Rabin Karp String Matching Algorithm in C

This is a C Program to implement RabinKarp method. Given a text txt[0..n-1] and a pattern pat[0..m-1], write a function search(char pat[], char txt[]) that prints all occurrences of pat[] in txt[]. You may assume that n > m.
Examples:
1) Input:
txt[] = “THIS IS A TEST TEXT”
pat[] = “TEST”
Output:
Pattern found at index 10

Here is source code of the C Program to Implement Rabin-Karp Method for String Matching. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include<stdio.h>
```
```
#include<string.h>
```
```
 
```

// d is the number of characters in input alphabet

```
#define d 256
```
```
 
```
```
/*  pat  -> pattern
```
```
    txt  -> text
```
```
    q    -> A prime number
```
```
*/
```

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);
```
```
        }
```
```
    }
```
```
}
```
```
 
```

/* Driver program to test above function */

```
int main()
```
```
{
```

    char *txt = "Sanfoundry C Programming Examples";

```
    char *pat = "San";
```
```
    int q = 101;  // A prime number
```
```
    search(pat, txt, q);
```
```
    return 0;
```
```
}
```

Output:

$ gcc RabinCarp.c
$ ./a.out
 
Pattern found at index 0

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