C program to check if a number is palindrome using recursion

A palindrome is a number (or a word, phrase, or sequence) that reads the same backward as forward. For example, the numbers 121, 1331, and 12321 are palindromes. In this article, we will focus on writing a C program to check if a given number is a palindrome using recursion. We will delve into the concept of recursion, outline the algorithm, and provide a complete C program with thorough explanations.

Understanding Palindromes and Recursion

To determine if a number is a palindrome, we need to check if the number reads the same from left to right as it does from right to left. Recursion, a technique where a function calls itself, can be particularly useful for such problems as it allows us to break down the problem into smaller, more manageable parts.

Algorithm to Check Palindrome Using Recursion

1. Base Case:
   • If the number has a single digit or no digits (when divided to 0), it is a palindrome.
2. Recursive Case:
   • Compare the first and last digits of the number.
   • If they are the same, remove these digits and check the remaining part of the number recursively.

Extracting Digits Using Recursion

To check if a number is a palindrome using recursion, we need a way to compare the first and last digits. Here’s the approach:

1. Determine the Number of Digits:
   • Use a helper function to calculate the number of digits.
2. Extract First and Last Digits:
   • The last digit can be obtained using the modulus operator % 10.
   • The first digit can be determined using division by the appropriate power of 10.
3. Recursive Reduction:
   • Remove the first and last digits and check the remaining number.

Write a C program to check if a number is palindrome using recursion

Below is a C program to check if a number is a palindrome using recursion:

#include <stdio.h>
#include <math.h>

// Recursive function to check if a number is palindrome
int isPalindrome(int num, int temp) {
    // Base case: If the number becomes 0
    if (num == 0)
        return temp;

    // Recursive case
    temp = temp * 10 + num % 10; // Reverse the number
    return isPalindrome(num / 10, temp);
}

int main() {
    int number, reverse = 0;

    // Input the number from the user
    printf("Enter a number: ");
    scanf("%d", &number);

    // Check if the number is a palindrome
    if (number == isPalindrome(number, reverse))
        printf("%d is a palindrome.", number);
    else
        printf("%d is not a palindrome.", number);

    return 0;
}

Output

Enter a number: 12321
12321 is a palindrome.

Explanation of the Code

1. Recursive Function isPalindrome:
   • The function isPalindrome takes two arguments: num (the number to check) and temp (a temporary variable to store the reversed number).
   • Base Case: If num is 0, the function returns temp, which contains the reversed number.
   • Recursive Case:
     • temp = temp * 10 + num % 10; updates temp with the last digit of num.
     • return isPalindrome(num / 10, temp); recursively calls isPalindrome with the remaining part of num.
2. Main Function main:
   • The user is prompted to enter a number.
   • The function isPalindrome is called with the initial number and reverse set to 0.
   • If the returned value matches the original number, it is a palindrome; otherwise, it is not.

Example Runs

Example 1

Enter a number: 1221
1221 is a palindrome.

Example 2

Enter a number: 12345
12345 is not a palindrome.