Check if a string is substring of another

Given two strings txt and pat, return the 0-based index of the first occurrence of the substring pat in txt. If pat is not found, return -1.

Examples: 

Input: txt = "GeeksForGeeks", pat = "Fr"
Output: -1
Explanation: "Fr" is not present in the string "GeeksForGeeks" as substring.

Input: txt = "GeeksForGeeks", pat = "For"
Output: 5
Explanation: "For" is present as substring in "GeeksForGeeks" from index 5 (0 based indexing).

Input: txt = "GeeksForGeeks", pat = "gr"
Output: -1
Explanation: "gr" is not present in the string "GeeksForGeeks" as substring.

Nested loops - O(n * m) Time and O(1) Space

The basic idea is to traverse the string txt and check whether pat matches starting from each index. For every index, use a nested loop to compare the characters of pat with the corresponding characters of txt.

• If all characters of pat match, return the starting index.
• If a mismatch occurs, move to the next index of txt.
• Return -1 if pat is not found in txt.

#include 
using namespace std;

// Function to find if pat is a substring of txt
int firstOccurence(string &txt, string &pat)
{
    int n = txt.length();
    int m = pat.length();

    // Iterate through txt
    for (int i = 0; i <= n - m; i++)
    {

        // Check for substring match
        int j;
        for (j = 0; j < m; j++)
        {

            // Mismatch found
            if (txt[i + j] != pat[j])
            {
                break;
            }
        }

        // If we completed the inner loop, we found a match
        if (j == m)
        {

            // Return starting index
            return i;
        }
    }

    // No match found
    return -1;
}

int main()
{
    string txt = "GeeksForGeeks";
    string pat = "For";

    cout << firstOccurence(txt, pat);

    return 0;
}

#include 
#include 

// Function to find if pat is a substring of txt
int firstOccurence(char *txt, char *pat)
{
    int n = strlen(txt);
    int m = strlen(pat);

    // Iterate through txt
    for (int i = 0; i <= n - m; i++)
    {

        // Check for substring match
        int j;
        for (j = 0; j < m; j++)
        {

            // Mismatch found
            if (txt[i + j] != pat[j])
            {
                break;
            }
        }

        // If we completed the inner loop, we found a match
        if (j == m)
        {

            // Return starting index
            return i;
        }
    }

    // No match found
    return -1;
}

int main()
{
    char txt[] = "geeksforgeeks";
    char pat[] = "eks";
    printf("%d", firstOccurence(txt, pat));

    return 0;
}

class GFG {

    // Function to find if pat is a substring of txt
    static int firstOccurence(String txt, String pat)
    {
        int n = txt.length();
        int m = pat.length();

        // Iterate through txt
        for (int i = 0; i <= n - m; i++) {

            // Check for substring match
            int j;
            for (j = 0; j < m; j++) {

                // Mismatch found
                if (txt.charAt(i + j) != pat.charAt(j)) {
                    break;
                }
            }

            // If we completed the inner loop, we found a
            // match
            if (j == m) {

                // Return starting index
                return i;
            }
        }

        // No match found
        return -1;
    }

    public static void main(String[] args)
    {
        String txt = "geeksforgeeks";
        String pat = "eks";
        System.out.println(firstOccurence(txt, pat));
    }
}

def firstOccurence(txt, pat):
    n = len(txt)
    m = len(pat)

    # Iterate through txt
    for i in range(n - m + 1):

        # Check for substring match
        j = 0
        while j < m and txt[i + j] == pat[j]:
            j += 1
        
        # If we completed the inner loop, we found a match
        if j == m:
            return i

    # No match found
    return -1

if __name__ == "__main__":
    txt = "geeksforgeeks"
    pat = "eks"
    print(firstOccurence(txt, pat))

using System;

class GFG {

    // Function to find if pat is a substring of txt
    static int firstOccurence(string txt, string pat)
    {
        int n = txt.Length;
        int m = pat.Length;

        // Iterate through txt
        for (int i = 0; i <= n - m; i++) {

            // Check for substring match
            int j;
            for (j = 0; j < m; j++) {

                // Mismatch found
                if (txt[i + j] != pat[j]) {
                    break;
                }
            }

