This document provides an overview of the Knuth-Morris-Pratt substring search algorithm. It defines the algorithm, describes its history and key components including the prefix function and KMP matcher. An example showing the step-by-step workings of the algorithm on a text and pattern is provided. The algorithm's linear runtime complexity of O(n+m) is compared to other string matching algorithms. Real-world applications including DNA sequence analysis and search engines are discussed.
Given presentation tell us about string, string matching and the navie method of string matching. Well this method has O((n-m+1)*m) time complexicity. It also tells the problem with naive approach and gives list of approaches which can be applied to reduce the time complexicity
Given presentation tell us about string, string matching and the navie method of string matching. Well this method has O((n-m+1)*m) time complexicity. It also tells the problem with naive approach and gives list of approaches which can be applied to reduce the time complexicity
Here i discuss 3 algorithm about String matching.
Those algorithm are:
1. The naive algorithm.
2. The Rabin-Krap algorithm.
3. The Knuth-Morris-Pratt algorithm.
i hope,by readinng this slide, it is easy to undarstand those algorithm.
P, NP, NP-Complete, and NP-Hard
Reductionism in Algorithms
NP-Completeness and Cooks Theorem
NP-Complete and NP-Hard Problems
Travelling Salesman Problem (TSP)
Travelling Salesman Problem (TSP) - Approximation Algorithms
PRIMES is in P - (A hope for NP problems in P)
Millennium Problems
Conclusions
Merge sort is a sorting technique based on divide and conquer technique. With worst-case time complexity being Ο(n log n), it is one of the most respected algorithms.
Merge sort first divides the array into equal halves and then combines them in a sorted manner.
Here i discuss 3 algorithm about String matching.
Those algorithm are:
1. The naive algorithm.
2. The Rabin-Krap algorithm.
3. The Knuth-Morris-Pratt algorithm.
i hope,by readinng this slide, it is easy to undarstand those algorithm.
P, NP, NP-Complete, and NP-Hard
Reductionism in Algorithms
NP-Completeness and Cooks Theorem
NP-Complete and NP-Hard Problems
Travelling Salesman Problem (TSP)
Travelling Salesman Problem (TSP) - Approximation Algorithms
PRIMES is in P - (A hope for NP problems in P)
Millennium Problems
Conclusions
Merge sort is a sorting technique based on divide and conquer technique. With worst-case time complexity being Ο(n log n), it is one of the most respected algorithms.
Merge sort first divides the array into equal halves and then combines them in a sorted manner.
WHAT IS PATTERN RECOGNISITION ?
Given a text string T[0..n-1] and a pattern P[0..m-1], find all occurrences of the pattern within the text.
Example: T = 000010001010001 and P = 0001, the occurrences are:
first occurrence starts at T[1]
second occurrence starts at T[5]
third occurrence starts at T[11]
APPLICATION :
Image preprocessing
• Computer vision
• Artificial intelligence
• Radar signal classification/analysis
• Speech recognition/understanding
• Fingerprint identification
• Character (letter or number) recognition
• Handwriting analysis
• Electro-cardiographic signal analysis/understanding
• Medical diagnosis
• Data mining/reduction
ALGORITHM DESCRIPTION
The Knuth-Morris-Pratt (KMP) algorithm:
It was published by Donald E. Knuth, James H.Morris and Vaughan R. Pratt, 1977 in: “Fast Pattern Matching in Strings.“
To illustrate the ideas of the algorithm, consider the following example:T = xyxxyxyxyyxyxyxyyxyxyxxy
And P = xyxyyxyxyxx
it considers shifts in order from 1 to n-m, and determines if the pattern matches at that shift. The difference is that the KMP algorithm uses information gleaned from partial matches of the pattern and text to skip over shifts that are guaranteed not to result in a match.
The text and pattern are included in Figure 1, with numbering, to make it easier to follow.
1.Consider the situation when P[1……3] is successfully matched with T[1……..3]. We then find a mismatch: P[4] = T[4]. Based on our knowledge that P[1…… 3] =T[1…… 3], and ignoring symbols of the pattern and text after position 3, what can we deduce about where a potential match might be? In this case, the algorithm slides the pattern 2 positions to the right so that P[1] is lined up with T[3]. The next comparison is between P[2] and T[4].
P: x y x y y x y x y x x
q: 1 2 3 4 5 6 7 8 9 10 11
_(q): 0 0 1 2 0 3
Table 1: Table of values for pattern P.
Time Complexity :
The call to compute prefix is O(m)
using q as the value of the potential function, we argue in the same manner as above to show the loop is O(n)
Therefore the overall complexity is O(m + n)
Boyer-Moore algorithm:
It was developed by Bob Boyer and J Strother Moore in 1977. The algorithm preprocesses the pattern string that is being searched in text string.
