subsequences of length n from list performance

Question

I implemented a version of this answer https://stackoverflow.com/a/9920425/1261166 (I don\'t know what was intended by the person answering)

sublistofsize 0          


        

Answer 1

An optimization which should help is to keep track of whether there are enough elements in the list to form the rest of the subsequence. This can be done very efficiently by keeping track of a pointer which is n-1-elements ahead of xs and advancing them both as you recurse.

An implementation:

  nthtail 0 xs = xs
  nthtail _ [] = []
  nthtail n (x:xs) = nthtail (n-1) xs

  subseq 0 _ = [[]]
  subseq n xs =
    if null t
      then []
      else go n xs t
    where
      t = nthtail (n-1) xs  -- n should always be >= 1 here
      go 0 _ _  =  [[]]
      go _ _ [] = []
      go n xs@(x:xt) t = withx ++ withoutx
        where withx = map (x:) $ go (n-1) xt t
              withoutx = go n xt (tail t)