# Difference between revisions of "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
```

Or we can use the `permutations` function from `Data.List`:

```import System.Random (getStdGen, randomRIO)
import Data.List (permutations)

rndElem :: [a] -> IO a
rndElem xs = do
gen <- getStdGen
index <- randomRIO (0, length xs - 1)
return \$ xs !! index

rndPermutation :: [a] -> IO [a]
rndPermutation xs = rndElem . permutations \$ xs
```

WARNING: this may choke long lists