README.md

Day 30 - Search and Sort Algorithms Part C: Naive Pattern Search

So this week is for searching and sorting algorithms, and we did linear search and binary search yesterday, and today's algorithm would be the Naive Search.

Question

Given a sentence (string), and a pattern, write a function that returns the index of found pattern. Since it is used for pattern searching, after doing it also have a look at KMP algorithm for pattern searching (We already did the KMP algorithm while string problems series, but since today's topic is searching, it deserves a place here as well 😁)

Example

input:
  str = "Hello World, Coding is beautiful"
  pattern = "World"

output: 6 (start index of the found pattern)

Solution

JavaScript Implementation

to be added

Java Implementation

Solution

/**
 * @date 30/01/19
 * @author SPREEHA DUTTA
 */
import java.util.*;
public class naive {
    public static void main(String []args)
    {
        Scanner sc=new Scanner(System.in);
        String s,p;int i,j;String str;int c=0;
        s=sc.next();
        System.out.println("Enter pattern ");
        p=sc.next();
        for(i=0;i<s.length()-p.length();i++)
        {
            str=s.substring(i,i+p.length());
            if(str.equals(p))
            {
                System.out.println(i); c=1;
                break;
            }
        }
        if(c==0)
            System.out.println("Pattern not found");
    }
}

C++ Implementation

/*
* @author : imkaka
* @date   : 1/2/2019
*/

#include<iostream>
#include<string>

using namespace std;

void computeLPS(string, int, int []);

// KMP Algorithm
void KMPsearch(string text, string pat){
    // Length
    int N = text.size();
    int M = pat.size();

    // Define LPS (Longest Proper Prefix)
    int lps[M];

    //Preprocess
    computeLPS(pat, M, lps);

    int i = 0, j = 0;
    while(i < N){
        //While Match
        if(pat[j] == text[i]){
            i++;
            j++;
        }

        if(j == M){
            cout << "Pattern Found At " << (i-j) << endl;
            j = lps[j-1];
        }

        else if(i < N && pat[j] != text[i]){
            if(lps[j] != 0)
                j = lps[j-1];
            else
                i++;
        }
    }
}

void computeLPS(string pat, int M, int lps[]){
    int len = 0; //Track len of longest common prefix which is suffix also.

    lps[0] = 0;
    int i = 1;

    while(i < M){
        if(pat[i] == pat[len]){
            len++;
            lps[i] = len;
            i++;
        }
        else{
            if(len != 0){
                len = lps[len-1]; //Don't increment i
            }
            else{
                lps[i] = 0;
                i++;
            }
        }
    }
}

int main(){

    string txt = "ABABDABACDABABCABAB";
    string pat = "ABABCABAB";

    KMPsearch(txt, pat);
    return 0;
}