C program to implement a custom strstr() function to check if one string is a substring of another

🔶 Custom strstr() Function in C

#include <stdio.h>

// Custom strstr function
int my_strstr(char *str1, char *str2) {
    int i, j;
    for (i = 0; str1[i] != '\0'; i++) {
        for (j = 0; str2[j] != '\0'; j++) {
            if (str1[i + j] != str2[j]) {
                break;
            }
        }
        // If full substring matched
        if (str2[j] == '\0') {
            return 1;
        }
    }
    return 0;
}

int main() {
    char str1[100], str2[100];

    printf("Enter string1: ");
    scanf("%s", str1);

    printf("Enter string2: ");
    scanf("%s", str2);

    if (my_strstr(str1, str2)) {
        printf("String2 is a substring of string1\n");
    } else {
        printf("String2 is not a substring of string1\n");
    }

    return 0;
}
  

📘 Explanation:

This C program defines a custom version of the strstr() function to check whether one string is a substring of another.

🔹 The outer loop goes through each character of the main string str1.
🔹 The inner loop checks if all characters of str2 match starting from current index in str1.
🔹 If all characters of str2 match (end of str2 is reached), the function returns 1 (true).
🔹 Otherwise, it continues to the next position.
🔹 If no match is found by the end of the loop, it returns 0 (false).

🔍 Sample Output:

Enter string1: hello
Enter string2: ell
String2 is a substring of string1

Enter string1: openai
Enter string2: bot
String2 is not a substring of string1
    

🏷️ Keywords:

custom strstr in C, substring check program, string functions in C, how to check substring manually in C, C interview string question, nested loop string match