User define function in c programming

• C allows you to define functions according to your need. These functions are known as user-defined functions.
•  For example:  let, you need to create a circle and color it depending upon the radius and color. You can create two functions to solve this problem:

        createCircle() function
       color() function
       Example: User-defined function
       Here is a example to add two integers. 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
     }

Function prototype...

• A function prototype is simply the declaration of a function that specifies function's name, parameters and return type. It doesn't contain function body.
• A function prototype gives information to the compiler that the function may later be used in the program.

Syntax of function prototype

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

      In the above example, int addNumbers(int a, int b); is the function prototype which provides               following information to the compiler:

      name of the function is addNumbers()
      return type of the function is int
      two arguments of type int are passed to the function
      The function prototype 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.
         A function is a group of statements that together perform a task. Every C program has at least              one function, which is main(), and all the most trivial programs can define additional functions.

Defining a Function...

• The general form of a function definition in C programming language is as follows −

           return_type function_name( parameter list ) {
          body of the function
              }

• A function definition in C programming consists of a function header and a function body. Here are all the parts of a function −

Return Type 

• − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.

• Function Name − It is the actual name of the function. The function name and the parameter list together constitute the function signature.

• Parameters − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actualparameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.

• Function Body − The function body contains acollection of statements that define what the function does.

           Example
           Given below is the source code fora function called max(). This function takes two parameters             num1 and num2 and returns the maximum value between the two −

          /* function returning the max between two numbers */
          int max(int num1, int num2) {

         /* local variable declaration */
           int result;

           if (num1 > num2)
           result = num1;
           else
           result = num2;

          return result;
           }

 Function Declarations

• A function declaration says the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

           A function declaration has the following parts −

           return_type function_name( parameter list );
           For the above defined function max(), the function declaration is as follows −

        int max(int num1, int num2);
        Parameter names are not important in function declaration only their type is required, so the              
       following is also a valid declaration −

       int max(int, int);
       Function declaration is required when you define a function inone source file and you call that            function in another file. In such case, you should declare the function at the top of the file calling       the function.

Calling a Function

• When creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.
• When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.
• To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value. For example −

         #include <stdio.h>

          /* function declaration */
          int max(int num1, int num2);

        int main () {

         /* local variable definition */
        int a = 100;
        int b = 200;
        int ret;

        /* calling a function to get max value */
          ret = max(a, b);

        printf( "Max value is : %d\n", ret );

        return 0;
        }

       /* function returning the max between two numbers */
       int max(int num1, int num2) {

      /* local variable declaration */
       int result;

      if (num1 > num2)
       result = num1;
    else
      result = num2;

      return result;
      }
      We have kept max() along with main() and compiled the source code. While running the final             executable, it would produce the following result −

     Max value is : 200
     Function Arguments
     If a function is to use arguments, it must declare variables that accept the values of the arguments.     These variables are called the formal parameters of the function.

    here ,Formal parameters behave like other local variables inside the function and are created upon       entry       into the function and destroyed upon exit.

   While calling a function, there are two ways in which arguments can be passed to a function −

   S.N. Call Type & Description

1. Call by value...

• In This method copies the actual value of an argument into the formal parameter of the function. here In this case, changes made to the parameter inside the function have willl  no effect on the argument.

2. Call by reference...

• In This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.
•  here By default, C uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function.