[Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints
michael rice
nowgate at yahoo.com
Tue Jun 14 13:35:51 CEST 2011
I had the same thought, since my interval [1..10000] is rather large.
Thanks,
MIchael
--- On Tue, 6/14/11, Jonas Almström Duregård <jonas.duregard at chalmers.se> wrote:
From: Jonas Almström Duregård <jonas.duregard at 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 at yahoo.com>
Cc: "Felipe Almeida Lessa" <felipe.lessa at gmail.com>, haskell-cafe at 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 at 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 at gmail.com> wrote:
From: Felipe Almeida Lessa <felipe.lessa at 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 at yahoo.com>
Cc: haskell-cafe at haskell.org
Date: Monday, June 13, 2011, 9:38 PM
On Mon, Jun 13, 2011 at 8:56 PM, michael rice <nowgate at 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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