Types of User-defined Functions in C Programming

In this tutorial, you will learn about different approaches you can take to solve a single problem using functions.
User-defined function with different argument and return type

For better understanding of arguments and return value from the function, user-defined functions can be categorized as:

The 4 programs below checks whether an integer entered by the user is a prime number or not. And, all these programs generate the same output.

Example #1: No arguments passed and no return Value

#include <stdio.h>

void checkPrimeNumber();

int main()
{
    checkPrimeNumber();    // no argument is passed to prime()
    return 0;
}

// return type of the function is void becuase no value is returned from the function
void checkPrimeNumber()
{
    int n, i, flag=0;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    for(i=2; i <= n/2; ++i)
    {
        if(n%i == 0)
        {
            flag = 1;
        }
    }
    if (flag == 1)
        printf("%d is not a prime number.", n);
    else
        printf("%d is a prime number.", n);
}

The empty parentheses in checkPrimeNumber(); statement inside the main() function indicates that no argument is passed to the function.

The return type of the function is void. Hence, no value is returned from the function.

The checkPrimeNumber() function takes input from the user, checks whether it is a prime number or not and displays it on the screen.

Example #2: No arguments passed but a return value

#include <stdio.h>
int getInteger();

int main()
{
    int n, i, flag = 0;

    // no argument is passed to the function
    // the value returned from the function is assigned to n
    n = getInteger();

    for(i=2; i<=n/2; ++i)
    {
        if(n%i==0){
            flag = 1;
            break;
        }
    }

    if (flag == 1)
        printf("%d is not a prime number.", n);
    else
        printf("%d is a prime number.", n);

    return 0;
}

// getInteger() function returns integer entered by the user
int getInteger()
{
    int n;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    return n;
}

The empty parentheses in n = getInteger(); statement indicates that no argument is passed to the function. And, the value returned from the function is assigned to n.

Here, the getInteger() function takes input from the user and returns it. The code to check whether a number is prime or not is inside the main() function.

Example #3: Argument passed but no return value

#include <stdio.h>
void checkPrimeAndDisplay(int n);

int main()
{
    int n;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    // n is passed to the function
    checkPrimeAndDisplay(n);

    return 0;
}

// void indicates that no value is returned from the function
void checkPrimeAndDisplay(int n)
{
    int i, flag = 0;

    for(i=2; i <= n/2; ++i)
    {
        if(n%i == 0){
            flag = 1;
            break;
        }
    }
    if(flag == 1)
        printf("%d is not a prime number.",n);
    else
        printf("%d is a prime number.", n);
}

The integer value entered by the user is passed to checkPrimeAndDisplay() function.

Here, the checkPrimeAndDisplay() function checks whether the argument passed is a prime number or not and displays the appropriate message.

Example #4: Argument passed and a return value

#include <stdio.h>
int checkPrimeNumber(int n);

int main()
{
    int n, flag;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    // n is passed to the checkPrimeNumber() function
    // the value returned from the function is assigned to flag variable
    flag = checkPrimeNumber(n);

    if(flag==1)
        printf("%d is not a prime number",n);
    else
        printf("%d is a prime number",n);

    return 0;
}

// integer is returned from the function
int checkPrimeNumber(int n)
{
    /* Integer value is returned from function checkPrimeNumber() */
    int i;

    for(i=2; i <= n/2; ++i)
    {
        if(n%i == 0)
            return 1;
    }

    return 0;
}

The input from the user is passed to checkPrimeNumber() function.

The checkPrimeNumber() function checks whether the passed argument is prime or not. If the passed argument is a prime number, the function returns 0. If the passed argument is a non-prime number, the function returns 1. The return value is assigned to flag variable.

Then, the appropriate message is displayed from the main() function.

Which approach is better?

Well, it depends on the problem you are trying to solve.  In case of this problem, the last approach is better.

A function should perform a specific task. The checkPrimeNumber() function doesn't take input from the user nor it displays the appropriate message. It only checks whether a number is prime or not, which makes code modular, easy to understand and debug.