C Program to Reverse a Number Using Recursion

• Write a C program to reverse digits of a number using recursion
• Write a C program to reverse a number using recursion

Reversing the digits of a number means reversing the sequence of digits in a number. After reversing, least significant digit will become most significant digit of number and most significant digit becomes least significant digit and so on.
Let the total number of digits in a number is N. So a digit at i^th position from left will become i^th digit from right or N-i+1^th digit from left.

For Example

Befor reverse : 25346
After reverse : 64352

Algorithm to reverse a number using recursion

• N%10 returns the last digit(least significant) of N. For Example : 354%10 = 4.
• N/10 return the number after removing least significant digit of N(rightmost digit of N). For Example: 2345/10 = 234.
• log10(N) + 1 returns the number of digits in N. log10(N) is logarithm of N with base 10. For Example: log10(2311) + 1 = 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)

For Example

• getReversedNumber(8645) = lastDigitOf(8645) x power(10, numberOfDigits(864)) + getReversedNumber(864)
• getReversedNumber(8645) = lastDigitOf(8645) x 1000 + getReversedNumber(864)
• getReversedNumber(8645) = 5×1000 + getReversedNumber(864)
• getReversedNumber(8645) = 5×1000 + 468
• getReversedNumber(8645) = 5468

C program to reverse the digits of a number recursively

This program first take an integer as input form user, then reverse it’s digits using modulus(%), division(/), multiplication(*) and logarithmic function inside a recursive function. After reversing it prints the reversed number.

/*
* 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\n");
    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
6542
Reversed number : 2456