Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints

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 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 *wrote:

From: Felipe Almeida Lessa Subject: Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints To: "michael rice" Cc: haskell-cafe@haskell.org Date: Monday, June 13, 2011, 9:38 PM

On Mon, Jun 13, 2011 at 8:56 PM, michael rice 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/Contr...

-- Felipe.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe