Example: Palindrome Check using Two Pointers in C

// Palindrome Check using Two Pointers
#include <stdio.h>

// Function to manually calculate string length
int calculate_length(char* str) {
    int length = 0;
    while (str[length] != '\0' && str[length] != '\n') { // Account for newline from fgets
        length++;
    }
    return length;
}

// Function to check if a string is a palindrome
int is_palindrome_two_pointers(char* str) {
    int length = calculate_length(str);
    int start = 0;
    int end = length - 1;

    // Remove potential trailing newline if it's there
    if (length > 0 && str[end] == '\n') {
        str[end] = '\0'; // Replace newline with null terminator
        length--;        // Adjust length
        end--;
    }
    
    // If the string is empty or has one character, it's a palindrome
    if (length <= 1) {
        return 1; 
    }

    while (start < end) {
        if (str[start] != str[end]) {
            return 0; // Not a palindrome
        }
        start++;
        end--;
    }
    return 1; // It is a palindrome
}

int main() {
    char input_string[100]; // Buffer to store the input string

    // Step 1: Prompt user for input
    printf("Enter a string: ");
    
    // Step 2: Read string safely using fgets
    // fgets reads up to size-1 characters or until a newline, storing the newline
    if (fgets(input_string, sizeof(input_string), stdin) == NULL) {
        printf("Error reading input.\n");
        return 1;
    }

    // Step 3: Check if it's a palindrome and print result
    if (is_palindrome_two_pointers(input_string)) {
        printf("'%s' is a palindrome.\n", input_string);
    } else {
        printf("'%s' is not a palindrome.\n", input_string);
    }

    return 0;
}