Palindrome Program in C – Check Number (Iterative, Function, Recursive)

A palindrome program in C checks whether a number reads the same forwards and backwards. Numbers like 121, 1331, and 12321 are palindromes; 12345 is not. The standard technique is to reverse the digits and compare with the original.

This page covers three versions: a simple iterative solution, a clean function-based version, and a recursive approach — plus a variant that validates exactly five digits.

Logic — How to Check a Palindrome Number in C

1. Save a copy of the original number.
2. Reverse the number digit by digit: extract the last digit with % 10, shift it into the reversed value, then drop it with / 10.
3. After the loop, compare the reversed number to the original. Equal → palindrome.

Example trace for 12321:

Step 1: digit = 12321 % 10 = 1,  rev = 0*10 + 1 = 1,  num = 1232
Step 2: digit = 1232  % 10 = 2,  rev = 1*10 + 2 = 12, num = 123
Step 3: digit = 123   % 10 = 3,  rev = 12*10+ 3 = 123, num = 12
Step 4: digit = 12    % 10 = 2,  rev = 123*10+2 = 1232,num = 1
Step 5: digit = 1     % 10 = 1,  rev = 1232*10+1=12321,num = 0
Result: rev == original (12321) → Palindrome

Palindrome Program in C — Iterative Version

#include <stdio.h>

int main(void)
{
    int num, original, reversed = 0, digit;

    printf("Enter an integer: ");
    scanf("%d", &num);

    original = num;

    while (num > 0) {
        digit    = num % 10;
        reversed = reversed * 10 + digit;
        num      = num / 10;
    }

    if (original == reversed)
        printf("%d is a palindrome.\n", original);
    else
        printf("%d is not a palindrome.\n", original);

    return 0;
}

How to Compile and Run

gcc -ansi -Wall -o palindrome palindrome.c
./palindrome

Sample Output

Enter an integer: 12321
12321 is a palindrome.

Enter an integer: 12345
12345 is not a palindrome.

Enter an integer: 7
7 is a palindrome.

Function-Based Version — isPalindrome()

Separating the palindrome logic into a function makes the code reusable and easier to test:

#include <stdio.h>

int isPalindrome(int n)
{
    int original = n, reversed = 0, digit;

    while (n > 0) {
        digit    = n % 10;
        reversed = reversed * 10 + digit;
        n        = n / 10;
    }
    return original == reversed;
}

int main(void)
{
    int num;
    printf("Enter an integer: ");
    scanf("%d", &num);

    if (isPalindrome(num))
        printf("%d is a palindrome.\n", num);
    else
        printf("%d is not a palindrome.\n", num);

    return 0;
}

Recursive Palindrome Check in C

The recursive approach uses a helper function that builds the reversed number on the call stack:

#include <stdio.h>

int reverseNum(int n, int rev)
{
    if (n == 0)
        return rev;
    return reverseNum(n / 10, rev * 10 + n % 10);
}

int isPalindromeRecursive(int n)
{
    return n == reverseNum(n, 0);
}

int main(void)
{
    int num;
    printf("Enter an integer: ");
    scanf("%d", &num);

    if (isPalindromeRecursive(num))
        printf("%d is a palindrome.\n", num);
    else
        printf("%d is not a palindrome.\n", num);

    return 0;
}

How the recursion works: reverseNum(12321, 0) calls itself with reverseNum(1232, 1), then reverseNum(123, 12), and so on. Each call appends one digit to rev until n reaches 0, at which point it returns the fully reversed number.

Five-Digit Palindrome Variant

If you need to validate that the input is exactly five digits before checking:

#include <stdio.h>

int main(void)
{
    int num, original, reversed = 0, digit, count = 0;

    printf("Enter a 5-digit number: ");
    scanf("%d", &num);

    original = num;

    while (num > 0) {
        digit    = num % 10;
        reversed = reversed * 10 + digit;
        num      = num / 10;
        count++;
    }

    if (count != 5)
        printf("Please enter exactly a five-digit number.\n");
    else if (original == reversed)
        printf("%d is a palindrome.\n", original);
    else
        printf("%d is not a palindrome.\n", original);

    return 0;
}

Time and Space Complexity

Both visit each digit exactly once. The iterative version uses constant space; the recursive version adds one stack frame per digit.

What This Program Teaches

• The % 10 / / 10 pattern for extracting and consuming digits — a building block for many number-manipulation programs
• Why you must save the original value before the loop modifies it
• How to convert an iterative digit-reversal into a tail-recursive form
• Integer overflow: for large numbers, reversed can overflow int. Using long or checking for overflow is necessary in production code

Related Programs

• C Program to Check Whether a String is a Palindrome
• Anagram Program in C
• C Aptitude Questions and Answers

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Recommended Book

Loops, integer arithmetic, and the modulo operator are all covered in chapter 1 of The C Programming Language by Kernighan & Ritchie. Also on Amazon.com.