Reverse Number Program in C

The reverse of a number 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 programming language. For example, if number num = 12548, the reverse of number num is 84521.

Algorithm to Reverse an Integer

Input: num
(1) Initialize rev_num = 0
(2) Loop while num > 0
     (a) Multiply rev_num by 10 and add remainder of num
          divide by 10 to rev_num
               rev_num = rev_num*10 + num%10;
     (b) Divide num by 10
(3) Return rev_num

C Program to Reverse an Integer

C

// C program to implement
// the above approach
#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;
}

// Driver code
int main()
{
    int num = 4562;
    printf("Reverse of  is %d", reverseDigits(num));

    getchar();
    return 0;
}

Output

Reverse of no. is 2654

Complexity Analysis

Time Complexity: O(log(n)), where n is the input number. 
Auxiliary Space: O(1)

Explanation

The above program takes an integer num as input. We 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. rev_num is returned, which contains the reversed digits of the original number.

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

Flow of Program To Reverse a Number