String pattern matching - Boyer-Moore:
This algorithm uses fail-functions to shift the pattern efficiently. Boyer-Moore starts however at the end of the pattern, which can result in larger shifts.Two heuristics are used:1: if we encounter a mismatch at character c in Q, we can shift to the first occurrence of c in P from the right:
Q a b c a b c d g a b c e a b c d a c e d
P a b c e b c d
a b c e b c d
(restart here)
Time Complexity:
• performs the comparisons from right to left;
• preprocessing phase in O(m+ ) t
string searching algorithms. Given two strings P and T over the same alphabet E, determine whether P occurs as a substring in T (or find in which position(s) P occurs as a substring in T). The strings P and T are called pattern and target respectively.
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Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
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2. Outline
Definition
History
Components of KMP
Algorithm
Example
Run-Time Analysis
Complexity comparison of String Matching Algorithms
Advantages and Disadvantages
Real Time Applications
References
2
3. What is Pattern Searching ?
Suppose you are reading a text document.
You want to search for a word.
You click CTRL + F and search for that word.
The word processor scans the document and shows the position of
occurrence.
What exactly happens is that, word i.e. pattern is searched inside the
text document.
3
4. Definition
Best known for linear time for exact matching. Compares from left to right.
Shifts more than one position.
Preprocessing approach of Pattern to avoid trivial comparisions.
Avoids recomputing matches.
4
5. History
This algorithm was conceived by Donald Knuth and Vaughan Pratt and independently by
James H.Morris in 1977.
Knuth, Morris and Pratt discovered first linear time string-matching algorithm by
analysis of the naive algorithm.
It keeps the information that naive approach wasted gathered during the scan of the
text. By avoiding this waste of information, it achieves a running time of O(m + n).
The implementation of Knuth-Morris-Pratt algorithm is efficient because it minimizes
the total number of comparisons of the pattern against the input string.
5
6. Naïve Approach
The naïve approach is to check whether the pattern matches the string at
every possible position in the string.
P= Pattern (word) of length m
T= Text (document) of length n
Naive string matching algorithm
takes time O((n-m+1)m) or
O(mn)
6
7. The KMP Algorithm - Motivation
x
j
. . a b a a b . . . . .
a b a a b a
a b a a b a
No need to
repeat these
comparisons
Resume
comparing
here
Knuth-Morris-Pratt’s algorithm
compares the pattern to the text
in left-to-right, but shifts the
pattern more intelligently than
the brute-force algorithm.
When a mismatch occurs, what
is the most we can shift the
pattern so as to avoid redundant
comparisons?
Answer: the largest prefix of
P[0..j] that is a suffix of P[1..j]
7
8. Components of KMP algorithm
The prefix function, Π
The prefix function,Π for a pattern encapsulates knowledge about how the pattern
matches against shifts of itself. This information can be used to avoid useless shifts of
the pattern ‘p’. In other words, this enables avoiding backtracking on the text ‘T’.
The KMP Matcher
With text ‘T’, pattern ‘p’ and prefix function ‘Π’ as inputs, finds the occurrence of ‘p’ in
‘T’ and returns the number of shifts of ‘p’ after which occurrence is found.
8
9. The prefix function, Π
Following pseudocode computes the prefix function, Π:
Compute-Prefix-Function (p)
1 m length[p] //’p’ pattern to be matched
2 Π[1] 0
3 k 0
4 for q 2 to m
5 do while k > 0 and p[k+1] != p[q]
6 do k Π[k]
7 If p[k+1] = p[q]
8 then k k +1
9 Π[q] k
10 return Π
9
10. Example: compute Π for the pattern ‘p’ below:
a b a b a c a
Initially: m = length[p] = 7
Π[1] = 0
k = 0
Step 1: q = 2, k=0
Π[2] = 0
Step 2: q = 3, k = 0,
Π[3] = 1
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1
10
p
10
11. Contd…
11
Step 3: q = 4, k = 1
Π[4] = 2 q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2
Step 4: q = 5, k =2
Π[5] = 3
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2 3
12. Step 5: q = 6, k = 3
Π[6] = 0
Step 6: q = 7, k = 0
Π[7] = 1
After iterating 6 times, the prefix function
computation is complete:
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2 3 0
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2 3 0 1
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2 3 0 1
The running time of the prefix function is O(m).
12Contd…..
13. The KMP Matcher
Input: The KMP Matcher, with pattern ‘p’, text ‘T’and prefix function ‘Π’, finds a match of p in T.
Following pseudocode computes the matching component of KMP algorithm:
KMP-Matcher(T,p)
1 n length[T]
2 m length[p]
3 Π Compute-Prefix-Function(p)
4 q 0 //number of characters matched
5 for i 1 to n //scan T from left to right
6 do while q > 0 and p[q+1] != T[i]
7 do q Π[q] //next character does not match
8 if p[q+1] = T[i]
9 then q q + 1 //next character matches
10 if q = m //is all of p matched?
11 then print “Pattern occurs with shift” i – m
12 q Π[ q] // look for the next match
Note: KMP finds every occurrence of a ‘p’in ‘T’. That is why KMP does not terminate in step 12, rather it searches
remainder of ‘T’for any more occurrences of ‘p’.
