On 10/25/05, Jon Fairbairn
Well, you can do this:
import Array
calc::String->[(Char, Int)]
calc = filter (\p-> snd p > 0) . assocs . accumArray (+) 0 (minBound, maxBound) . (map (\x->(x,1)))
You can import whatever sort of array you want, so long as it has assocs and accumArray. You don't want to change minBound and maxBound to toEnum x, since they are the bounds of Char, which might or might not be 0 and 255.
Is there anything wrong with: import Data.List count :: (Ord a) => [a] -> [(a, Int)] count = map (\x -> (head x, length x)) . group . sort Unless I'm missing something, generating and filtering a list like \NUL..\111411 (which is what calc does on my machine) is going to be more expensive than playing around with lists for all but the longest strings. As far as I can tell, count is O(n log n) in the length of the string whereas calc has such a massive constant (producing and filtering the list of \NUL..\111411, most of which will be 0's), that it will overwhelm the cost of the counting for most strings. Cheers, Thomas Sutton