Python Program for Rabin-Karp Algorithm for Pattern Searching
Last Updated :
10 Feb, 2023
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:
Input: txt[] = "THIS IS A TEST TEXT"
pat[] = "TEST"
Output: Pattern found at index 10
Input: txt[] = "AABAACAADAABAABA"
pat[] = "AABA"
Output: Pattern found at index 0
Pattern found at index 9
Pattern found at index 12
The Naive String Matching algorithm slides the pattern one by one. After each slide, one by one checks characters at the current shift, and if all characters match then print the match. Like the Naive Algorithm, the Rabin-Karp algorithm also slides the pattern one by one. But unlike the Naive algorithm, the Rabin Karp algorithm matches the hash value of the pattern with the hash value of the current substring of text, and if the hash values match then only it starts matching individual characters. So Rabin Karp’s algorithm needs to calculate hash values for the following strings.1) Pattern itself.2) All the substrings of the text of length m.
Python
d = 256
def search(pat, txt, q):
M = len(pat)
N = len(txt)
i = 0
j = 0
p = 0
t = 0
h = 1
for i in xrange(M-1):
h = (h * d)% q
for i in xrange(M):
p = (d * p + ord(pat[i]))% q
t = (d * t + ord(txt[i]))% q
for i in xrange(N-M + 1):
if p == t:
for j in xrange(M):
if txt[i + j] != pat[j]:
break
j+= 1
if j == M:
print "Pattern found at index " + str(i)
if i < N-M:
t = (d*(t-ord(txt[i])*h) + ord(txt[i + M]))% q
if t < 0:
t = t + q
txt = "GEEKS FOR GEEKS"
pat = "GEEK"
q = 101
search(pat, txt, q)
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Output:
Pattern found at index 0
Pattern found at index 10
Time Complexity: O(nm), where m is the length of the pattern and n is the length
Auxiliary Space: O(1).
Please refer complete article on Rabin-Karp Algorithm for Pattern Searching for more details!
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