# 99 questions/Solutions/25

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< 99 questions | Solutions(Difference between revisions)

Line 18: | Line 18: | ||

rest <- rnd_permu xs | rest <- rnd_permu xs | ||

return $ let (ys,zs) = splitAt rand rest | return $ let (ys,zs) = splitAt rand rest | ||

− | in ys++(x:zs) | + | 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 | ||

</haskell> | </haskell> |

## Revision as of 06:31, 25 November 2010

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