[Haskell-cafe] Efficient functional idiom for histogram

[Paul Moore]

2009/7/31 Gregory Collins <greg at gregorycollins.net>:
> 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
>      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.