Output: Enter Text:Mr. Antony saw an Ant on Aunt's hand. Enter Pattern to Search:Ant Pattern found at index 4 Pattern found at index 18
This C++ program is successfully compiled and tested on our system. The program output is given below.
/*
* C++ program implements the Rabin-Karp method for string matching.
*/
#include<stdio.h>
#include<string.h>
#include<iostream>
#include<vector>
using namespace std;
// d is the number of characters in input alphabet
#define d 256
/* pat -> pattern
txt -> text
q -> A prime number
*/
void get_input(vector<char>& a)
{
char c;
while (1)
{
c = getchar();
if (c == '\n')
break;
a.push_back(c);
}
return;
}
void search(char *pat, char *txt, int q)
{
int M = strlen(pat);
int N = strlen(txt);
int i, j;
int p = 0; // hash value for pattern
int t = 0; // hash value for txt
int h = 1;
// The value of h would be "pow(d, M-1)%q"
for (i = 0; i < M-1; i++)
h = (h*d)%q;
// Calculate the hash value of pattern and first window of text
for (i = 0; i < M; i++)
{
p = (d * p + pat[i]) % q;
t = (d * t + txt[i]) % q;
}
// Slide the pattern over text one by one
for (i = 0; i <= N - M; i++)
{
// Check the hash values of current window of text and pattern
// If the hash values match then only check for characters on by one
if ( p == t )
{
/* Check for characters one by one */
for (j = 0; j < M; j++)
{
if (txt[i + j] != pat[j])
break;
}
if (j == M) // if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
{
printf("Pattern found at index %d \n", i);
}
}
// Calculate hash value for next window of text: Remove leading digit,
// add trailing digit
if ( i < N - M )
{
t = (d * (t - txt[i] * h) + txt[i + M]) % q;
// We might get negative value of t, converting it to positive
if (t < 0)
t = (t + q);
}
}
}
int main()
{
vector<char> txt;
vector<char> pat;
cout<<"Enter Text:";
get_input(txt);
cout<<"Enter Pattern to Search:";
get_input(pat);
char *text,*pattern;
text=&txt[0];
text[txt.size()]='\0';
pattern=&pat[0];
pattern[pat.size()]='\0';
int q = 101; // A prime number
search(pattern, text, q);
getchar();
return 0;
}
Output: Enter Text:Mr. Antony saw an Ant on Aunt's hand. Enter Pattern to Search:Ant Pattern found at index 4 Pattern found at index 18