            // If we completed the inner loop, we found a
            // match
            if (j == m) {

                // Return starting index
                return i;
            }
        }

        // No match found
        return -1;
    }

    static void Main()
    {
        string txt = "geeksforgeeks";
        string pat = "eks";
        Console.WriteLine(firstOccurence(txt, pat));
    }
}

function firstOccurence(txt, pat)
{
    const n = txt.length;
    const m = pat.length;

    // Iterate through txt
    for (let i = 0; i <= n - m; i++) {

        // Check for substring match
        let j;
        for (j = 0; j < m; j++) {

            // Mismatch found
            if (txt[i + j] !== pat[j]) {
                break;
            }
        }

        // If we completed the inner loop, we found a match
        if (j === m) {

            // Return starting index
            return i;
        }
    }

    // No match found
    return -1;
}

// Driver code
const txt = "geeksforgeeks";
const pat = "eks";
console.log(firstOccurence(txt, pat));

5

Pattern Searching Algorithms - O(n + m) Time

There are several pattern searching algorithms, such as:

• KMP Algorithm,
• Rabin-Karp Algorithm,
• Aho-Corasick Algorithm,

Please refer to Introduction to Pattern Searching to learn more about them.

In-built library functions

This approach uses a built-in function to find the first occurrence of pat in txt.

Most programming languages provide built-in functions to find the first occurrence of a substring. The commonly used functions are:

• C: strstr()
• C++: string::find()
• Java: String.indexOf()
• Python: str.find()
• C#: String.IndexOf()
• JavaScript: String.indexOf()

#include 
using namespace std;

int firstOccurence(string &txt, string &pat)
{
    // Find the first occurrence of pat in txt.
    size_t idx = txt.find(pat);

    // Return the index if pat is found, otherwise return -1.
    if (idx != string::npos)
        return idx;

    return -1;
}

int main()
{
    string txt = "GeeksForGeeks";
    string pat = "For";

    cout << firstOccurence(txt, pat);

    return 0;
}

#include 
#include 

int firstOccurence(char *txt, char *pat)
{
    // Find the first occurrence of pat in txt.
    char *idx = strstr(txt, pat);

    // Return the index if pat is found, otherwise return -1.
    if (idx != NULL)
        return (int)(idx - txt);

    return -1;
}

int main()
{
    char txt[] = "GeeksForGeeks";
    char pat[] = "For";

    printf("%d", firstOccurence(txt, pat));

    return 0;
}

public class GFG {
    public static int firstOccurence(String txt, String pat)
    {
        // Find the first occurrence of pat in txt.
        int idx = txt.indexOf(pat);

        // Return the index if pat is found, otherwise
        // return -1.
        if (idx != -1)
            return idx;

        return -1;
    }

    public static void main(String[] args)
    {
        String txt = "GeeksForGeeks";
        String pat = "For";

        System.out.println(firstOccurence(txt, pat));
    }
}

def firstOccurence(txt, pat):
    # Find the first occurrence of pat in txt.
    idx = txt.find(pat)

    # Return the index if pat is found, otherwise return -1.
    if idx != -1:
        return idx

    return -1


if __name__ == "__main__":
    txt = "GeeksForGeeks"
    pat = "For"

    print(firstOccurence(txt, pat))

using System;

class GFG {
    static int firstOccurence(string txt, string pat)
    {
        // Find the first occurrence of pat in txt.
        int idx = txt.IndexOf(pat);

        // Return the index if pat is found, otherwise
        // return -1.
        if (idx != -1)
            return idx;

        return -1;
    }

    static void Main()
    {
        string txt = "GeeksForGeeks";
        string pat = "For";

        Console.WriteLine(firstOccurence(txt, pat));
    }
}

function firstOccurence(txt, pat)
{
    // Find the first occurrence of pat in txt.
    let idx = txt.indexOf(pat);

    // Return the index if pat is found, otherwise return
    // -1.
    if (idx !== -1)
        return idx;

    return -1;
}

// Driver Code
let txt = "GeeksForGeeks";
let pat = "For";

console.log(firstOccurence(txt, pat));

5

Note: The time and space complexity of in-built functions may vary across different languages and implementations.