20 Dec
2007
20 Dec
'07
10:05 a.m.
Chris Kuklewicz wrote:
Unfortunately, permutations5 has an error:
*Main> permutations5 [1..3] [[1,2,3],[2,1,3],[3,2,1],[3,3,1],[3,2,2],[3,3,2]] *Main> permutations6 [1..3] [[1,2,3],[2,1,3],[3,2,1],[2,3,1],[3,1,2],[1,3,2]]
Oops, the "map (x:) ..." in interleave should be "map (y:) ...", permutations5 xs = xs : perms xs [[]] where perms [] ps = [] perms (x:xs) ps = concatMap interleave' ps ++ perms xs (concatMap interleave ps) where interleave [] = [[x]] interleave (y:ys) = (x:y:ys) : map (y:) (interleave ys) interleave' [] = [] interleave' (y:ys) = (x:y:ys++xs) : map (y:) (interleave' ys) Twan