Re: [Haskell-cafe] Efficient functional idiom for histogram

31 Jul 2009


      Gregory Collins <greg@gregorycollins.net> writes:
...
Paul Moore <p.f.moore@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
Here "foldl" should be "foldl'" (d'oh!)

G
-- 
Gregory Collins <greg@gregorycollins.net>