Functions

A function is a module or block of program code which deals with a particular task. Making functions is a way of isolating one block of code from other independent blocks of code. Functions serve two purposes. They allow a programmer to say:"this piece of code does a specific job which stands by itself and should not be mixed up with anything else", and they make a block of code reusable since a function can be reused in many different contexts without repeating parts of the program text. In previous modules, main( ) itself is a pre-defined function.

Functions help us to organize a program in a simple way

Each function has a name or identifier by which is used to refer to it in a program. A function can accept a number of parameters or values which pass information from outside, and consists of a number of statements and declarations, enclosed by curly braces { }, which make up the doing part of the object. The declarations and `type of parameter' statements are formalities which will be described in good time.

Some Rules for writing functions:
The name of a function in C can be anything from a single letter to a long word. The name of a function must begin with an alphabetic letter or the underscore _ character but the other characters in the name can be chosen from the following groups:

a .. z       (any letter from a to z)
A .. Z     (any letter from A to Z)
0 .. 9       (any digit from 0 to 9)
_             (the underscore character)

Uses of C functions:

• C functions are used to avoid rewriting same logic/code again and again in a program.
• There is no limit in calling C functions to make use of same functionality wherever required.
• We can call functions any number of times in a program and from any place in a program.
• A large C program can easily be tracked when it is divided into functions.
• The core concept of C functions are, re-usability, dividing a big task into small pieces to achieve the functionality and to improve understandability of very large C programs.

C function declaration, function call and function definition:

There are 3 aspects in each C function. They are,

• Function declaration or prototype  - This informs compiler about the function name, function parameters and  return value’s data type.

                       Syntax: return_type function_name ( argument list );

• Function call – This calls the actual function

                        Syntax: function_name ( arguments list );

• Function definition – This contains all the statements to be executed.

                        Syntax:  return_type function_name ( arguments list )
                                      { Body of function; }

Example:

#include
// function prototype, also called function declaration
float square ( float x );                              
 
// main function, program starts from here
int main( )              
{
 
        float m, n ;
        printf ( "\nEnter some number for finding square \n");
        scanf ( "%f", &m ) ;
        // function call
        n = square ( m ) ;                      
        printf ( "\nSquare of the given number %f is %f",m,n );
 
}
 
float square ( float x )   // function definition
{
        float p ;
        p = x * x ;
        return ( p ) ;
}

Output:
Enter some number for finding square
2
Square of the given number 2.000000 is 4.000000