C Program to Reverse Digits of a Number

  • Write a c program to reverse digits of a number
  • Write a c program to reverse a number.

This program first take an integer as input form user, then reverse it's digits using modulus(%), division(/) and multiplication(*) operator inside loop. After reversing it prints the reversed number. Inside loop, we first get the least significant digit(right most digit) of number using (number % 10) then append it at the end of reverse number. Finally we remove right most digits from number in number = number/10 statement.

C program to reverse digit of a number using loop

Algorithm to reverse digits of a number
  • Get the least significant digit(right most digit) of the number.
    rightDigit = number%10;
  • Append it at the end of reverse number.
    reverse = (reverse * 10) + rightDigit;
  • Remove right most digit from number.
    number = number/10;
  • Repeat this process till number is greater than zero.
This program used while loop, we can similarly use for loop.
 /*
* C Program to reverse the digits of a number
*/
#include <stdio.h>
#include <conio.h>

int main(){
    int number, reverse = 0, rightDigit;
    printf("Enter a number \n");
    scanf("%d", &number);
    while(number != 0){
        rightDigit = number % 10;
        reverse = (reverse * 10) + rightDigit;
        number = number/10;
    }    
    printf("Reversed number : %d\n", reverse);
    
    getch();
    return 0;
}
Program Output
Enter a number : 12345
Reversed number : 54321

C program to reverse digits of a number using recursion

We can use recursion to reverse the digits if a number because it follows recursive sub-problem property.


getReversedNumber(1234) = (4*1000) + getReversedNumber(123)

This program uses a function getReversedNumber which takes a number as input and returns reversed number.


Recursive algorithm to reverse a number
  • N%10 returns the least significant digit of N(354%10 = 4).
  • N/10 return the number after removing least significant digit of N(2345/10 = 234).
  • log10(N) + 1 returns the number of digits in N. log10(N) is logarithm of N with base 10(log10(2311) + = 4).
  • Let getReversedNumber(N) is a function, which returns reverse of N. Then, we can use recursion to reverse the digits if a number using below mention recursive equation.
    • getReversedNumber(N) = (N%10)X pow(10, (log10(N/10) + 1)) + getReversedNumber(N/10)
/*
* C Program to reverse the digits of a number using recursion
*/
#include <stdio.h>
#include <conio.h>
#include <math.h>
 
int getReversedNumber(int number);
int main(){
    int number, reverse = 0;
    printf("Enter a number :");
    scanf("%d", &number);
   
    reverse = getReversedNumber(number);
   
    printf("Reversed number : %d\n", reverse);
    getch();
    return 0;
}
 
/*
 * Function to reverse the digits of a number
 */
int getReversedNumber(int number){
    int lastDigit, numberOfDigits, sign = 1;
   
    if(number < 0){
        number = number * -1;
        sign = -1;
    }
    if(number < 10)
        return number*sign;
       
    lastDigit = number % 10;
    number = number / 10;
    /* log10(number) + 1 gives number of digits in number */
    numberOfDigits = log10(number) + 1;
    /*
     * for Example: getReversedNumber(1234) = ((4*1000) + getReversedNumber(321))*1
     */
    return (lastDigit*pow(10,numberOfDigits) + getReversedNumber(number)) * sign;
}
Program Output
Enter a number : 23148
Reversed number : 84132

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