This C++ program implements the Kunth-Morris-Pratt Algorithm for string matching. A text and a pattern is given as input. The pattern is searched for in the text and all instances of the pattern are given as output.
This C++ program is successfully compiled and run on Code::Blocks 10.05, a C++ compiler. The program output is given below.
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
void computeLPSArray(char *pat, int M, int *lps);
void KMPSearch(char *pat, char *txt)
{
int M = strlen(pat);
int N = strlen(txt);
// create lps[] that will hold the longest prefix suffix values for pattern
int *lps = (int *)malloc(sizeof(int)*M);
int j = 0; // index for pat[]
// Preprocess the pattern (calculate lps[] array)
computeLPSArray(pat, M, lps);
int i = 0; // index for txt[]
while(i < N)
{
if(pat[j] == txt[i])
{
j++;
i++;
}
if (j == M)
{
printf("Found pattern at index %d \n", i-j);
j = lps[j-1];
}
// mismatch after j matches
else if(pat[j] != txt[i])
{
// Do not match lps[0..lps[j-1]] characters,
// they will match anyway
if(j != 0)
j = lps[j-1];
else
i = i+1;
}
}
free(lps); // to avoid memory leak
}
void computeLPSArray(char *pat, int M, int *lps)
{
int len = 0; // lenght of the previous longest prefix suffix
int i;
lps[0] = 0; // lps[0] is always 0
i = 1;
// the loop calculates lps[i] for i = 1 to M-1
while(i < M)
{
if(pat[i] == pat[len])
{
len++;
lps[i] = len;
i++;
}
else // (pat[i] != pat[len])
{
if( len != 0 )
{
// This is tricky. Consider the example AAACAAAA and i = 7.
len = lps[len-1];
// Also, note that we do not increment i here
}
else // if (len == 0)
{
lps[i] = 0;
i++;
}
}
}
}
// Driver program to test above function
int main()
{
char *txt = "ABABDABACDABABCABAB";
char *pat = "ABABCABAB";
KMPSearch(pat, txt);
return 0;
}
Output Found pattern at index 10
Sanfoundry Global Education & Learning Series – 1000 C++ Programs.
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