[Haskell-beginners] List -> Map -> List, to add duplicates?
Jonas Almström Duregård
jonas.duregard at chalmers.se
Tue Jun 7 13:47:39 CEST 2011
Is this what you want? (note that it does not preserve ordering, but neither
noDups :: (Ord b, Num a) => [(a,b)] -> [(a,b)]
noDups = map sumGroup . groupBy ((==) `on` snd) . sortBy (compare `on` snd)
sumGroup xs = (sum $ map fst xs, snd $ head xs)
If you use the fact that the list has at most one value per currency,
perhaps you should just keep the values in a map instead.
On 7 June 2011 03:43, Tom Murphy <amindfv at gmail.com> wrote:
> Hi All,
> This seems like an inefficient way to do what I'm trying to do.
> I'd really appreciate any suggestions or comments:
> import qualified Data.Map as Map
> data Currency = Dollar
> | Yen
> | XP
> | Health
> | Street_Cred
> | Peso
> deriving (Show, Eq, Ord) -- why ord?
> withDups = [(30, Dollar), (-20, Street_Cred), (-2, Dollar), (30, XP),
> (15, Peso), (30, XP)]
> flipAssoc (a, b) = (b, a)
> noDups = Map.fromListWith (+) (map flipAssoc withDups)
> final = map flipAssoc $ Map.toList noDups
> Thanks for your time!
> Beginners mailing list
> Beginners at haskell.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Beginners