🚀 DSA with C – Day 5: Strings in C

🎯 Goal:

• Understand how strings work in C

• Perform common string operations:

  • Reverse a string

  • Check for palindrome

  • Count character frequency

  • Check if two strings are anagrams

---

🧠 What is a String in C?

A string is a character array ending with a null character (\0).

🔧 Declaration

char str[100]; // Character array

📥 Input

scanf("%s", str); // No spaces
fgets(str, sizeof(str), stdin); // With spaces

---

👨‍💻 1. Reverse a String

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

int main() {
    char str[100];
    printf("Enter a string: ");
    scanf("%s", str); // Use fgets if you want spaces

    int len = strlen(str);
    for (int i = 0; i < len / 2; i++) {
        char temp = str[i];
        str[i] = str[len - i - 1];
        str[len - i - 1] = temp;
    }

    printf("Reversed string: %s\n", str);
    return 0;
}

---

👨‍💻 2. Check if a String is a Palindrome

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

int main() {
    char str[100];
    int flag = 1;

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

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

    if (flag)
        printf("Palindrome\n");
    else
        printf("Not a palindrome\n");

    return 0;
}

---

👨‍💻 3. Character Frequency Count

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

int main() {
    char str[100];
    int freq[256] = {0}; // ASCII chars

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

    for (int i = 0; str[i] != '\0'; i++) {
        freq[(int)str[i]]++;
    }

    printf("Character frequencies:\n");
    for (int i = 0; i < 256; i++) {
        if (freq[i] > 0)
            printf("%c: %d\n", i, freq[i]);
    }

    return 0;
}

---

👨‍💻 4. Check if Two Strings Are Anagrams

Two strings are anagrams if they contain the same characters in any order.

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

int isAnagram(char str1[], char str2[]) {
    int count[256] = {0};

    if (strlen(str1) != strlen(str2))
        return 0;

    for (int i = 0; str1[i]; i++) {
        count[(int)str1[i]]++;
        count[(int)str2[i]]--;
    }

    for (int i = 0; i < 256; i++) {
        if (count[i] != 0)
            return 0;
    }

    return 1;
}

int main() {
    char str1[100], str2[100];
    printf("Enter two strings: ");
    scanf("%s %s", str1, str2);

    if (isAnagram(str1, str2))
        printf("Anagram\n");
    else
        printf("Not anagram\n");

    return 0;
}

---

🧪 Practice Problems:

1. Count vowels and consonants in a string.

2. Convert lowercase letters to uppercase.

3. Remove duplicate characters from a string.

4. Check if two strings are rotations of each other (e.g., abc and cab).

---

📘 Homework for Day 5:

• Implement your own versions of:

  • strlen()

  • strcpy()

  • strcmp()

• Try a string problem from LeetCode: Valid Anagram