Digit Sum, Reverse and Palindrome Check in C

Algorithm

1. Read a positive integer and copy it into a working variable.
2. Repeatedly take the last digit with % 10, add it to the sum and append it to the reversed number, then divide by 10.
3. Compare the reversed number with the original.
4. Print the digit sum, the reversed number and whether they are equal.

C program

#include <stdio.h>

int main(void) {
    int number, copy, digit, sum = 0, reversed = 0;

    printf("Enter a positive integer:\n");
    if (scanf("%d", &number) != 1 || number < 1) {
        printf("Error: enter a positive integer\n");
        return 1;
    }

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

    printf("Sum of digits: %d\n", sum);
    printf("Reversed: %d\n", reversed);
    if (reversed == number) {
        printf("%d is a palindrome\n", number);
    } else {
        printf("%d is not a palindrome\n", number);
    }
    return 0;
}

Sample input and output

121

Enter a positive integer:
Sum of digits: 4
Reversed: 121
121 is a palindrome

1234

Enter a positive integer:
Sum of digits: 10
Reversed: 4321
1234 is not a palindrome

How the program works

The loop peels off the last digit with the remainder operator and removes it with integer division by 10. Building reversed as reversed × 10 + digit appends each digit to the right end of the new number.

A number is a palindrome when it reads the same backwards, so comparing the reversed value with the original answers the question directly. Trailing zeros lose their place in a reversal, e.g. 120 reverses to 21, which is a known limitation of this method.

Viva questions

Common mistakes

• Comparing the loop variable with the original after the loop has already consumed it.
• Expecting a number with trailing zeros to keep them after reversal; 120 reverses to 21.