C Program to Reverse a Number Using Function

Howdy readers, today you will learn how to write a program to reverse a number using function in the C Programming language.

The below program prompts the user to enter an integer, then this program reverses the entered integer using function, while loop and Modulus (%) operator.

So, without any delay, let’s begin this tutorial.

C Program to Reverse a Number Using Function

C Program

// C Program to Reverse a Number Using Function
#include <stdio.h>
int reverse(int);

int main(){
    int num, rev_num, rem;
    
    // Asking for input
    printf("Enter an integer: ");
    scanf("%d", &num);
    
    // Calling out user-defined function 
    rev_num = reverse(num);
    
    // Displaying result
    printf("Reversed number is: %d", rev_num);
    
    return 0;
}

int reverse(int n){
    int rem, rev = 0;
    
    while (n != 0){
        rem = n % 10;
        rev = rev * 10 + rem;
        n = n / 10;
    }
    return rev;
}

Output

Enter an integer: 789
Reversed number is: 987

Explanation

    int num, rev_num, rem;

We have declared three int data type variables named num, rev_num, and rem.

    // Asking for input
    printf("Enter an integer: ");
    scanf("%d", &num);

The user is asked to enter an integer which he/she wants to reverse. This number gets stored in the num named variable.

    // Calling out user-defined function 
    rev_num = reverse(num);

Then, we called the user-defined function named reverse() in the main function.

int reverse(int n){
    int rem, rev = 0;
    
    while (n != 0){
        rem = n % 10;
        rev = rev * 10 + rem;
        n = n / 10;
    }
    return rev;
}

Suppose, the user enters a number 789, then:

• 1st While Loop Iteration: while (789 != 0), here the condition is True.
  • rem = n % 10 = 789 % 10 = 9
  • rev = rev * 10 + rem = 0 * 10 + 9 = 0 + 9 = 9
  • n = n / 10 = 789 % 10 = 78

• 2nd While Loop Iteration: while (78 != 0), the condition is True.
  • rem = n % 10 = 78 % 10 = 8
  • rev = rev * 10 + rem = 9 * 10 + 8 = 90 + 8 = 98
  • n = n / 10 = 78 % 10 = 7

• 3rd While Loop Iteration: while (7 != 0), the condition is True.
  • rem = n % 10 = 7 % 10 = 7
  • rev = rev * 10 + rem = 98 * 10 + 7 = 980 + 7 = 987
  • n = n / 10 = 7 % 10 = 0

• 4th While Loop Iteration: while (0! = 0), here the condition is False, so the loop terminates and returns 987 which is the reversed number.

    // Displaying result
    printf("Reversed number is: %d", rev_num);

The reverse number is displayed on the screen using printf() function.

Conclusion

I hope after reading this tutorial, you understand how to write a program to reverse a number using a function in the C Programming language.

If you have any queries regarding the program, then let us know in the comment section. We will be pleased to solve all of your queries.