Re: [Haskell-cafe] how can I select all the 3-element-combination out of a list efficiently

Jules Bean wrote:

In the spirit of multiple implementations; another approach is to note that you're really asking for all 3-element sublists:

power [] = [[]] power (x:xs) = power xs ++ map (x:) (power xs)

triples1' l = [ t | t <- power l, length t == 3]

(this implementation also preserves sorting)

...but is exponentially slower than necessary, and fails on infinite lists. Try this one: sublistsN 0 _ = [[]] sublistsN n (x:xs) = map (x:) (sublistsN (n-1) xs) ++ sublistsN n xs sublistsN _ _ = [] triples = sublistsN 3 BR, -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---