Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies
- Selected Reading
- UPSC IAS Exams Notes
- Developer's Best Practices
- Questions and Answers
- Effective Resume Writing
- HR Interview Questions
- Computer Glossary
- Who is Who
C Program for Rabin-Karp Algorithm for Pattern Searching
Pattern matching in C − We have to find if a string is present in another string, as an example, the string "algorithm” is present within the string "naive algorithm". If it is found, then its location (i.e. position it is present at) is displayed. We tend to create a function that receives 2character arrays and returns the position if matching happens otherwise returns -1.
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 searching algorithm. It is the string matching algorithm that was proposed by Rabin and Karp to find the pattern in a more efficient way. Like the Naive Algorithm, it also checks the pattern by moving the window one by one, but without checking all characters for all cases, it finds the hash value. When the hash value is matched, then only it proceeds to check each character. In this way, there is only one comparison per text subsequence making it a more efficient algorithm for pattern searching.
Preprocessing time- O(m)
The time complexity of the Rabin-Karp Algorithm is O(m+n), but for the worst case, it is O(mn).
Algorithm
rabinkarp_algo(text, pattern, prime)
Input − The main text and the pattern. Another prime number of find hash location
Output − locations, where the pattern is found
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 – text[i]*h)+text[i+patLen]) mod prime, then if strHash < 0, then strHash := strHash + prime End
Example
#include<stdio.h>
#include<string.h>
int main (){
char txt[80], pat[80];
int q;
printf ("Enter the container string
");
scanf ("%s", &txt);
printf ("Enter the pattern to be searched
");
scanf ("%s", &pat);
int d = 256;
printf ("Enter a prime number
");
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
", 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
2K+ Views
