C Program to reverse a given number

In this article, we will learn how to write a C program to reverse a number. For example, if the given number is 1234 then program should return number 4321 as output. We will see two different ways to accomplish this.

Example 1: C Program to reverse a given number using Recursion

In this program, we are using a recursive function reverse_function to reverse a user entered number. The explanation of the program is at the end of the following code.

#include <stdio.h>
int revNum=0, rem;
int reverse_function(int num){
  if(num){
    rem=num%10;
    revNum=revNum*10+rem;
    reverse_function(num/10);
  }
  else
    return revNum;
  return revNum;
}
int main(){
  int num, reverse_number;

  //Take the number as an input from user
  printf("Enter any number:");
  scanf("%d",&num);

  //Calling user defined function to perform reverse
  reverse_number=reverse_function(num);
  printf("The reverse of entered number is :%d",reverse_number);
  return 0;
}

Output:

reverse_function(): In this function, we are dividing the input number by 10 and assigning the remainder in a variable sum, the remainder is the last digit of the number. This function then calls itself for the number/10, the number/10 ensures that the number without last digit is passed to the same function again.

In the next iteration the second last digit becomes the second digit of revNum (because of this logic revNum=revNum*10+rem). This process keeps going on until all digits are removed from original number in reverse order and appended to the variable revNum. At the end of the recursion, this revNum variable contains the reverse of the original number.

Example 2: C Program to reverse a number using While loop

In this program, we are reversing the input number using while loop. The explanation of the while loop is available at the end of the program.

#include <stdio.h>

int main() {
    int num, revNum = 0, remainder;

    // Prompt user to enter an integer number
    printf("Enter an integer: ");

    // store the input number in variable num
    scanf("%d", &num);

    // loop to reverse the entered number
    while (num != 0) {
        // Get the last digit
        remainder = num % 10; 

        // Append it to the revNum
        revNum = revNum * 10 + remainder; 

        // Remove the last digit from the original number
        num /= 10; 
    }

    // print the revNum as output
    printf("Reversed Number: %d\n", revNum);

    return 0;
}

Output:

Enter an integer: 1234
Reversed Number: 4321

How the loop works:

• The num % 10 operation fetches the last digit of num.
• The revNum = revNum * 10 + remainder operation appends the last digit to the reversed number.
• The num /= 10 operation removes the last digit from num.
• The loop continues until num becomes 0.