KMP & RabinKarp in Strings

String/PatternMatching/KMPalgorithm.c++

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+#include "bits/stdc++.h"
+using namespace std;
+
+void findLpsArray(char* pattern, int* lps, int m) 
+{
+
+    int len = 0;
+
+    lps[0] = 0;   
+    int i = 1;
+
+    while (i < m) 
+    {
+        if (pattern[i] == pattern[len]) 
+        {
+            len++;
+            lps[i] = len;
+            i++;
+        }
+        else
+        {
+           if (len != 0) 
+           {
+                len = lps[len - 1];
+           }
+            else
+            {
+                lps[i] = 0;
+                i++;
+            }
+        }
+    }
+}
+
+void patternSearchKMP(char* pattern, char* text) 
+{
+    int m = strlen(pattern);
+    int n = strlen(text);
+
+    int lps[m]; //longest-prefix-suffix array
+
+    findLpsArray(pattern, lps, m); //calling findLpsArray function
+
+    int i = 0; 
+    int j = 0;
+
+    while (i < n) 
+    {
+        if (pattern[j] == text[i]) 
+        {
+            j++;
+            i++;
+        }
+
+        if (j == m) 
+        {
+            cout<<"Pattern found at index: "<<i - j;
+            j = lps[j - 1];
+        }
+
+        else if (i < n && pattern[j] != text[i]) 
+        {
+            if (j != 0)
+                j = lps[j - 1];
+            else
+                i = i + 1;
+        }
+    }
+}
+
+int main()
+{
+    char pattern[100], text[100];
+
+    cout<<"Enter the pattern to be found: "<<endl;
+    cin>>pattern;
+
+    cout<<"Enter the string in which the pattern is to be found: "<<endl;
+    cin>>text;
+
+    patternSearchKMP(pattern, text); //Function call
+
+    return 0;
+}
+

String/PatternMatching/README.md

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+KMP Algorithm:
+
+The Knuth-Morris-Pratt algorithm is used to find a given text from a specific word. 
+
+This algorithm uses the degenerating property which primarily works based on sub-strings. 
+
+The basic idea of this algorithm is to not match a character which is already known to match for sure.
+
+Example:
+
+Output: Enter the pattern to be found:
+
+Input: cktober
+
+Output : Enter the string in which the pattern is to be found: 
+
+Input: hacktoberfest
+
+Output/Answer: Pattern found at index: 2
+
+Rabin Karp Algorithm
+
+The Rabin karp Algorithm is used to search a text from a given word using hash function.
+
+This algorithm doesnt search slide through every character but filters the unlike characters and then does the comparing.
+
+Example:
+
+Enter the pattern to be found: 
+
+program
+
+Enter the string in which the pattern is to be found: 
+
+competitveprogramming
+
+Pattern found at index: 10
+
+
+

String/PatternMatching/RabinKarpAlgorithm.c++

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+#include "bits/stdc++.h"
+using namespace std;
+
+void rabinKarpAlgo(char* text, char* pattern)
+{
+    int m = strlen(text);
+    int n = strlen(pattern);
+
+    int prime = 31;
+    int mod = 1e9 + 9;
+
+    vector<long long> p_pow(m);
+    p_pow[0] = 1; 
+
+    for (int i = 1; i < m; i++) 
+    {
+        p_pow[i] = (p_pow[i-1] * prime) % mod;
+    }
+
+    vector<long long> h(m + 1, 0); 
+
+    for (int i = 0; i < m; i++)
+    {
+        h[i+1] = (h[i] + (text[i] - 'a' + 1) * p_pow[i]) % mod;
+    }
+
+    long long hash_pattern = 0; 
+
+    for (int i = 0; i < n; i++) 
+    {
+        hash_pattern = (hash_pattern + (pattern[i] - 'a' + 1) * p_pow[i]) % mod; 
+    }
+
+    for (int i = 0; i + n - 1 < m; i++) 
+    { 
+        long long curr_hash = (h[i+n] + mod - h[i]) % mod; 
+
+        if (curr_hash == hash_pattern * p_pow[i] % mod)
+            cout<<"Pattern found at index: "<<i<<endl;
+    }
+}
+
+int main()
+{
+    char pattern[100], text[100];
+
+    cout<<"Enter the pattern to be found: "<<endl;
+    cin>>pattern;
+
+    cout<<"Enter the string in which the pattern is to be found: "<<endl;
+    cin>>text;
+
+    rabinKarpAlgo(text, pattern); //Function call
+
+    return 0;
+}