EXOR or symmetric difference for the Counter class

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Fri Aug 13 07:36:15 CEST 2010


On Thu, 12 Aug 2010 13:20:19 -0700, Paddy wrote:

> I find myself needing to calculate the difference between two Counters
> or multisets or bags.

Is this collections.Counter from Python 3.1? If so, you should say so, 
and if not, you should tell us which Counter class this is. It will save 
people (by which I mean *me*) from spending an unrewarding few minutes 
trying to import Counter in Python 2.5 and writing up a sarcastic 
response ... :)

 
> I want those items that are unique to each bag. I know how to calculate
> it:
> 
>     >>> b = Counter(a=1, b=2)
>     >>> c = Counter(a=3, b=1)
>     >>> diff = (b - c) + (c - b)
>     >>> (b - c)
>     Counter({'b': 1})
>     >>> (c - b)
>     Counter({'a': 2})
>     >>> diff
>     Counter({'a': 2, 'b': 1})
> 
> But thought why doesn't this operation appear already as a method of the
> class?

Don't know. Perhaps you should put in a feature request.



-- 
Steven



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