User defined Functions

Function Declarations

 Prototype to perform this task, a user-defined function add Numbers () is defined.

#include <stdio.h>

int addNumbers(int a, int b);         // function prototype

int main()

{

    int n1,n2,sum;

    printf("Enters two numbers: ");

    scanf("%d %d",&n1,&n2);

    sum = addNumbers(n1, n2);        // function call

    printf("sum = %d",sum);

    return 0;

}

int addNumbers(int a,int b)         // function definition  

{

    int result;

    result = a+b;

    return result;                  // return statement

}

A function Declarations is simply the declaration of a function that specifies function's name, parameters and return type. It doesn't contain function body.

A function Declarations gives information to the compiler that the function may later be used in the program.

Syntax of function Declarations

Declarations In the above example, 

int addNumbers(int a, int b);

 is the function Declarations which provides following information to the compiler:

1.   name of the function is addNumbers()

2.   return type of the function is int

3.   two arguments of type int are passed to the function

The function Declarations is not needed if the user-defined function is defined before the main() function.

Calling a function

Control of the program is transferred to the user-defined function by calling it.

Syntax of function call

functionName(argument1, argument2, ...);

In the above example, function call is made using addNumbers(n1,n2); statement inside the main().

 

Function definition

Function definition contains the block of code to perform a specific task i.e. in this case, adding two numbers and returning it.

Syntax of function definition

returnType functionName(type1 argument1, type2 argument2, ...)

{

    //body of the function

}

When a function is called, the control of the program is transferred to the function definition. And, the compiler starts executing the codes inside the body of a function.

Passing arguments to a function

In programming, argument refers to the variable passed to the function. In the above example, two variables n1 and n2 are passed during function call.

The parameters a and b accepts the passed arguments in the function definition. These arguments are called formal parameters of the function.