs.
In the call to the function "square", we have an added
feature, namely the variable "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 "square", we find that another variable name is
enclosed in its parentheses, namely 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, the
line 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 "numsq" for use only within the function
itself, (more about that later) and proceed with the
required calculations. We set "numsq" equal to the square
of number, then add numsq to the current total stored in
"sum". Remember that "sum += numsq" is the same as "sum =
sum + numsq" from the last lesson. We print the number and
its square, and return to the main program.
MORE ABOUT PASSING A VALUE TO A FUNCTION
When we passed the value of "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
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Chapter 5 - Functions, variables, and prototypes
called function. We could have modified the variable
"number" in any way we wished in the function "square", and
when we returned to the main program, "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 "ending". This call simply calls
the last function which has no local variables defined. It
prints out a message with the value of "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 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 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 "squ" function
taking along the value of "x" as a variable. Looking ahead
to the function itself we find that the function prefers to
call the variable "in" and it proceeds to square the value
of "in" and call the result "square". Finally, a new kind
of a statement appears, 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 "y" is
then set equal to that value. If "x" were therefore
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Chapter 5 - Functions, variables, and prototypes
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 "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 which the function call is placed in the print
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.
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 return. It begins by defining
a global floating point variable 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 the 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 "sqr" near the center of the
listing and you will see that the function name is preceded
by the name "float". This is an indication to the compiler
that this 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".
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Chapter 5 - Functions, variables, and prototypes
The next function, namely "glsqr", will also return a
"float" type variable, but it uses a global variable for
input. It also 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.
THERE IS A BUG IN THE FIRST VERSION OF TURBO C
When you run this program, using version 1.0 of Turbo
C, you will find that the function named "sqr()" will return
a value of zero. This is because it receives a zero from
the calling program. The program is written correctly but
the compiler has a bug in it. A call to Borland resulted in
a fix by simply using the "modern" method of function
definition rather than the "classic" which has been used to
this point. 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. We will return to
this program and show how to fix it so that it will work in
spite of the bug.
Note that this bug is fixed in TURBO C version 1.5.
SCOPE OF VARIABLES
Load the next program, SCOPE.C, and display it for a
discussion of the scope of variables in a program. Ignore
the 4 statements in lines 2 through 5 of this program for a
few moments, and we will discuss them later.
The first variable defined is a global variable "count"
which is available to any function in the program since it
is defined before any of the functions. In addition, 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 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.
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Chapter 5 - Functions, variables, and prototypes
Return to the main program and you will see the
variable "index" defined as an integer. 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, namely "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 "index" 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. 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 to a function to the next
call, the value of an automatic variable is not preserved
and must therefore be reinitialized.
WHAT ARE STATIC VARIABLES?
An additional variable type must be mentioned at this
point, the "static" variable. By putting the reserved word
"static" in front of a variable declaration within a
function, the variable or variables in that declaration are
static variables and will stay in existence from call to
call of the particular function.
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Chapter 5 - Functions, variables, and prototypes
By putting the same reserved word 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 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 if possible in order to speed up the execution of
the program. Turbo C allows you to use 2 register variables
and will ignore additional requests if you request more than
2.
Turbo C allows you to use register variables for short
int and int types along with their unsigned counterparts. It
also allows you to use register storage for two byte
pointers which we will study in about 3 chapters.
WHERE DO I DEFINE VARIABLES?
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 program. Other
variables used in the function are defined at the beginning
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Chapter 5 - Functions, variables, and prototypes
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 the real thing and when
programming in Turbo C, 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 should be released late in 1987, and will contain
prototyping as part of its recommended standard. Every good
C compiler will have prototyping available, so you should
learn to use it.
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 "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.
You should enable prototype checking with Turbo C at
this point to see how well it works. In the "Option" menu
select "Compiler", then "Errors", followed by "Less common
errors", then turn on "F: Call to function with no
prototype" by hitting the "F" key. The program SCOPE.C will
not produce any warnings, but RECURSON.C which is next will
result in some warning messages.
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.
Compile and run this program and, when you return to
the Integrated Environment, observe that there are no
warning messages.
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Chapter 5 - Functions, variables, and prototypes
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 Turbo C compiler writer for all of
the functions that are included with your compiler. A few
minutes spent studying your Turbo C Reference Guide will
give you an insight in where the prototypes are defined for
each of the functions.
In addition, 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
the Turbo C compiler on your own. The entire Turbo C
Reference Guide is devoted to defining functions. 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 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.
