function
A function is an independent
section
of program code that performs a certain task and has been assigned a name.
By referencing
a function’s name, your program can execute the code in the function.
The
program
also can send information, called arguments, to the function, and the function
can
return information to the main part of the program. The two types of C
functions are
library
functions, which are a part of the C compiler package, and user-defined
functions,
which
you, the programmer, create.
For
example
#include
<stdio.h>
int val1,
val2, val3;
int
product(int x, int y);
int main( void ) // line 8
{
/* Get the first number */
printf("Enter a number between 1 and
100: ");
scanf("%d", &val1); /* Get the second number */
printf("Enter another number between 1
and 100: ");
scanf("%d", &val2);
/* Calculate and display the product */
val3 = product(val1, val2);
printf ("%d times %d = %d\n",
val1, val2, val3);
return 0;
} //
line23
int product(int x, int y)
{
return (x * y);
}
Enter a
number between 1 and 100: 35
Enter
another number between 1 and 100: 23
35 times 23
= 805
The main() Function (Lines 8 Through 23)
The only
component that is required in every executable C program is the main() func-tion. In its simplest form,
the main() function consists of the name
main followed by a
pair of
parentheses containing the word void ((void) ) and a pair of braces ({}).
You can
leave
the word void out and the program will still work with most compilers. The ANSI
standard
states that you should include the word
void so that you know there is nothing
being
sent to the main function.
Within
the braces are statements that make up the main body of the program. Under
nor-mal circumstances, program execution starts at the first statement in
main() and termi-nates at the last statement in
main() .
The #include Directive (Line 2)
The
#include directive instructs the C compiler to add the contents of an include
file
into your program during compilation. An
include file is a separate disk file
that
contains information that can be used by your program or the compiler. Several
of
these
files (sometimes called header files )
are supplied with your compiler. You rarely
need to
modify the information in these files; that’s why they’re kept separate from
your
source
code. Include files should all have an .h extension (for example, stdio.h).
You use
the #include directive to instruct the
compiler to add a specific include file to
your
program during compilation. In Listing 2.1, the #include directive is
interpreted to
mean
“Add the contents of the file stdio.h.” You will almost always include one or
more
include
files in your C programs.
The
Variable Definition (Line 4)
A
variable is a name assigned to a location in memory used to store information.
Your
program uses variables to store various kinds of information during pro-gram
execution. In C, a variable must be defined before it can be used. A variable
defini-tion informs the compiler of the variable’s name and the type of
information the variable
is to
hold. In the sample program, the definition on line 4, int val1, val2, val3;,
defines
three variables—named val1, val2, and
val3—that will each hold an integer
value
The
Function Prototype (Line 6)
A
function prototype provides the C compiler with the name and arguments of
the
functions contained in the program. It appears before the function is used. A
function
prototype is distinct from a function
definition, which contains the actual state-ments that make up the function.
(Function definitions are discussed in more detail later
today.
Program
Statements (Lines 11, 12, 15, 16, 19, 20, 22,
and
28)
The real
work of a C program is done by its statements. C statements display information
on-screen,
read keyboard input, perform mathematical operations, call functions, read
disk
files, and all the other operations that a program needs to perform. Most of
this book
s
devoted to teaching you the various C statements. For now, remember that in your
source
code, C statements are generally written one per line and always end with a
semi-
colon.
The statements in multiply.c are explained briefly in the following sections.
The
printf() Statement
The
printf() statement (lines 11, 15, and 20) is a library function that displays
inform
tion
on-screen. The printf() statement can display a simple text message (as in
lines
and 15)
or a message and the value of one or more program variables (as in line 20).
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