99 questions/Solutions/25

Generate a random permutation of the elements of a list.

rnd_permu xs = diff_select' (length xs) xs

Uses the solution for the previous problem. Choosing N distinct elements from a list of length N will yield a permutation.

Or we can generate the permutation recursively:

import System.Random (randomRIO)

rnd_permu :: [a] -> IO [a]
rnd_permu []     = return []
rnd_permu (x:xs) = do
    rand <- randomRIO (0, (length xs))
    rest <- rnd_permu xs
    return $ let (ys,zs) = splitAt rand rest
             in ys++(x:zs)

rnd_permu' [] = return []
rnd_permu' xs = do
    rand <- randomRIO (0, (length xs)-1)
    rest <- let (ys,(_:zs)) = splitAt rand xs
            in rnd_permu' $ ys ++ zs
    return $ (xs!!rand):rest