From: Jonas Almström Duregård <jonas.duregard@chalmers.se>
Subject: Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints
To: "michael rice" <nowgate@yahoo.com>
Cc: "Felipe Almeida Lessa" <felipe.lessa@gmail.com>, haskell-cafe@haskell.org
Date: Tuesday, June 14, 2011, 5:17 AM

> Shuffle [1..20], then take 5?

> Yes, so simple, I'm embarrassed I didn't think of it.

That works well for small numbers, but I'm guessing it will evaluate the entire list so it should not be used for large inputs. If you have a large interval and use a relatively small part of it, the following function should be significantly faster (it builds a random permutation lazily):  

import System.Random

randomOrder :: (Ord a, Num a, Random a, RandomGen g) => (a,a) -> g -> [a]

randomOrder (low,high) g 

 | low > high  = []

 | otherwise   = let

   (a,g') = randomR (low,high) g

   (gl,gr) = split g'

   in a : mergeRandom (a-1-low,randomOrder (low,a-1) gl) 

                      (high-a-1, randomOrder (a+1,high) gr) g'

  where

    mergeRandom (_,[]) (_,xs) _       = xs

    mergeRandom (_,xs) (_,[]) _       = xs

    mergeRandom (lx,x:xs) (ly,y:ys) g = let

      (pick,g') = randomR (1,lx + ly) g

      in if pick <= lx 

        then x : mergeRandom (lx-1,xs) (ly,y:ys) g'

        else y : mergeRandom (lx,x:xs) (ly-1,ys) g'

On 14 June 2011 04:31, michael rice <nowgate@yahoo.com> wrote:

Thanks, all. It seemed like something like this should exist in a prob/stat package, and if so, didn't want to reinvent the wheel. Shuffle [1..20], then take 5? Yes, so simple, I'm embarrassed I didn't think of it. Michael --- On Mon, 6/13/11, Felipe Almeida Lessa <felipe.lessa@gmail.com> wrote: From: Felipe Almeida Lessa <felipe.lessa@gmail.com> Subject: Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints To: "michael rice" <nowgate@yahoo.com> Cc: haskell-cafe@haskell.org Date: Monday, June 13, 2011, 9:38 PM On Mon, Jun 13, 2011 at 8:56 PM, michael rice <nowgate@yahoo.com> wrote: > Is there an (existing) way to select 5 Ints randomly (no duplicates) from a population, say 1-20 (inclusive)? Yes, already implemented in the monte-carlo package as sampleSubset [1],   sampleSubset :: MonadMC m => [a] -> Int -> m [a] Complete example code for your example:   evalMC (sampleSubset [1..20] 5) (mt19937 0) Cheers! [1] http://hackage.haskell.org/packages/archive/monte-carlo/0.4.1/doc/html/Control-Monad-MC-Class.html#v:sampleSubset -- Felipe.

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