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Haskell 99 Problems - Problem 23

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problem 23, Extract a given number of randomly selected elements from a list.
Example:

Prelude System.Random>rnd_select "abcdefgh" 3 >>= putStrLn

"eda"

Two problems: 1) How to return a list, and 2) how to sample without duplication

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  1. -- ah, randomization. Always fun in Haskell.
  2. -- this selects only one letter. Not what we want...
  3. rnd_select_one :: [a] -> IO [a]
  4. rnd_select_one xs = do
  5.     let len = (length xs) - 1
  6.     n <- randomRIO (0, len)
  7.     return [(xs!!n)]
  8.  
  9. -- this version allows resampling. 
  10. rnd_select :: [a] -> Int -> IO [a]
  11. rnd_select xs n = do
  12.     g <- sequence (replicate n (rnd_select_one xs))
  13.     return (concat g)
  14.  
  15. -- Here's how to prevent re-sampling, using partition from Data.List
  16. -- like so: 
  17. -- partition (`elem` "g") "abcdefghijklmn"
  18. --  --> ("g", "abcdefhijklmn")
  19. --
  20. -- It would also be fairly trivial to use removeAt from Problem 20
  21.  
  22.  
  23. rnd_select_one' :: Eq a => ([a], [a]) -> Int -> IO ([a], [a])
  24. rnd_select_one' (acc, xs) 0 = return (acc, xs)
  25. rnd_select_one' (acc, []) _ = return (acc, [])
  26. rnd_select_one' (acc, xs) n = do
  27.     let len = (length xs) - 1
  28.     d <- randomRIO (0, len)
  29.     let el = (xs!!d)
  30.     let (g, ys) = partition (`elem` [el]) xs
  31.     let ret = (acc ++ g, ys)
  32.     rnd_select_one' ret (n-1) 
  33.  
  34. rnd_select' :: Eq a => [a] -> Int -> IO [a]
  35. rnd_select' xs n = do
  36.     j <- rnd_select_one' ([], xs) n
  37.     return $ fst j

URL: http://haskell.org/haskellwiki/99_questions/21_to_28

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