2009/7/31 Gregory Collins <g...@gregorycollins.net>: > Paul Moore <p.f.mo...@gmail.com> writes: > >> How would I efficiently write a function in Haskell to count >> occurrences of unique elements in a (potentially very large) list? For >> example, given the list [1,2,3,4,5,3,4,2,4] I would like the output >> [[1,1], [2,2], [3,2], [4,3], [5,1]] (or some equivalent >> representation). > > import qualified Data.Map as Map > import Data.Map (Map) > > histogram :: Ord a => [a] -> [(a,Int)] > histogram = Map.assocs . foldl f Map.empty > where > f m k = Map.insertWith (+) k 1 m
Right. I see how that works, and can work out how to think about this sort of thing from your example. Thanks very much. BTW, I did know that Haskell had an efficient map implementation, I just wasn't sure how to use it "functionally" - I probably should have searched a bit harder for examples before posting. Thanks for the help in any case. Paul. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
