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Dynamic Programming (Longest Common Subsequence)

Dynamic Programming (Longest Common Subsequence). Subsequence. String Z is a subsequence of string X if Z’s characters appear in X following the same left-to-right order. X = < A, B, C, T, D, G, N, A, B > Z = < B, D, A >. Longest Common Subsequence (LCS).

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Dynamic Programming (Longest Common Subsequence)

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  1. Dynamic Programming(Longest Common Subsequence)

  2. Subsequence • String Z is a subsequence of string X if Z’s characters appear in X following the same left-to-right order X = < A, B, C, T, D, G, N, A, B > Z = < B, D, A >

  3. Longest Common Subsequence (LCS) • String Z is a common subsequence of strings X and Y if Z’s characters appear in both X & Y following the same left-to-right order X = < A, B, C, T, B, D, A, B > Y = < B, D, C, A, B, A > • < B, C, A > is a common subsequence of both X and Y. • < B, C, B, A > or < B, C, A, B > is the Longest Common Subsequence (LCS) of X and Y. LCS is used to measure the similarity between two strings X and Y. The longer the LCS , the more similar X and Y

  4. LCS Problem Definition • We are given two sequences • X= <x1,x2,...,xm>,and • Y = <y1,y2,...,yn> • We need to find the LCS between X and Y Very common in DNA sequences

  5. Characterization of LCS

  6. Characterization of LCS (Cont’d)

  7. Recursive Nature of LCS • Implications of Theorem 15.1

  8. Recursive Equation • Input X = <x1, x2, …., xm> Y = <y1, y2, ………, yn> • Assume C[i, j] is the LCS for the first i positions in X with the first j positions in Y • C[i,j] = LCS(<x1, x2, …., xi>, <y1, y2, ………, yj>) Our goal is to compute C[m,n]

  9. Dynamic Programming for LCS Initialization step

  10. Dynamic Programming for LCS If matching, go diagonal

  11. Dynamic Programming for LCS Else select the larger of top or left

  12. Dynamic Programming for LCS Note that array c  keeps track of the cost, Array b  keeps track of the parent (to backtrack)

  13. Summary of the Main Strategies

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