📌 Find Substring Position Using strstr() in C

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🔎 Find a Substring in a String Using strstr() in C

This tutorial shows how to find the position of a substring within a main string using the strstr() function from <string.h>.

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✅ Problem Statement

Input two strings from the user: the main string and the substring to search for. Print the position of the substring’s first occurrence if found; otherwise, show a message saying it was not found.

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✅ Method 1: Using a Custom Function

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

int findSubstring(char *mainStr, char *subStr) {
    char *pos = strstr(mainStr, subStr);
    if (pos != NULL) {
        return (int)(pos - mainStr);  // zero-based index
    } else {
        return -1; // substring not found
    }
}

int main() {
    char mainStr[200], subStr[100];
    int index;

    printf("Enter the main string: ");
    fgets(mainStr, sizeof(mainStr), stdin);

    printf("Enter the substring to find: ");
    fgets(subStr, sizeof(subStr), stdin);

    // Remove newline characters if present
    size_t lenMain = strlen(mainStr);
    if (lenMain > 0 && mainStr[lenMain - 1] == '\n') {
        mainStr[lenMain - 1] = '\0';
    }
    size_t lenSub = strlen(subStr);
    if (lenSub > 0 && subStr[lenSub - 1] == '\n') {
        subStr[lenSub - 1] = '\0';
    }

    index = findSubstring(mainStr, subStr);

    if (index != -1) {
        printf("Substring found at position: %d\n", index + 1);
    } else {
        printf("Substring not found in the main string.\n");
    }

    return 0;
}

🛠️ Method 2: Without Function (All in main())

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

int main() {
    char mainStr[200], subStr[100];
    char *pos;

    printf("Enter the main string: ");
    fgets(mainStr, sizeof(mainStr), stdin);

    printf("Enter the substring to find: ");
    fgets(subStr, sizeof(subStr), stdin);

    // Remove newline characters if present
    size_t lenMain = strlen(mainStr);
    if (lenMain > 0 && mainStr[lenMain - 1] == '\n') {
        mainStr[lenMain - 1] = '\0';
    }
    size_t lenSub = strlen(subStr);
    if (lenSub > 0 && subStr[lenSub - 1] == '\n') {
        subStr[lenSub - 1] = '\0';
    }

    pos = strstr(mainStr, subStr);

    if (pos != NULL) {
        printf("Substring found at position: %ld\n", pos - mainStr + 1);
    } else {
        printf("Substring not found in the main string.\n");
    }

    return 0;
}

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📌 Summary

The strstr() function returns a pointer to the first occurrence of the substring in the main string or NULL if not found. This example covers both function and inline approaches.

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