C Program to check if a string is a palindrome

A palindrome is a word, phrase, number, or other sequence of characters that reads the same forward and backwards. Checking if a string is a palindrome is a common task in programming that helps understand string manipulation and algorithmic thinking.

In this article, we'll explore a C program that checks if a given simple string (without spaces, punctuation, or mixed case) is a palindrome. We'll discuss the logic behind the program and provide the code implementation.

Examples

1. Input: madam
   • Output: The string is a palindrome.
   • Explanation: "madam" reads the same forwards and backwards.
2. Input: hello
   • Output: The string is not a palindrome.
   • Explanation: "hello" does not read the same forwards and backwards.

Logic Behind the Program

To determine if a string is a palindrome:

1. Reverse the String: Generate a reverse of the original string.
2. Compare Strings: Compare the original string with its reversed version.
3. Check for Palindromic Nature: If the strings match, the input is a palindrome; otherwise, it's not.

We will not actually reverse the string, but check if the string will remain the same after reversal or not by traversing.

Write a C Program to check if a string is a palindrome

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

int main() {
    char str[100];
    int length, left, right, flag;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove newline character if present
    str[strcspn(str, "\\n")] = '\\0';

    length = strlen(str);
    left = 0;
    right = length - 1;

    // Check for palindrome
    flag = 1;
    while (left < right) {
        if (str[left] != str[right]) {
            flag = 0;
            break;
        }
        left++;
        right--;
    }

    if (flag) {
        printf("%s is a palindrome.", str);
    } else {
        printf("%s is not a palindrome.", str);
    }

    return 0;
}

Output

Enter a string: madam
madam is a palindrome.

Explanation of the Code

Certainly! Let's break down the provided code snippet that checks if a string is a palindrome:

flag = 1;
while (left < right) {
    if (str[left] != str[right]) {
        flag = 0;
        break;
    }
    left++;
    right--;
}

Explanation

1. Initialization of the flag variable:

   flag = 1;
   

   • Here, flag is initialized to 1. This variable acts as an indicator of whether the string is a palindrome or not.
   • 1 means we assume the string is a palindrome unless proven otherwise.
   • 0 will indicate that the string is not a palindrome.

2. While Loop:

   while (left < right) {
   

   • This loop will continue to run as long as left index is less than right index.
   • left and right are indices used to compare characters from the beginning and end of the string, respectively.

3. Character Comparison:

   if (str[left] != str[right]) {
       flag = 0;
       break;
   }
   

   • Inside the loop, it compares the characters at the left and right indices.
   • If the characters at these positions are not equal (str[left] != str[right]), it sets flag to 0 (indicating the string is not a palindrome) and breaks out of the loop.
   • The break statement stops further execution of the loop as we have already determined that the string is not a palindrome.

4. Increment and Decrement:

   left++;
   right--;
   

   • If the characters at the left and right indices are equal, the left index is incremented by 1 and the right index is decremented by 1.
   • This moves the indices towards the center of the string for the next comparison.

Summary

• The code starts by assuming the string is a palindrome (flag = 1).
• It then compares characters from the outermost edges towards the center.
• If any pair of characters doesn't match, it sets flag to 0 and exits the loop, indicating that the string is not a palindrome.
• If the loop completes without finding any mismatched characters, the string is confirmed to be a palindrome (as flag remains 1).

Example Walk-through

Let's illustrate this with an example string "madam":

1. Initialization:
   • str = "madam"
   • left = 0
   • right = 4 (index of the last character)
   • flag = 1
2. First Iteration:
   • str[left] is 'm' and str[right] is 'm' (both are equal)
   • Increment left to 1
   • Decrement right to 3
3. Second Iteration:
   • str[left] is 'a' and str[right] is 'a' (both are equal)
   • Increment left to 2
   • Decrement right to 2
4. Third Iteration:
   • Now, left is not less than right (left == right), so the loop terminates.
   • Since flag is still 1, the string is confirmed to be a palindrome.

This simple but effective algorithm ensures that we can determine if a string is a palindrome by comparing characters from both ends towards the centre, stopping as soon as we find a mismatch.