Reverse Number Program in C

The reverse of a number of means reversing the order of digits of a number. In this article, we will learn how to reverse the digits of a number in C language.

Example:

Input: 12354
Output: 45321
Explanation: The number 12354 when reversed gives 45321

Input: 623
Output: 326
Explanation: The number 623 when reversed gives 326.

Reverse a Number in C

The simplest method to reverse a string is by extracting its digits one by one using (%) modulo and (/) division operator and form the number by rearrange them in the reverse.

#include <stdio.h>

// Iterative function to
// reverse digits of num
int reverseDigits(int num)
{
    int rev_num = 0;
    while (num > 0) {
        rev_num = rev_num * 10 + num % 10;
        num = num / 10;
    }
    return rev_num;
}

int main()
{
    int num = 4562;
    printf("Given number: %d\n", num);
    printf("Revers of the number: %d",
        reverseDigits(num));

    getchar();
    return 0;
}

Reverse of no. is 2654

Explanation

The above program use a while loop to iterate until the value of num becomes 0. Inside the loop, the last digit of num is extracted using the modulo operator (num % 10). This digit is then added to rev_num after multiplying it by 10, which means the existing digits of rev_num are shifted one place to the left.

The value of num is updated by dividing it by 10, (num = num / 10). This removes the last digit of num in each iteration, and terminates the loop when num becomes 0.

num = 4562 
rev_num = 0
rev_num = rev_num *10 + num%10 = 2 
num = num/10 = 456
rev_num = rev_num *10 + num%10 = 20 + 6 = 26 
num = num/10 = 45
rev_num = rev_num *10 + num%10 = 260 + 5 = 265 
num = num/10 = 4
rev_num = rev_num *10 + num%10 = 2650 + 4 = 2654 
num = num/10 = 0