# [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>
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>
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!

--
Felipe.

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