[Haskell-cafe] Efficient functional idiom for histogram

Gregory Collins greg at gregorycollins.net
Fri Jul 31 16:52:10 EDT 2009

Paul Moore <p.f.moore at 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
        f m k = Map.insertWith (+) k 1 m

Gregory Collins <greg at gregorycollins.net>

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