Search the pattern in given String

Given two strings, text and pattern, of size N and M (N > M)respectively, the task is to print all occurrences of pattern in text. 

Examples:

Input: text = “This is a dummy text”,  pattern = “This”
Output: Pattern found at indices: 0
Explanation: The pattern “This” starts from index 0 in the given text.

Input: text = “Welcome to Geeks for Geeks”,  pattern = “Geeks”
Output: Pattern found at indices: 21 11
Explanation: The pattern “Geeks” starts from the 11th and 21st index (considering the white spaces).

Approach: The approach for this problem is based on the following idea:

Find the possible starting indices of all the starting points in the text. Then for all those indices check if their adjacents match with the next elements of the pattern.

The above idea can be implemented using the queue. Follow the steps mentioned below to implement the idea.

• First of all, use a 256 sized array of unordered_set. Traverse through the text and insert every index to the set of the respective character.

C++

for (int i = 0; i < st.length(); i++)
    // Insert every index to the hash set using character
    // ASCII.
    structured_text[st[i]].insert(i);

•  Search for the first character of the pattern in the array and push every index contained in the hash set into a queue.

C++

for (int ind : structured_text[pattern[0]])
    q_indices.push(ind);

• Then traverse through pattern from i = 1 to M-1:
  • If the index in structured_text[pattern[i]] is adjacent to any of the characters present in pat[i-1], push it to the queue for next iteration.
  • Otherwise, continue checking for other positions.

C++

for (int i = 1; i < pattern.length(); i++) {
    char ch = pattern[i];
    int q_size = q_indices.size();
    /*  the queue contains the number of occurrences of the
    previous character. traverse the queue for q_size times
    Check the next character of the pattern found or not. */
    while (q_size--) {
        int ind = q_indices.front();
        q_indices.pop();
        if (structured_text[ch].find(ind + 1)
            != structured_text[ch].end())
            q_indices.push(ind + 1);
    }
}

Python3

for i in range(1, pat_len):
        ch = pattern[i]
        q_size = len(q_indices)

        ## The queue contains the
        ## number of occurrences of
        ## the previous character.
        ## Traverse the queue for
        ## q_size times.
        ## Check the next character of
        ## the pattern found or not.
        while q_size > 0:
            q_size -= 1
            ind = q_indices[0]
            q_indices.pop(0)

            if ((ind + 1) in structured_text[ord(ch)]):
                q_indices.append(ind + 1)

                # This code is contributed by akashish_.

• If the whole pattern is found then return those indices.

Below is the implementation for the above approach:

C++

// C++ code for the above approach:

#include <bits/stdc++.h>
using namespace std;

// Using a 256 sized array of
// hash sets.
unordered_set<int> structured_text[256];

// Function to perform the hashing
void StringSearch(string st)
{
    // Structure the text. It will be
    // helpful in pattern searching
    for (int i = 0; i < st.length(); i++)

        // Insert every index to the
        // hash set using character ASCII.
        structured_text[st[i]].insert(i);
}

// Function to search the pattern
void pattern_search(string st, string pattern)
{
    StringSearch(st);

    // Queue contain the indices
    queue<int> q_indices;

    for (int ind : structured_text[pattern[0]])
        q_indices.push(ind);

    // Pattern length
    int pat_len = pattern.length();
    for (int i = 1; i < pat_len; i++) {
        char ch = pattern[i];
        int q_size = q_indices.size();

        // The queue contains the
        // number of occurrences of
        // the previous character.
        // Traverse the queue for
        // q_size times.
        // Check the next character of
        // the pattern found or not.
        while (q_size--) {
            int ind = q_indices.front();
            q_indices.pop();

            if (structured_text[ch].find(ind + 1)
                != structured_text[ch].end())
                q_indices.push(ind + 1);
        }
    }
    cout << "Pattern found at indexes:";
    while (!q_indices.empty()) {