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
"index" is set to 8, and is used as the argument to the
function "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.
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Chapter 5 - Functions, variables, and prototypes
For purposes of understanding you can think of it as
having 8 copies of the function "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 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.
Compile and run this program. If you left the
prototype checking on from the last program you will find
several "warnings" in the message window when you return
to the Integrated Environment. Study these for a few
minutes. One of the suggested exercises at the end of this
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Chapter 5 - Functions, variables, and prototypes
chapter is to modify this program to eliminate the prototype
warnings.
ANOTHER EXAMPLE OF RECURSION
The program named BACKWARD.C is another example of
recursion, so load it and display 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 name looks more like the
corresponding line in Pascal, Modula 2, or Ada.
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
on, and observe the results.
HOW TO WORK AROUND THE TURBO C (v1.0) BUG
Load and display the program named FLOATSQ2.C which is
an exact copy of the program FLOATSQ.C which we considered
earlier with prototyping added. The return of the erroneous
zeros is now repaired and the correct values are returned.
Apparently, when the coders of Turbo C at Borland were
testing this compiler, they used prototyping and never found
this bug. The use of prototyping is a good practice for all
C programmers to get into. Note that if you are using
version 1.5 of TURBO C this file will also compile correctly
so you can use either method of programming. It would be to
your advantage to learn the newer method using prototypes.
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
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Chapter 5 - Functions, variables, and prototypes
earlier in this chapter. They can be included in line 12,
but they are not required to be.
Notice also that the type of the variable "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
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
the Turbo C compiler will allow it.
3. Add prototyping to the program named RECURSON.C to
eliminate the warnings.
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CHAP6.TXT
Chapter 6 - Defines and Macros
DEFINES AND MACROS ARE AIDS TO CLEAR PROGRAMMING
Load and display the file named DEFINE.C for your first
look at some defines and macros. Notice the first four
lines 2 through 5 of the program, each starting with the
word "#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 simply 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 word "START" appears in
a text string or a comment, it will be ignored and
unchanged.
It should be clear to you by now 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 the "START", it would be a completely
different matter. If you wanted to change all of the
"START"s in the program to a new number, it would be simple
to change the one #define, but 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.
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
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Chapter 6 - Defines and Macros
"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 Turbo C, the EOF is defined in the STDIO.H file
on line 44. You can observe this for yourself by listing
this file.
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 4 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 definition. Thus
the first term will replace every "A" in the second
definition and the second term will replace every "B" in the
second definition. When line 13 of the program is reached,
"index" will be substituted for every "A", and "count" will
be substituted for every "B". But this replaceing will not
take place in string literals or comments. 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". The
results are then printed out. 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 program.
Compile and run DEFINE.C.
LETS 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 its use. The second
line 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 =
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Chapter 6 - Defines and Macros
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.
Inspection of line 24 will reveal that we are
evaluating "5*(i) + (i)" which is 6 if "i" is one, and in
the second case "5*((i) + (i))" which is 10 if "i' is one.
The parentheses around the entire expression assure that the
value will be evaluated correctly.
WHAT IS AN ENUMERATION VARIABLE?
Load and display the program named ENUM.C for an
example of how to use the "enum" type variable. Line 4
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 "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 "result" is used just like any
"int" type variable would be used and 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
that 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. The remainder of the program is simple and will be
left to your inspection and understanding.
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.
2. Add some print statements to MACRO.C to see the result
of the erroneous addition macro.
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CHAP7.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 titles and names, and
is esthetically 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 is a series of "char" type data
terminated by a NULL character, which is a zero.
When C is going to use a string of data in some way,
either to compare it with another, 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.
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 and display it on
your monitor. The first thing new is the line that 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 the variable "name". 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.
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
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Chapter 7 - Strings and Arrays
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 simply
print it out with some other string data in the output
statement.
The %s is the output definition to output a string and
the system will output characters starting with the first
one in "name" until it comes to the NULL character, and it
will quit. Notice that in the "printf" statement, only the
variable name "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 next "printf" illustrates that we can output any
single character of the string by using the "%c" and naming
the particular character of "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.
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. That is an incorrect conclusion because
strings are very easy to use as we will see in the next
example program.
Compile and run this program.
SOME STRING SUBROUTINES
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" function, or string copy. It copies
from one string to another until it comes to the NULL
character. Remember that the NULL is actually a "0" 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
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Chapter 7 - Strings and Arrays
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, 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, "Zeke" is copied into "name2" by the second
statement, then the "title" is copied. 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. 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 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
The last four statements have 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" is concatenated to the combination. The
result is printed out with both names in the one 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.
