[Haskell-cafe] Re: Grouping - Map / Reduce

GüŸnther Schmidt gue.schmidt at web.de
Tue Mar 24 21:20:10 EDT 2009

Hi Ketil,

Ketil Malde schrieb:
> "Gü?nther Schmidt" <gue.schmidt at web.de> writes:
>> let say I got an unordered lazy list of key/value pairs like
>> [('a', 99), ('x', 42), ('a', 33) ... ]
>> and I need to sum up all the values with the same keys.
>> So far I wrote a naive implementation, using Data.Map, foldl and insertWith.
> Data.Map.fromListWith (+)
>> The building of this map is of course a bottleneck, the successive
>> processing needs to wait until the entire list is eventually consumed
>> the Map is built and flattened again.
> Sure this is not an artifact of the laziness of foldl?

well I can't really see how the map could be consumed *while* it's still 
being built, I just don't see it. (I'm using foldl' and insertWith', sry 
for not saying so initially).

>> Is there another way of doing this, something more "streaming
>> architecture" like?
> I don't see how you can do this much better - for a small, fixed set
> of keys, you could use an (STU) array for the sums, but it depends if
> the added complexity is worth it.  You're already doing a single pass
> over the data.
> -k

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