C Program to Check Palindrome String

1. Introduction

A palindrome is a word, phrase, number, or another sequence of characters that reads the same forward as backward (ignoring spaces, punctuation, and capitalization). The word "radar" and the phrase "A man, a plan, a canal, Panama!" are famous examples of palindromes. 

In this tutorial, we will guide you through a C program to determine if a given string is a palindrome without using library functions.

2. Program Overview

The program will:

1. Take a string input from the user.

2. Check if the string is a palindrome.

3. Display the result accordingly.

3. Code Program

#include <stdio.h>  // Include standard I/O library for input-output operations.

int main() {  // Begin main function.

    char str[100], temp[100];  // Define arrays to store the string and its copy.
    int start = 0, end, length = 0;  // Define start and end indices and length variable for the string.

    printf("Enter a string: ");  // Prompt user for input.
    scanf("%s", str);  // Store user input in 'str' array.

    // Find the length of the string.
    for (end = 0; str[end] != '\0'; end++) {
        length++;
    }

    end = length - 1;  // Set 'end' to the last character of the string.

    // Reverse the string into 'temp'.
    for (int i = 0; i < length; i++) {
        temp[i] = str[end];
        end--;
    }

    temp[length] = '\0';  // Append null character to the end of 'temp'.

    // Check if 'str' and 'temp' are the same.
    for (int i = 0; i < length; i++) {
        if (str[i] != temp[i]) {
            printf("The string is not a palindrome.\n");
            return 0;  // Exit the program if the strings are different.
        }
    }

    printf("The string is a palindrome.\n");  // Display the result.

    return 0;  // End the program.

}

Output:

Enter a string: radar
The string is a palindrome.

4. Step By Step Explanation

1. Header Inclusion: The standard input-output library #include <stdio.h> provides us with essential functions like printf and scanf.

2. Main Function: Every C program execution starts from the main() function.

3. Variable Declaration:

- str and temp are character arrays to store the original string and its reversed copy, respectively.

- start, end, and length are integer variables to keep track of positions and string length.

4. Collecting Input: We use printf to prompt the user for input and scanf to read the string into str.

5. Calculating String Length: A for loop calculates the string length, and the end variable points to the last character in the string.

6. Reversal Process:

- Another for loop is used to reverse the string from str into temp.

- The null character is then added at the end of temp to ensure it`s a valid string.

7. Palindrome Check:

- A loop compares each character of str with the corresponding character of temp.

- If theres a mismatch, the program prints that the string isnt a palindrome and exits.

- If the loop completes without finding a mismatch, the program concludes that the string is a palindrome.

8. Displaying Result: The result, whether the string is a palindrome or not, is displayed using printf.

This method, though not the most optimized, offers clarity and helps beginners grasp the fundamental logic behind palindrome checking.