## Write a C program to reverse digits of a number

**ITERATIVE WAY**

Algorithm:

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

Example:

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 = 265 + 4 = 2654

num = num/10 = 0

**Program:**

#include <stdio.h> /* Iterative function to reverse digits of num*/ int reversDigits(int num) { int rev_num = 0; while(num > 0) { rev_num = rev_num*10 + num%10; num = num/10; } return rev_num; } /*Driver program to test reversDigits*/ int main() { int num = 4562; printf("Reverse of no. is %d", reversDigits(num)); getchar(); return 0; }

**Time Complexity:** O(Log(n)) where n is the input number.

**RECURSIVE WAY**

Thanks to Raj for adding this to the original post.

#include <stdio.h>; /* Recursive function to reverse digits of num*/ int reversDigits(int num) { static int rev_num = 0; static int base_pos = 1; if(num > 0) { reversDigits(num/10); rev_num += (num%10)*base_pos; base_pos *= 10; } return rev_num; } /*Driver program to test reversDigits*/ int main() { int num = 4562; printf("Reverse of no. is %d", reversDigits(num)); getchar(); return 0; }

**Time Complexity:** O(Log(n)) where n is the input number

Note that above above program doesn’t consider leading zeroes. For example, for 100 program will print 1. If you want to print 001 then see this comment from Maheshwar.

Try extensions of above functions that should also work for floating point numbers.

### Related Topics:

- Print squares of first n natural numbers without using *, / and -
- How to efficiently implement k stacks in a single array?
- Connect n ropes with minimum cost
- Data Structure for Dictionary and Spell Checker?
- Count ways to reach the n’th stair
- An Interesting Method to Generate Binary Numbers from 1 to n
- Calculate the angle between hour hand and minute hand
- Dynamic Programming | Set 37 (Boolean Parenthesization Problem)

Writing code in comment?Please useideone.comand share the link here.