C Program to Check Palindrome String

A palindrome is a word, phrase, number, or string that reads the same backward as forward. In C programming, we can write a program to check if a string entered by the user is a palindrome. This tutorial explains how to create a simple program to perform this check using basic string manipulation techniques.

Understanding Palindrome Strings

Examples of palindrome strings include 'radar', 'level', and 'madam'. The main logic to check a palindrome involves comparing characters from the beginning and end of the string. If all corresponding characters match until the middle of the string, the string is a palindrome; otherwise, it is not.

Algorithm to Check Palindrome String

1. Start the program.
2. Declare a string to store user input and other required variables.
3. Prompt the user to enter a string.
4. Read the string using scanf or fgets.
5. Initialize two index variables: one starting from the beginning and one from the end of the string.
6. Compare characters at both indexes.
7. If all characters match, the string is a palindrome; else it is not.
8. Display the result.
9. End the program.

C Program Code

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

int main() {
    char str[100];
    int length, i, flag = 0;

    printf("Enter a string: ");
    scanf("%s", str);

    length = strlen(str);

    for(i = 0; i < length/2; i++) {
        if(str[i] != str[length-i-1]) {
            flag = 1;
            break;
        }
    }

    if(flag == 0)
        printf("The string '%s' is a palindrome.\n", str);
    else
        printf("The string '%s' is not a palindrome.\n", str);

    return 0;
}

Sample Output

Enter a string: radar
The string 'radar' is a palindrome.

Enter a string: hello
The string 'hello' is not a palindrome.

Step-by-Step Explanation

• The program starts by including the stdio.h and string.h header files. string.h is required for the strlen() function to calculate the length of the string.
• A character array 'str' is declared to store the input string, along with integer variables for loop control and a flag.
• The user is prompted to enter a string using printf, and the input is read using scanf.
• The strlen() function is used to determine the length of the string.
• A for loop iterates from the start to the middle of the string, comparing characters from the beginning and the end.
• If any pair of characters do not match, the flag variable is set to 1 and the loop breaks, indicating the string is not a palindrome.
• After the loop, the program checks the flag variable and prints whether the string is a palindrome or not.

Important Notes

• This program checks for case-sensitive palindromes. For example, 'Radar' will not be considered a palindrome. To make it case-insensitive, convert all characters to lowercase using tolower().
• The current program uses scanf(), which stops input at whitespace. To check multi-word phrases, use fgets() instead of scanf().
• The maximum string length is set to 100 characters. Adjust the size according to your needs.

Practice Exercises

• Modify the program to check multi-word palindromes with spaces (e.g., 'nurses run').
• Create a function isPalindrome() that returns 1 if the string is a palindrome and 0 otherwise.
• Write a program to check if a number is a palindrome instead of a string.
• Make the palindrome check case-insensitive and ignore punctuation and spaces.

Conclusion

Checking palindrome strings in C is a great way for beginners to practice string handling, loops, and conditional statements. The logic involves comparing characters from both ends of the string and using a simple flag to determine the result. By modifying and practicing this program, you can explore more advanced string manipulation tasks in C programming.