        // last_ind is the last index
        // of the pattern in the text
        int last_ind = q_indices.front();
        q_indices.pop();
        cout << " " << last_ind - (pat_len - 1);
    }
    cout << endl;
}

// Driver code
int main()
{
    // Passing the Text
    string text = "Welcome to Geeks for Geeks";
    string pattern = "Geeks";

    // Function call
    pattern_search(text, pattern);
    return 0;
}

Python3

# Python program for the above approach:

## Using a 256 sized array of
## hash sets.
structured_text = [set({}) for _ in range(256)]

## Function to perform the hashing
def StringSearch(st):

    ## Structure the text. It will be
    ## helpful in pattern searching
    global structured_text
    for i in range(len(st)):

        ## Insert every index to the
        ## hash set using character ASCII.
        structured_text[ord(st[i])].add(i)

## Function to search the pattern
def pattern_search(st, pattern):

    global structured_text
    StringSearch(st)

    ## Queue contain the indices
    q_indices = []

    for ind in structured_text[ord(pattern[0])]:
        q_indices.append(ind)

    ## Pattern length
    pat_len = len(pattern);
    for i in range(1, pat_len):
        ch = pattern[i]
        q_size = len(q_indices)

        ## The queue contains the
        ## number of occurrences of
        ## the previous character.
        ## Traverse the queue for
        ## q_size times.
        ## Check the next character of
        ## the pattern found or not.
        while q_size > 0:
            q_size -= 1
            ind = q_indices[0]
            q_indices.pop(0)

            if ((ind + 1) in structured_text[ord(ch)]):
                q_indices.append(ind + 1)

    print("Pattern found at indexes:", end="")
    while len(q_indices) > 0:

        ## last_ind is the last index
        ## of the pattern in the text
        last_ind = q_indices[0]
        q_indices.pop(0)
        print("", last_ind - (pat_len - 1), end="")
    print("")

## Driver code
if __name__ == '__main__':

    ## Passing the Text
    text = "Welcome to Geeks for Geeks"
    pattern = "Geeks"

    ## Function call
    pattern_search(text, pattern)

    # This code is contributed by subhamgoyal2014.

Java

import java.util.HashSet;
import java.util.Queue;
import java.util.LinkedList;

public class PatternSearch {
    // Using a 256 sized array of hash sets.
    private HashSet<Integer>[] structuredText = new HashSet[256];

    // Function to perform the hashing
    public void stringSearch(String st) {
        // Structure the text. It will be helpful in pattern searching
        for (int i = 0; i < 256; i++) {
            structuredText[i] = new HashSet<Integer>();
        }

        // Insert every index to the hash set using character ASCII.
        for (int i = 0; i < st.length(); i++) {
            structuredText[(int)st.charAt(i)].add(i);
        }
    }

    // Function to search the pattern
    public void patternSearch(String st, String pattern) {
        stringSearch(st);

        // Queue to contain the indices
        Queue<Integer> qIndices = new LinkedList<Integer>();

        for (int ind : structuredText[(int)pattern.charAt(0)]) {
            qIndices.add(ind);
        }

        // Pattern length
        int patLen = pattern.length();
        for (int i = 1; i < patLen; i++) {
            char ch = pattern.charAt(i);
            int qSize = qIndices.size();

            // The queue contains the number of occurrences of the previous character.
            // Traverse the queue for q_size times.
            // Check the next character of the pattern found or not.
            while (qSize > 0) {
                qSize--;
                int ind = qIndices.peek();
                qIndices.remove();

                if (structuredText[(int)ch].contains(ind + 1)) {
                    qIndices.add(ind + 1);
                }
            }
        }

        System.out.print("Pattern found at indexes: ");
        while (!qIndices.isEmpty()) {
            // lastInd is the last index of the pattern in the text
            int lastInd = qIndices.peek();
            qIndices.remove();
            System.out.print(lastInd - (patLen - 1) + " ");
        }
        System.out.println();
    }

    // Driver code
    public static void main(String[] args) {
        PatternSearch ps = new PatternSearch();