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Chapter 7 - Strings and Arrays
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 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
section. 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]". Next
we have a loop to assign nonsense, but well defined, data to
each of the 12 variables, then print all 12 out. Note
carefully that each element of the array is simply an "int"
type variable capable of storing an integer. The only
difference between the variables "index" and "values[2]",
for example, is in the way that 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 "float" type data.
This program has an extra feature to illustrate how strings
can be initialized. The first line of the program
illustrates to you 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 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.
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 data back from a function
by using an array, and that is true. Load the program
PASSBACK.C for an example of doing that. In this program,
we define an array of 20 variables named "matrix", then
assign some nonsense data to the variables, and print out
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Chapter 7 - Strings and Arrays
the first five. Then we call the function "dosome" taking
along the entire array by putting the name of the array in
the parentheses.
The function "dosome" 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 20 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 to
see if they did indeed get passed here. Then 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 in
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 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
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Chapter 7 - Strings and Arrays
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.
MULTIPLY DIMENSIONED ARRAYS
Load and display the file named MULTIARY.C for an
example of a program with doubly 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. Next "big[2][2]" is assigned the arbitrary value of
5, and this value is used for the subscripts of the next
assignment statement. The third assignment statement 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 so you can check the values to see if they did
get set the way you expected them to.
You should spend enough time to completely understand
arrays since they are used in many C programs in one form or
another. You will find many opportunities to use arrays, so
do not slight the material in this chapter.
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Chapter 7 - Strings and Arrays
PROGRAMMING EXERCISES
1. Write a program with three short strings, about 6
characters each, and use "strcpy" to copy "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]);
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CHAP8.TXT
Chapter 8 - Pointers
WHAT IS A POINTER?
Simply stated, a pointer is an address. Instead of
being a variable, it is a pointer 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 "index". This is no
surprise, we have been doing it for several programs now.
The next statement 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 "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 seven 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
ten of the program can be read as "The stored (starred)
value to which the pointer "pt1" points is assigned the
value 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.
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Chapter 8 - Pointers
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. Line 7 of the
program 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.
Line 10 modifies the value by using the pointer. Since
the pointer "pt1" points to the variable "index", then
putting a star in front of the pointer name refers to the
memory location to which it is pointing. Line 10 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 8, 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. Statement 8 assigns "pt2" the same address as "pt1",
so that now "pt2" also points to the variable "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 identical in meaning. This fact is
illustrated in the first "printf" statement since this
statement uses the three means of identifying the same
variable to print out the same variable three times.
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 next "printf" statement
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.
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Chapter 8 - Pointers
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 declare a
pointer. Refer to the third line 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.
A pointer must be defined to point to some 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.
Compile and run this program and observe that there is
only one variable and the single statement in line 10
changes the one variable which is displayed three times.
THE SECOND PROGRAM WITH POINTERS
In these few pages so far on pointers, we have covered
a lot of territory, but it is 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.
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 first we
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Chapter 8 - Pointers
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 the ninth line 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 defined to point to, as we will see in the next
section of the program.
Moving ahead to line 13, 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".
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. 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.
Since "there" is already a pointer, it can be assigned
the address of the eleventh element of "strg" by the
statement in line 17 of the program. Remember that since
"there" is a true pointer, 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
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Chapter 8 - Pointers
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 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.
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 run POINTER2.C and study the output.
You may recall that back in the lesson on functions we
mentioned that there were two ways to 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 program named TWOWAY.C for an example
of this.
FUNCTION DATA RETURN WITH A POINTER
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 "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
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Chapter 8 - Pointers
variable and a pointer to an integer variable. This turns
out to be very simple. Notice that the parameter
definitions in the function define "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 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
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.
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 data. 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
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Chapter 8 - Pointers
important in the area of input/output which is next on the
agenda.
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 to print out the string backwards by
pointing to the end and using a decrementing pointer.
Page 59
CHAP9.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 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 will notice is the second line of
the file, the #include "stdio.h" line. 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 Turbo C compiler. Each of the header files
has a specific purpose and any or all of them can be
included in any program.
The Turbo 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.
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 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
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Chapter 9 - Standard Input/Output
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 a reference to K & R. This refers to the book
written by Kernigan and Ritchie. You would be advised to
purchase a copy for reference.
