Haskell - Is there a standard function to provide a count of each item in a list?

Question

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Counting unique elements in a list
Count frequency of each element in a list

I tried Google, Hoogle, and here and didn't see anything obvious.

It should take ['a', 'b', 'e', 'c', 'e', 'a', 'e'] and return [('a', 2), ('b', 1), ('c', 1), ('e', 3)] or something to that effect.

score 6 · Answer 1 · edited Dec 07 '12 at 17:00

6

I would use a Map for counting:

import qualified Data.Map as M

countElems :: (Ord a) => [a] -> M.Map a Int
countElems = M.fromListWith (+) . flip zip (repeat 1)

edited Dec 07 '12 at 17:00

huon

answered Dec 07 '12 at 08:13

ertes

1

Every time this question is asked (and I see it all the time), there is this solution and the `group . sort` based solution. I prefer this one for basically no good reason. – luqui Dec 07 '12 at 10:18

score 2 · Answer 2 · edited Dec 07 '12 at 10:18

2

No. You can use e.g.

import Data.List
import Control.Arrow

map (head &&& length) $ group $ sort ['a', 'b', 'e', 'c', 'e', 'a', 'e']

Or consider using a MultiSet.

edited Dec 07 '12 at 10:18

luqui

answered Dec 07 '12 at 07:50

Landei

2 Answers2