        // Passing the Text
        String text = "Welcome to Geeks for Geeks";
        String pattern = "Geeks";

        // Function call
        ps.patternSearch(text, pattern);
    }
}

C#

using System;
using System.Collections.Generic;

class GFG {
    // Using a Dictionary of lists
    // for structuring the text
    static Dictionary<char, List<int> > structuredText
        = new Dictionary<char, List<int> >();

    // Function to perform the hashing
    static void StringSearch(string st)
    {
        for (int i = 0; i < st.Length; i++) {
            char ch = st[i];

            // Add the index to the list
            // of corresponding character
            if (structuredText.ContainsKey(ch)) {
                structuredText[ch].Add(i);
            }
            else {
                structuredText.Add(ch, new List<int>{ i });
            }
        }
    }

    // Function to search the pattern
    static void pattern_search(string st, string pattern)
    {
        StringSearch(st);

        // Queue to contain the indices
        Queue<int> q_indices = new Queue<int>();

        // Add the first index of the
        // pattern from the text
        if (structuredText.ContainsKey(pattern[0])) {
            q_indices = new Queue<int>(
                structuredText[pattern[0]]);
        }

        // Pattern length
        int pat_len = pattern.Length;

        for (int i = 1; i < pat_len; i++) {
            char ch = pattern[i];
            int q_size = q_indices.Count;

            // Traverse the queue for
            // q_size times.
            // Check the next character of
            // the pattern found or not.
            while (q_size-- > 0) {
                int ind = q_indices.Dequeue();

                if (structuredText.ContainsKey(ch)
                    && structuredText[ch].Contains(ind
                                                   + 1)) {
                    q_indices.Enqueue(ind + 1);
                }
            }
        }

        Console.Write("Pattern found at indexes:");
        while (q_indices.Count > 0) {
            // last_ind is the last index
            // of the pattern in the text
            int last_ind = q_indices.Dequeue();
            Console.Write(" " + (last_ind - (pat_len - 1)));
        }
        Console.WriteLine();
    }

    // Driver code
    static void Main(string[] args)
    {
        // Passing the Text
        string text = "Welcome to Geeks for Geeks";
        string pattern = "Geeks";

        // Function call
        pattern_search(text, pattern);
    }
}

Javascript

// Using a 256 sized array of hash sets
const structuredText = Array(256).fill().map(() => new Set());

// Function to perform the hashing
function stringSearch(st) {
  // Structure the text. It will be helpful in pattern searching
  for (let i = 0; i < st.length; i++) {
    // Insert every index to the hash set using character ASCII
    structuredText[st.charCodeAt(i)].add(i);
  }
}

// Function to search the pattern
function patternSearch(st, pattern) {
  stringSearch(st);

  // Queue contain the indices
  const indicesQueue = [];

  for (const ind of structuredText[pattern.charCodeAt(0)]) {
    indicesQueue.push(ind);
  }

  // Pattern length
  const patLen = pattern.length;
  for (let i = 1; i < patLen; i++) {
    const ch = pattern.charCodeAt(i);
    let qSize = indicesQueue.length;

    // The queue contains the number of occurrences of the previous character
    // Traverse the queue for qSize times
    // Check the next character of the pattern found or not
    while (qSize--) {
      const ind = indicesQueue.shift();

      if (structuredText[ch].has(ind + 1)) {
        indicesQueue.push(ind + 1);
      }
    }
  }

  console.log("Pattern found at indexes:");
  while (indicesQueue.length > 0) {
    // lastInd is the last index of the pattern in the text
    const lastInd = indicesQueue.shift();
    console.log(" ", lastInd - (patLen - 1));
  }
  console.log();
}

// Driver code
const text = "Welcome to Geeks for Geeks";
const pattern = "Geeks";

// Function call
patternSearch(text, pattern);

Output

Pattern found at indexes: 21 11

Time Complexity: O(N * logK), where K is the maximum occurrence of any character
Auxiliary Space: O(d), d represents a 256 sized array of unordered_set