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 does.
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 "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 "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. 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
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Chapter 9 - Standard Input/Output
the variable "c". The next function "putchar(c)", uses the
standard output device, the video monitor, and outputs the
character contained in the variable "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. In fact, 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 (OR GETTING IN THE WAY)
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, 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
because the capital X signalled program termination.
Compile and run "SIMPLEIO.C". After running the
program several times and feeling confident that you
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Chapter 9 - Standard Input/Output
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 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 "getch()" is a new function that is a "get
character" function. It differs from "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.
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 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
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
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Chapter 9 - Standard Input/Output
exactly as defined here. In the main program, after
outputting the character, we compare it to CR, and if it is
equal to CR, we also output a linefeed which is the LF. We
could have just left out the two #define statements and used
"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. Lets take a
look at each.
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 "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
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Chapter 9 - Standard Input/Output
in a simple program so its use will be up to you to study
when you need it.
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. 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 files, 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 probably 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 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.
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Chapter 9 - Standard Input/Output
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. 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 a higher level language such as Pascal, Modula-2, etc.
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. 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 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
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Chapter 9 - Standard Input/Output
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".
IN MEMORY I/O
The next operation may seem a little strange at first,
but you will probably see lots of 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 a "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 "line",
because that is the string name we inserted as the first
argument in the "sprintf" function. The spaces after the
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Chapter 9 - Standard Input/Output
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.
STANDARD ERROR OUTPUT
Sometimes it is desirable to redirect the output from
the standard output device to a 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
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Chapter 9 - Standard Input/Output
"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
More information about I/O redirection can be found in
your DOS manual. 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.
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.
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.
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 69
FILES816.TXT
Disk No 816
Program Title: TURBO C TUTORIAL version 1.5 (Disk 1 of 2)
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INTRO.TXT
Introduction to the Turbo C Tutorial
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 and specifically 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.
It 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 the Pascal compiler will
flag and call a fatal error. In C, you are very much on
your own as you will soon find.
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.
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 50 to 100% increase in runtime because no
language is as compact or 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
Page 1
Introduction to the Turbo C Tutorial
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, but even these differences can be
minimized by careful choice of programming means. Turbo C
follows the developing ANSI standard so well that your
programs developed with Turbo C should be very portable if
you should decide to use a different ANSI standard compiler.
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. Most of those constructs will use the
generic name "C", such as in the statement, "writing to a
file is easy in C after you master a few fundamentals." If
a particular construct is a Borland extension to the
language, it will be pointed out as such as an indication to
you that it may not be available on other compilers.
Turbo C is a very capable compiler, but due to the
tremendous versatility of this compiler, it could be very
difficult for you to learn how to use it effectively. If
you are a complete novice to programming, you will probably
find the installation instructions somewhat confusing. The
file on the SOURCE disk named COMPILER.DOC is provided just
for you since it will take you step by step through the
installation procedure and even show you how to compile and
run your first program. You should print out a copy of
COMPILER.DOC at this time and use it as an aid to installing
Turbo C on your system.
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 the more advanced
techniques of using the programming language C.
It would definitely be to your advantage to study the
Turbo C User's Guide at this point to gain experience in how
to edit, compile, and run a C program. You should read
chapter 2 to familiarize yourself with the Integrated
Environment and learn how to use it, and to learn how to use
Page 2
Introduction to the Turbo C Tutorial
the full screen editor which is a part of the Turbo C
Integrated Environment.
Remember to read the file COMPILER.DOC To aid you in
compiling your first program.
Page 3
TABCONT.TXT
CORONADO ENTERPRISES TURBO C TUTOR - Ver 1.50
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 entire tutorial
is distributed under the "Freeware" concept which means that
you are not required to pay for it. You are permitted to
copy the disks in their entirety and pass them on to a
friend or acquaintance. In fact, you are encouraged to do
so. You are permitted to charge a small fee to cover the
mechanical costs of duplication, but the software itself
must be distributed free of charge, and in its entirety.
If you find the tutorial and the accompanying example
programs useful, you may, if you desire, pay a small fee to
the author to help compensate him for his time and expense
in writing it. A payment of $10.00 is suggested as
reasonable and sufficient. If you don't feel the tutorial
was worth this amount, please do not make any payment, but
feel free to send in the questionnaire anyway.
