# Combinations of choosing n items from a list

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## Combinations of choosing n items from a list

 Hi all, This function took me a long time to write, getting my head around the double recursion. It returns a list of all possible sub-sets of a given length from a list. I have a couple of questions. 1. Can it be improved for efficiency or style? 2. Is there a library that already has this and other related functions? I assumed there would be but I couldn't find it on hoogle. combinations :: Int -> [a] -> [[a]] combinations _ [] = [] combinations 0 _ = [] combinations 1 x = map (:[]) x combinations n (x:xs) = (map (x:) \$ combinations (n-1) xs) ++ combinations n xs e.g. > combinations 3 [1..5] [[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5],[3,4,5]] Thanks, Peter
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## Combinations of choosing n items from a list

 On Sunday 13 November 2011, 23:42:25, Peter Hall wrote: > Hi all, > This function took me a long time to write, getting my head around the > double recursion. It returns a list of all possible sub-sets of a > given length from a list. > > I have a couple of questions. > > 1. Can it be improved for efficiency or style? > 2. Is there a library that already has this and other related > functions? I assumed there would be but I couldn't find it on hoogle. > > > combinations :: Int -> [a] -> [[a]] > combinations _ [] = [] > combinations 0 _ = [] That should be combinations 0 _ = [[]] since there is one sublist of length 0 of any list. And that equation should come before combinations _ [] = [] > combinations 1 x = map (:[]) x This equation is then superfluous. > combinations n (x:xs) = (map (x:) \$ combinations (n-1) xs) ++ > combinations n xs Some would prefer a list-comprehension here, combinations n (x:xs)     = [x:comb | comb <- combinations (n-1) xs]        ++ combinations n xs (indentation to prevent wrapping in mail-clients), but I would leave the decision to personal preference (well, I would use a list-comprehension, but I've no pains with the map. However, I submit also   map (x:) (combinations (n-1) xs) ++ combinations n xs for consideration). As for efficiency, yes, there's something you can do to dramatically improve performance, if you don't want to treat infinite lists (as it is, you'd only get sublists with identical (n-1)-long initial segment anyway, so support for infinite lists isn't ideal). Consider for example combinations 25 [1 .. 25] ~> [1:c | c <- ([2:d | d <- combinations 23 [3 .. 25]]                              ++ combinations 24 [3 .. 25])]          ++ combinations 25 [2 .. 25] For the second part, you go through all 2^24 sublists of [2 .. 25], only to find that all of them are too short. In the second part of the first part, you go through all 2^23 sublists of [3 .. 25] to find all of them too short. Continuing the splitting, you see that combinations 25 [1 .. 25] goes through all 2^25 sublists of [1 .. 25], finding all of them where an element was discarded anywhere too short. A lot of wasted work. If n > length xs, you know that xs has no sublists of length n, so you can abort the calculation immediately. Since calculating the length requires traversing the entire list, we'd rather not do that at each step, but carry the length as a parameter. combinations 0 _ = [[]] combinations _ [] = [] combinations n xs   | n < 0     = []   | n == 1    = map (:[]) xs  -- not needed, but a wee bit more efficient   | otherwise = helper n (length xs) xs     where       helper k l ys@(z:zs)         | k < l     = [z:ws | ws <- helper (k-1) (l-1) zs]                           ++ helper k (l-1) zs         | k == l    = [ys]         | otherwise = []   -- can only occur at the start Not so nice, but avoids a lot of futile work. The call to length prohibits infinite lists. You could, at some cost, treat infinite lists per combinations 0 _ = [[]] combinations _ [] = [] combinations n xs@(y:ys)   | n < 0     = []   | otherwise = case drop (n-1) xs of                   [ ] -> []                   [_] -> [xs]                   _   -> [y:c | c <- combinations (n-1) ys]                             ++ combinations n ys to get the same result as before for infinite lists. The finite case would be slower than the previous with the helper and length, but still avoid most of the futile work when n is close to length xs. > > e.g. > > > combinations 3 [1..5] > > [[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5] > ,[3,4,5]] > > Thanks, > Peter