Re: [Haskell-beginners] [Haskell-cafe] select :: [(Float, a)] -> a -- Weighted stochastic selection - help?

Am Freitag, 5. September 2008 18:54 schrieb Tim Millea:

I am already using the following function

pickOne xs g = (xs !! r, g') where (r, g') = Random.randomR (0, length xs - 1) g

but I need to weight the items in the list, e.g. [(0.1, 'a') , (0.9, 'b')] for stochastic selection in a purely functional way, i.e. no monads. Any ideas welcome.

Tim.

You could pair each item with the cumulative probability of all items up to and including it, that would be [(0.1,'a'),(1.0,'b')] in the above example, then pickOne prs g = (snd p,g') where (r,g') = Random.randomR (0,1.0) (smll,lrge) = span ((< r) . fst) prs p = case lrge of (x:_) -> x [] -> last smll