Whether or not you send any payment, feel free to write
to Coronado Enterprises and ask for the latest list of
available tutorials and a list of the known Public Domain
libraries that can supply you with this software for the
price of copying. Please enclose a self addressed stamped
envelope, business size preferred, for a copy of the latest
information. See the accompanying "READ.ME" file on the
disk for more information.
I have no facilities for telephone support of this
tutorial and have no plans to institute such. If you find
any problems, or if you have any suggestions, please write
to me at the address below.
Gordon Dodrill - Feb 1, 1988
Copyright (c) 1987, 1988 Coronado Enterprises
Coronado Enterprises
12501 Coronado Ave NE
Albuquerque, New Mexico 87122
TURBO C TUTORIAL - TABLE OF CONTENTS
Introduction --------------------------------------- Page 1
Chapter 1 - Getting started ----------------------- Page 4
FIRSTEX.C The first example program
Chapter 2 - Program Structure --------------------- Page 7
TRIVIAL.C The minimum program
WRTSOME.C Write some output
WRTMORE.C Write more output
ONEINT.C One integer variable
COMMENTS.C Comments in C
GOODFORM.C Good program style
UGLYFORM.C Bad program style
Chapter 3 - Program Control ----------------------- Page 13
WHILE.C The While loop
DOWHILE.C The Do-While loop
FORLOOP.C The For loop
IFELSE.C The If & If-Else construct
BREAKCON.C The Break & Continue
SWITCH.C The Switch construct
GOTOEX.C The Goto Statement
TEMPCONV.C The temperature conversion
DUMBCONV.C Poor program style
Chapter 4 - Assignment & Logical Compare ---------- Page 20
INTASIGN.C Integer assignments
MORTYPES.C More data types
LOTTYPES.C Lots of data types
COMPARES.C Logical compares
CRYPTIC.C The cryptic constructs
Chapter 5 - Functions, variables, & prototyping --- Page 31
SUMSQRES.C First functions
SQUARES.C Return a value
FLOATSQ.C Floating returns
SCOPE.C Scope of variables
RECURSON.C Simple Recursion Program
BACKWARD.C Another Recursion Program
FLOATSQ2.C Floating returns with prototypes
Chapter 6 - Defines & Macros ---------------------- Page 43
DEFINE.C Defines
MACRO.C Macros
ENUM.C The enum type variable
Chapter 7 - Strings and Arrays -------------------- Page 46
CHRSTRG.C Character Strings
STRINGS.C More Character strings
INTARRAY.C Integer Array
BIGARRAY.C Many Arrays
PASSBACK.C Getting data from Functions
MULTIARY.C Multidimensional arrays
TURBO C TUTORIAL - TABLE OF CONTENTS
Chapter 8 - Pointers ------------------------------ Page 53
POINTER.C Simple Pointers
POINTER2.C More pointers
TWOWAY.C Twoway Function Data
Chapter 9 - Standard Input/Output ----------------- Page 60
SIMPLEIO.C Simplest standard I/O
SINGLEIO.C Single character I/O
BETTERIN.C Better form of single I/O
INTIN.C Integer input
STRINGIN.C String input
INMEM.C In memory I/O conversion
SPECIAL.C Standard error output
Chapter 10 - File Input/Output --------------------- Page 70
FORMOUT.C Formatted output
CHAROUT.C Single character output
READCHAR.C Read single characters
READTEXT.C Read single words
READGOOD.C Better read and display
READLINE.C Read a full line
ANYFILE.C Read in any file
PRINTDAT.C Output to the printer
Chapter 11 - Structures ---------------------------- Page 77
STRUCT1.C Minimum structure example
STRUCT2.C Array of structures
STRUCT3.C Structures with pointers
NESTED.C Nested structure
UNION1.C An example union
UNION2.C Another Union example
BITFIELD.C A bitfield example
Chapter 12 - Dynamic Allocation -------------------- Page 87
DYNLIST.C Simple Dynamic Allocation
BIGDYNL.C Large Dynamic Allocation
DYNLINK.C Dynamic Linked List Program
Chapter 13 - Character and Bit Manipulation -------- Page 97
UPLOW.C Upper/Lower Case Text
CHARCLAS.C Character Classification
BITOPS.C Logical Bit Operations
SHIFTER.C Bit Shifting Operations
Chapter 14 - Example programs --------------------- Page 100
DOSEX.C DOS call examples
WHATNEXT.C Ask Question in Batch File
LIST.C Source Code Lister
VC.C Visual Calculator
Directory of PC-SIG Library Disk #0816
Volume in drive A has no label
Directory of A:\
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