Palindrome Program in C

A palindrome is a term that retains the same spelling, regardless of whether it's read backwards or forward. An example of a palindrome is the word "radar", "madam", or "racecar".

In the realm of computer science, checking for palindromes serves as a common programming challenge, testing our understanding of different programming concepts.

In this comprehensive guide, we'll explore how to write a C program to check if a given string is a palindrome. We'll delve into palindrome in C using C's standard library functions as well as manually implementing the logic via loops.

Methods to Check Palindrome Strings

There are several strategies in C program to check if a given string is palindrome or not. These may include a C program to check palindrome without using string functions, by using recursion and more. We will examine each method in detail, presenting the algorithm, code, explanation, and complexity.

1. Using the Standard (simple) Method

This method uses a two-pointer technique. We compare the characters at the beginning and end of the string and move the pointers towards the centre of the string. If, at any point, the characters don't match, the string is not a palindrome.

Algorithm to check whether a string is palindrome or not in c:

1. Initialise a start pointer at the beginning of the string and an end pointer at the end of the string.
2. Compare the characters at the start and end pointers.
3. If the characters match, increment the start pointer and decrement the end pointer. Go back to step 2.
4. If the characters don't match, the string is not a palindrome.
5. If the start pointer is greater than or equal to the end pointer, all characters have been matched, and the string is a palindrome.

Code:

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

int main() {
    char str[100];
    int start, end;

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

    start = 0;
    end = strlen(str) - 2;  // Subtract 2 to avoid '\n' from fgets() and '\0'

    while(start < end) {
        if(str[start] != str[end]) {
            printf("%s is not a Palindrome.\n", str);
            return 0;
        }
        start++;
        end--;
    }

    printf("%s is a Palindrome.\n", str);
    return 0;
}

Complexity Analysis:

This method has a time complexity of O(n), where n is the string’s length. This is because, in the worst-case scenario, we're traversing half of the string. The space complexity is O(1), as we're not using any extra space that scales with the input size.

2. Using Function in C

In this method, we'll encapsulate the logic of checking a palindrome inside a function. This makes our code modular and readable.

Algorithm:

1. Follow the same algorithm as the standard method.
2. Encapsulate the logic in a function that takes a string as input and returns whether the string is a palindrome.

Code:

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

int isPalindrome(char str[]) {
    int start = 0;
    int end = strlen(str) - 2;

    while(start < end) {
        if(str[start] != str[end]) {
            return 0;
        }
        start++;
        end--;
    }

    return 1;
}

int main() {
    char str[100];

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

    if(isPalindrome(str))
        printf("%s is a Palindrome.\n", str);
    else
        printf("%s is not a Palindrome.\n", str);

    return 0;
}

Complexity Analysis:

The time and space complexities are the same as the standard method. The time complexity is O(n), and the space complexity is O(1).

3. Using String Library Function Compare in C

This method leverages the string compare function strcmp() in C's standard library to compare the original string with its reversed string.

Algorithm:

1. Reverse the string.
2. Compare the original string with the reversed string using strcmp(). If they're equal, the string is a palindrome.

Code:

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

int main() {
    char str[100], revStr[100];

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

    strcpy(revStr, str);
    strrev(revStr);

    if(strcmp(str, revStr) == 0)
        printf("%s is a Palindrome.\n", str);
    else
        printf("%s is not a Palindrome.\n", str);

    return 0;
}

Complexity Analysis:

The time complexity is O(n) because the strcpy(), strrev(), and strcmp() functions each take O(n) time. The space complexity is O(1) as no additional space is used that scales with the input size.

4. Using Recursion

This method uses recursion to compare the characters at the beginning and end of the string.

Algorithm:

1. If the string has one or zero characters, it's a palindrome.
2. If the first and last characters of the string are unequal, it's not a palindrome.
3. Recursively check if the substring that excludes the first and last characters is palindrome.

Code:

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

int isPalindrome(char str[], int start, int end) {
    if(start >= end)
        return 1;
    if(str[start] != str[end])
        return 0;
    return isPalindrome(str, start+1, end-1);
}

int main() {
    char str[100];

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

    int len = strlen(str) - 2;

    if(isPalindrome(str, 0, len))
        printf("%s is a Palindrome.\n", str);
    else
        printf("%s is not a Palindrome.\n", str);

    return 0;
}

Complexity Analysis:

The time complexity is O(n) as we're recursively traversing the string until the base condition is met. The space complexity is O(n) because of the stack space required for recursion, which is proportional to the depth of recursion, i.e., the string’s length.

The above methods offer a glimpse into the versatility and power of C programming. Understanding and implementing these approaches solidifies fundamental concepts like loops, string manipulation, recursion, and function utilisation. Detecting palindromes, despite being a relatively simple task, provides a stepping stone to more complex data analysis and pattern recognition tasks.

It's important to note that while the four methods described here are standard approaches, they're by no means the only ones. As one's mastery of C programming deepens, alternative techniques may be discovered or invented. Some methods might leverage data structures, and others could explore the unique properties of palindromes.

With the foundational knowledge of detecting palindromes in C, you can explore more sophisticated problems and algorithms. To take your learning journey further, consider trying out Full Stack Software Development Bootcamp provided by upGrad. With a wide array of programming and data science courses, upGrad offers resources that can help you elevate your programming skills and delve deeper into the field of efficient development.

Conclusion

Implementing a palindrome program in C is an excellent way to solidify an understanding of various fundamental concepts. These include the usage of loops, string manipulation, and recursion. We've taken a comprehensive look at several methods to accomplish this, including using the standard method, creating a custom function, leveraging string library functions, and utilising recursion.

Remember, the best way to learn is by doing. So, why not give these methods a try? And if you're interested in exploring more about programming, upGrad programs can help you practice through a wide range of courses to enhance your skills and knowledge!

FAQs

1. Can a palindrome program in C handle both uppercase and lowercase letters?

Yes, it can. However, C is case-sensitive. You might need to convert all characters to the same case (either lower or upper) before comparison.

2. How to handle spaces in a string when checking for palindromes in C?

Generally, spaces are ignored when checking for palindromes. For sentences or phrases, consider removing spaces before performing the palindrome check.

3. Can special characters affect the palindrome checking in C?

Yes, they can. If the string contains special characters, they will be considered part of the string when checking for a palindrome. To ignore them, remove these characters before performing the palindrome check.

4. Is there a limitation to the size of the string that can be checked for a palindrome in C?

The maximum size of the string depends on how the string is declared in the program. In the examples given in this article, the string size is defined as 100 characters. This size can be modified based on the requirements of your specific application.

5. How many loops are required to check if string is palindrome?

You generally require to use only one loop to evaluate if a string is palindrome or not. However, the number of loops may vary depending on the method of approach you opt for to analyse the string. For instance, the iterative method uses a single loop, but the reverse and comparing method takes two.

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