Difference between revisions of "Talk:99 questions/Solutions/23"

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(Created page with "The more elegant solution using randomRs does not save the new random state and is thus dangerous to use if randomness is important.")
 
(→‎some notes: new section)
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The more elegant solution using randomRs does not save the new random state and is thus dangerous to use if randomness is important.
  
The more elegant solution using randomRs does not save the new random state and is thus dangerous to use if randomness is important.
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== some notes ==
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the signature of removeAt is:
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removeAt :: Int -> [a] -> (a, [a])
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so we have to exchange the orders of parameters in the calls of remove at from:
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removeAt l (k+1)
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to:
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removeAt (k+1) l
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and we have to extract the remaining list from the result as well:
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snd $ removeAt (k+1) l
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so the full version should be:
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rnd_select2 :: RandomGen g => [a] -> Int -> g -> ([a], g)
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rnd_select2 _ 0 gen = ([], gen)
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rnd_select2 [] _ gen = ([], gen)
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rnd_select2 l count gen
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| count == (length l) = (l, gen)
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| otherwise = rnd_select2 ('''snd $ removeAt (k+1) l''') count gen'
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where (k, gen') =
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randomR (0, (length l) - 1) gen
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rnd_selectIO :: [a] -> Int -> IO [a]
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rnd_selectIO l count = getStdRandom $ rnd_select2 l count

Revision as of 04:11, 27 July 2017

The more elegant solution using randomRs does not save the new random state and is thus dangerous to use if randomness is important.

some notes

the signature of removeAt is: 

   removeAt :: Int -> [a] -> (a, [a])

so we have to exchange the orders of parameters in the calls of remove at from: 

   removeAt l (k+1)

to: 

   removeAt (k+1) l

and we have to extract the remaining list from the result as well: 

   snd $ removeAt (k+1) l

so the full version should be: 

   rnd_select2 :: RandomGen g => [a] -> Int -> g -> ([a], g)
   rnd_select2 _ 0 gen = ([], gen)
   rnd_select2 [] _ gen = ([], gen)
   rnd_select2 l count gen
      | count == (length l) = (l, gen)
      | otherwise           =  rnd_select2 (snd $ removeAt (k+1) l) count gen'
                           where (k, gen') =
                                   randomR (0, (length l) - 1) gen

   rnd_selectIO :: [a] -> Int -> IO [a]
   rnd_selectIO l count = getStdRandom $ rnd_select2 l count
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