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

geniusfat wrote:

hi dear haskell lover ;) what I want to do is simply this: select3 :: [a] -> [(a, a, a)] and how can it be done efficiently? thanks in advance!

If, given [1,2,3,4,5,6,7,8,9,10,11,12] you want [(1,2,3),(4,5,6),(7,8,9)....] then: map (take 3) . iterate (drop 3) is very nearly what you need. Two problems: (a) it gives you [[1,2,3],[4,5,6]..] instead (b) it carries on with an infinite number of [] empty lists you can fix both of these: map (\[a,b,c]->(a,b,c)) . takeWhile (not.null) . map (take 3) . iterate (drop 3) Prelude> map (\[a,b,c] -> (a,b,c)) . takeWhile (not.null) . map (take 3) . iterate (drop 3) $ [1..12] [(1,2,3),(4,5,6),(7,8,9),(10,11,12)] Hope that helps. Jules