C program of Rabin-Karp algorithm for pattern search

Pattern Matching in C- We have to find if a string exists in another string, for example, the string "algorithm" exists in the string in "naive algorithm". If it is found, then its location (i.e. where it is located) is displayed. We tend to create a function that takes an array of 2 characters and returns the position if there is a match Otherwise, -1 is returned.

Input: txt = "HERE IS A NICE CAP"
   pattern = "NICE"
Output: Pattern found at index 10
Input: txt = "XYZXACAADXYZXYZX"
   pattern = "XYZX"
Output: Pattern found at index 0
   Pattern found at index 9
   Pattern found at index 12

Rabin-Karp is another pattern search algorithm. Just string matching Algorithm proposed by Rabin and Karp for finding patterns more efficiently Way. Like the naive algorithm, it also checks for patterns by moving the window It looks for hashes one by one, but doesn't need to check all characters in all cases. When the hashes match, each character is checked. This way, there is only one comparison per text subsequence, making it a more efficient pattern search algorithm.

Preprocessing time - O(m)

The time complexity of Rabin-Karp algorithm is O(m n), but for the worst Case, It is O(mn).

Algorithm

rabinkarp_algo(text,pattern,prime)

Input

rabinkarp_algo(text,pattern,prime)

Input strong>− Text and pattern. Find another prime number at the hash position

Output− Find the position of the pattern

Start
   pat_len := pattern Length
   str_len := string Length
   patHash := 0 and strHash := 0, h := 1
   maxChar := total number of characters in character set
for index i of all character in the pattern, do
   h := (h*maxChar) mod prime
for all character index i of pattern, do
   patHash := (maxChar*patHash + pattern[i]) mod prime
   strHash := (maxChar*strHash + text[i]) mod prime
for i := 0 to (str_len - pat_len), do
   if patHash = strHash, then
      for charIndex := 0 to pat_len -1, do
         if text[i+charIndex] ≠ pattern[charIndex], then
         break
if charIndex = pat_len, then
   print the location i as pattern found at i position.
if i < (str_len - pat_len), then
   strHash := (maxChar*(strHash &ndash; text[i]*h)+text[i+patLen]) mod prime, then
   if strHash < 0, then
   strHash := strHash + prime
End

Example

Live demonstration

#include<stdio.h>
#include<string.h>
int main (){
   char txt[80], pat[80];
   int q;
   printf ("Enter the container string </p><p>");
   scanf ("%s", &txt);
   printf ("Enter the pattern to be searched </p><p>");
   scanf ("%s", &pat);
   int d = 256;
   printf ("Enter a prime number </p><p>");
   scanf ("%d", &q);
   int M = strlen (pat);
   int N = strlen (txt);
   int i, j;
   int p = 0;
   int t = 0;
   int h = 1;
   for (i = 0; i < M - 1; i++)
      h = (h * d) % q;
   for (i = 0; i < M; i++){
      p = (d * p + pat[i]) % q;
      t = (d * t + txt[i]) % q;
   }
   for (i = 0; i <= N - M; i++){
      if (p == t){
         for (j = 0; j < M; j++){
            if (txt[i + j] != pat[j])
            break;
         }
         if (j == M)
            printf ("Pattern found at index %d </p><p>", i);
      }
      if (i < N - M){
         t = (d * (t - txt[i] * h) + txt[i + M]) % q;
         if (t < 0)
            t = (t + q);
      }
   }
   return 0;
}

Output

Enter the container string
tutorialspointisthebestprogrammingwebsite
Enter the pattern to be searched
p
Enter a prime number
3
Pattern found at index 8
Pattern found at index 21

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