Check Palindrome of a String

Introduction

A palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward (ignoring spaces, punctuation, and capitalization). Examples include “radar”, “level”, and “madam”. This C program checks if a given string is a palindrome.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

#define MAX_LENGTH 100

int isPalindrome(char str[]) {
    int left = 0;
    int right = strlen(str) - 1;

    while (left < right) {
        // Ignore non-alphanumeric characters
        while (left < right && !isalnum(str[left])) {
            left++;
        }
        while (left < right && !isalnum(str[right])) {
            right--;
        }

        // Compare characters, case insensitive
        if (tolower(str[left]) != tolower(str[right])) {
            return 0; // Not a palindrome
        }
        left++;
        right--;
    }
    return 1; // Is a palindrome
}

int main() {
    char str[MAX_LENGTH];

    printf("Enter a string: ");
    fgets(str, MAX_LENGTH, stdin);
    
    // Remove the newline character if present
    str[strcspn(str, "\n")] = '\0';

    if (isPalindrome(str)) {
        printf("'%s' is a palindrome.\n", str);
    } else {
        printf("'%s' is not a palindrome.\n", str);
    }

    return 0;
}

Explanation

1. Libraries Used:
   • stdio.h: For standard input/output functions.
   • string.h: For string manipulation functions.
   • ctype.h: For character handling functions like tolower() and isalnum().
2. Function isPalindrome:
   • Takes a string as input and initializes two pointers: left at the start and right at the end of the string.
   • It uses a loop to compare characters from both ends, ignoring non-alphanumeric characters and comparing them in a case-insensitive manner.
   • If characters do not match, it returns 0 (not a palindrome); if all characters match, it returns 1 (is a palindrome).
3. Main Function:
   • Prompts the user to enter a string.
   • Uses fgets to read the string, allowing spaces.
   • Removes the newline character that fgets might include.
   • Calls isPalindrome to check and prints the result.

Input/Output

Input Example:

Enter a string: A man, a plan, a canal: Panama

Output Example:

'A man, a plan, a canal: Panama' is a palindrome.

Conclusion

This C program effectively checks if a given string is a palindrome by comparing characters from both ends while ignoring spaces, punctuation, and capitalization. It showcases basic string manipulation and control flow in C. The program can be easily modified to enhance functionality, such as handling more complex input or providing detailed error messages.