Longest Common Subsequence Python 2 function

Question

When I run LCS( 'human', 'chimp' ), I'm getting "h" instead of "hm". When I run LCS( 'gattaca', 'tacgaacta' ), I'm getting "g" instead of "gaaca". When I run LCS( 'wow', 'whew' ), I'm getting "ww" which is correct. When I run LCS( '', 'whew' ), I'm getting "" which is correct. When I run LCS( 'abcdefgh', 'efghabcd' ), I'm getting "a" instead of "abcd". What am I doing incorrectly?

Here is my code:

def LCS(S, T):
  array = ''
  i = 0
  j = 0
  while i < len(S):
    while j < len(T):
      if S[i] == T[j]:
        array += S[i]
      j += 1
    i += 1
  return array

@RahulMadhavan: Nope, still totally wrong. This code isn't really salvageable; any fixed version would essentially be a complete rewrite. — user2357112, Aug 17 '16 at 20:19
Well, it would work if you reset it to the last matched value of j. But yes, thats essentially a rewrite. — Rahul Madhavan, Aug 17 '16 at 20:29

Dmitry Torba · Answer 1 · 2016-08-17T20:32:22.150

0

That is not how you write LCS. This is how you write very weird function that calculates the number of letters in the second string that are equal to the first letter of the first string.

I believe what you meant to write is wrong, so that does not matter, but if I correctly guessed what you meant to write, you forgot to assign 0 to j inside the outer while loop.

edited Aug 17 '16 at 20:32

answered Aug 17 '16 at 20:18

Dmitry Torba

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score 0 · Accepted Answer · answered Aug 20 '16 at 13:53

Figured it out thanks to the people next to me in the lab! It would also be nice to not run into snooty people on Stack Overflow every now and then.

def LCS(S, T):
  # If either string is empty, stop
  if len(S) == 0 or len(T) == 0:
    return ""

  # First property
  if S[-1] == T[-1]:
    return LCS(S[:-1], T[:-1]) + S[-1]

  # Second proprerty
  # Last of S not needed:
  result1 = LCS(S[:-1], T)
  # Last of T not needed
  result2 = LCS(S, T[:-1])
  if len(result1) > len(result2):
    return result1
  else:
    return result2

Longest Common Subsequence Python 2 function

2 Answers2