13
14. Illustration: given a Text ‘T’ and pattern ‘p’ as follows:
T
b a c b a b a b a b a c a c a
p a b a b a c a
Let us execute the KMP algorithm to find whether ‘p’ occurs in ‘T’.
For ‘p’the prefix function, Π was computed previously and is as follows:
q 1 2 3 4 5 6 7
p a b a b a c a
Π 0 0 1 2 3 0 1 14
14
15. b a c b a b a b a b a c a a b
a b a b a c a
Initially: n = size of T = 15;
m = size of p = 7
Step 1: i = 1, q = 0
comparing p[1] with T[1]
T
p
P[1] does not match with T[1]. ‘p’ will be shifted one position to the right.
15
15
Contd…
Step 2: i = 2, q = 0
comparing p[1] with T[2]
T
p
a b a b a c a
P[1] matches T[2]. Since there is a match, p is not shifted.
b a c b a b a b a b a c a a b
16. Contd…
16
T b a c b a b a b a b a c a a b
a b a b a c ap
p[2] does not match with T[3]
Backtracking on p, comparing p[1] and T[3]
Step 3: i = 3, q = 1
Comparing p[2] with T[3]
a b a b a c a
T
p
Step 4: i = 4, q = 0 comparing p[1] with T[4] p[1] does not match with T[4]
b a c b a b a b a b a c a a b
17. b a c b a b a b a b a c a a b
a b a b a c a
T
p
Step 5: i = 5, q = 0
comparing p[1] with T[5] p[1] matches with T[5]
17
17
Step 6: i = 6, q = 1 Comparing p[2] with T[6] p[2] matches with T[6]
T
p
b a c b a b a b a b a c a a b
a b a b a c a
Contd…
18. b a c b a b a b a b a c a a b
b a c b a b a b a b a c a a b
a b a b a c a
a b a b a c a
T
p
Step 7: i = 7, q = 2 Comparing p[3] with T[7]
p[3] matches with T[7]
Step 8: i = 8, q = 3 Comparing p[4] with T[8]
p[4] matches with T[8]
T
p
18
18
Contd…
19. Step 9: i = 9, q = 4
Comparing p[5] with T[9]
Comparing p[6] with T[10]Step 10: i = 10, q = 5
T
p
b a c b a b a b a b a c a a b
b a c b a b a b a b a c a a b
a b a b a c a
a b a b a c a
p[6] doesn’t match with T[10]
Backtracking on p, comparing p[4] with T[10] because after mismatch q = Π[5] = 3
p[5] matches with T[9]
19
19
T
p
Contd…
20. 20
Step 11: i = 11, q = 4
Comparing p[5] with T[11] p[5] matches with T[11]
T
p
b a c b a b a b a b a c a a b
a b a b a c a
Contd…
Step 12: i = 12, q = 5 Comparing p[6] with T[12] p[6] matches with T[12]
a b a b a c ap
b a c b a b a b a b a c a a bT
21. b a c b a b a b a b a c a a b
a b a b a c a
Comparing p[7] with T[13]
T
p
Step 13: i = 13, q = 6 p[7] matches with T[13]
Pattern ‘p’ has been found to completely occur in text ‘T’. The total number of shifts
that took place for the match to be found are: i – m = 13 – 7 = 6 shifts.
The running time of the KMP-Matcher function is O(n).
21
21
Contd…
22. Complexity
O(m) - It is to compute the prefix function values.
O(n) - It is to compare the pattern to the text.
Total of O(n + m) run time.
22
24. Advantage and Disadvantage
Advantages:
1.The running time of the KMP algorithm is optimal (O(m + n)), which is very fast.
2.The algorithm never needs to move backwards in the input text T. It makes the
algorithm good for processing very large files.
Disadvantages:
Doesn’t work so well as the size of the alphabets increases. By which more chances
of mismatch occurs.
24
25. Real time Applications
Good for plagiarism analysis.
search engines
language syntax checker
database queries
music content retrieval
25
26. Real time Applications
26
DNA sequences analysis :
• It is mainly composed of nucleotides of four types. The four bases in DNA are
Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). A DNA sequence is a
representation of a string of nucleotides contained in a strand of DNA.
• DNA sequences analysis of various diseases which are stored in database for retrieval
and comparison. This system compares similarity values with threshold value and
stores particular result which is diseased or not.
• For example: ATTCGTAACTAGTAAGTTA. The DNA sequencing techniques have
allowed the vast amount of data to be analyzed in a short span of time. So, pattern
matching techniques plays a vital role in computational biology for data analysis
related to biological data such as DNA sequences
27. References
Thomas H.Cormen; Charles E.Leiserson., Introduction to algorithms second edition , “The Knuth-Morris-Pratt
Algorithm”, year = 2001.
https://pdfs.semanticscholar.org/fe41/52465f96d09c94b46b86a3b6408dae5dbe13.pdf
http://research.ijcaonline.org/volume115/number23/pxc3902734.pdf
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