On Tue, Aug 4, 2009 at 9:23 PM, Daniel Fischer<[email protected]> wrote: > Am Dienstag 04 August 2009 19:48:25 schrieb Slavomir Kaslev: >> A friend mine, new to functional programming, was entertaining himself by >> writing different combinatorial algorithms in Haskell. He asked me for some >> help so I sent him my quick and dirty solutions for generating variations >> and >> >> permutations: >> > inter x [] = [[x]] >> > inter x yys@(y:ys) = [x:yys] ++ map (y:) (inter x ys) >> > >> > perm [] = [[]] >> > perm (x:xs) = concatMap (inter x) (perm xs) >> > >> > vari 0 _ = [[]] >> > vari _ [] = [] >> > vari k (x:xs) = concatMap (inter x) (vari (k-1) xs) ++ vari k xs >> >> After that I found out that nowadays there is a permutation function in the >> >> Data.List module: >> > permutations :: [a] -> [[a]] >> > permutations xs0 = xs0 : perms xs0 [] >> > where >> > perms [] _ = [] >> > perms (t:ts) is = foldr interleave (perms ts (t:is)) (permutations >> > is) where interleave xs r = let (_,zs) = interleave' id xs r in zs >> > interleave' _ [] r = (ts, r) >> > interleave' f (y:ys) r = let (us,zs) = interleave' (f . (y:)) >> > ys r in (y:us, f (t:y:us) : zs) >> >> I was surprised to find that not only my version is much simpler from the >> one in Data.List but it also performs better. Here are some numbers from my >> rather old ghc 6.8.1 running ubuntu on my box: >> >> *Main> length $ permutations [1..10] >> 3628800 >> (10.80 secs, 2391647384 bytes) >> *Main> length $ perm [1..10] >> 3628800 >> (8.58 secs, 3156902672 bytes) > > But you compare *interpreted* code here, that's not what counts. > > Prelude Perms> length $ perm [1 .. 10] > 3628800 > (1.20 secs, 1259105892 bytes) > Prelude Perms> length $ permutations [1 .. 10] > 3628800 > (0.56 secs, 551532668 bytes) > Prelude Perms> length $ perm [1 .. 11] > 39916800 > (13.18 secs, 14651808004 bytes) > Prelude Perms> length $ permutations [1 .. 11] > 39916800 > (4.30 secs, 5953485728 bytes)
Which version of ghc are you testing on? I guess, it's more recent than mine. > Apparently the library code is more amenable to the optimiser (note that the > actual > library is faster still: > > Prelude Data.List> length $ permutations [1 .. 10] > 3628800 > (0.49 secs, 551532812 bytes) > Prelude Data.List> length $ permutations [1 .. 11] > 39916800 > (3.73 secs, 5953485816 bytes) > > I have no idea why). Probably because it's compiled (and not interpreted) in this case. -- Slavomir Kaslev _______________________________________________ Glasgow-haskell-users mailing list [email protected] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
