strstr() in C/C++

strstr() is a predefined C/C++ string function used to find the first occurrence of one string inside another string. It is declared in the <string.h> (or <cstring>) header file.

• It takes two strings as arguments and searches for the first occurrence of the second string in the first string.
• If the substring is found, it returns a pointer to its first occurrence; otherwise, it returns NULL. The null character ('\0') is not included in the matching process.

#include <iostream>
using namespace std;

int main() {
    char s1[] = "GeeksforGeeks";
    char s2[] = "for";

    if (strstr(s1, s2))
        cout << "String found";
    else
        cout << "String not found";

    return 0;
}

String found

• Time Complexity: O(n × m), where n is the length of s1 and m is the length of s2.
• Auxiliary Space: O(1), as strstr() does not use any significant extra memory.

Syntax

char *strstr (const char *s1, const char *s2);

Parameters:

The strstr() function takes two strings as arguments to perform substring searching.

• s1: The main string in which the search is performed.
• s2: The substring that needs to be searched within s1.

Return Value:

The function returns the position of the first occurrence of the substring in the main string.

• Returns a pointer to the first character of s2 found in s1; otherwise, returns NULL.
• If s2 is an empty string (""), the function returns s1.

The strstr() function is used for locating a substring within a string.

Application: Replace a String with Another

In this example with the help of strstr() function we first search for the occurrence of substring "STL" in s1 and after that replace that word with "Strings".

#include <cstring>
#include <iostream>
using namespace std;
int main()
{
    // Take any two strings
    char s1[] = "Fun with STL";
    char s2[] = "STL";
    char* p;

    // Find first occurrence of s2 in s1
    p = strstr(s1, s2);

    // Prints the result
    if (p) {
        strcpy(p, "Strings");
        cout << s1;
    }
    else {
        cout << "String not found" << endl;
    }

    return 0;
}

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

int main()
{
    // Take any two strings
    char s1[] = "Fun with STL";
    char s2[] = "STL";
    char* p;

    // Find first occurrence of s2 in s1
    p = strstr(s1, s2);

    // Prints the result
    if (p) {
        strcpy(p, "Strings");
        printf("%s", s1);
    }
    else
        printf("String not found\n");

    return 0;
}

Fun with Strings

Time Complexity: O(n + m), where n is the size of s1 and m is the size of s2.
Auxiliary Space: O(m), where m is the size of s2.