Functions needed

Why are Functions Needed?

Let us analyse the reasons why segmenting a program into manageable chunks is an important aspect of programming.

* Dividing the program into separate well-defined functions facilitates each function to be written and tested separately. This simplifies the process of getting the total program to work.

* Understanding, coding, and testing multiple separate functions is easier than doing the same for one big function.

* If a big program has to be developed without using any function other than main(), then there will be countless lines in the main() function and maintaining that program will be a difficult task.

* All the libraries in C contain a set of functions that the programmers are free to use in their programs. These functions have been pre-written and pre-tested, so the programmers can use them without worrying about their code details. This speeds up program development, by allowing the programmer to concentrate only on the code that he has to write.

* Like C libraries, programmers can also write their own functions and use them from different points in the main program or any other program that needs its functionalities.

* When a big program is broken into comparatively smaller functions, then different programmers working on that project can divide the workload by writing different functions.

Using Functions

A function can be compared to a black box that takes in inputs, processes it, and then outputs the result. However, we may also have a function that does not take any inputs at all, or a function that does not return any value at all. While using functions, we will be using the following terminologies:

* A function f that uses another function g is known as the calling function, and g is known as the called function.

* The inputs that a function takes are known as arguments.

* When a called function returns some result back to the calling function, it is said to returnthat result.

* The calling function may or may not pass parameters to the called function. If the called function accepts arguments, the calling function will pass parameters, else not.

* Function declaration is a declaration statement that identifies a function’s name, a list of arguments that it accepts, and the type of data it returns.

* Function definition consists of a function header that identifies the function, followed by the body of the function containing the executable code for that function.

Function Declaration

Before using a function, the compiler must know the number of parameters and the type of parameters that the function expects to receive and the data type of value that it will return to the calling program. Placing the function declaration statement prior to its use enables the compiler to make a check on the arguments used while calling that function.

The general format for declaring a function that accepts arguments and returns a value as result can be given as:

return_data_type function_name(data_type variable1, data_type variable2,..);

Here, function_name is a valid name for the function. Naming a function follows the same rules that are followed while naming variables. A function should have a meaningful name that must specify the task that the function will perform.

eturn_data_type specifies the data type of the value that will be returned to the calling function as a result of the processing performed by the called function.

(data_type variable1, data_type variable2, ...) is a list of variables of specified data types.

These variables are passed from the calling function to the called function. They are also known as arguments or parameters that the called function accepts to perform its task.

Note A function having void as its return type cannot return any value. Similarly, a function having void as its parameter list cannot accept any value.

Function Definition

When a function is defined, space is allocated for that function in the memory. A function definition comprises of two parts:

* Function header

* Function body

The syntax of a function definition can be given as:

return_data_type function_name(data_type variable1, data_type variable2,..)

{

.............

statements

.............

return(variable);

}

Note that the number of arguments and the order of arguments in the function header must be the same as that given in the function declaration statement.

While return_data_type function_name(data_type variable1, data_type variable2,...) is known as the function header, the rest of the portion comprising of program statements within the curly brackets { } is the function body which contains the code to perform the specific task.

Note that the function header is same as the function declaration. The only difference between the two is that a function header is not followed by a semi-colon.

Function Call

The function call statement invokes the function. When a function is invoked, the compiler jumps to the called function to execute the statements that are a part of that function. Once the called function is executed, the program control passes back to the calling function. A function call statement has the following syntax:

function_name(variable1, variable2, ...);

The following points are to be noted while calling a function:

*Function name and the number and the type of arguments in the function call must be same as that given in the function declaration and the function header of the function definition.

* Names (and not the types) of variables in function declaration, function call, and header of function definition may vary.

* Arguments may be passed in the form of expressions to the called function. In such a case, arguments are first evaluated and converted to the type of formal parameter and then the body of the function gets executed.

* If the return type of the function is not void, then the value returned by the called function may be assigned to some variable as given below.

variable_name = function_name(variable1, variable2, ...);

Write a program to find whether a number is even or odd using functions.

#include <stdio.h>

int evenodd(int); //FUNCTION DECLARATION

int main()

{

int num, flag;

printf("\n Enter the number : ");

scanf("%d", &num);

flag = evenodd(num); //FUNCTION CALL

if (flag == 1)