EXOR or symmetric difference for the Counter class

Paddy paddy3118 at googlemail.com
Mon Aug 16 19:21:12 CEST 2010


On 14 Aug, 18:14, Raymond Hettinger <pyt... at rcn.com> wrote:
> On Aug 12, 1:20 pm, Paddy <paddy3... at googlemail.com> wrote:
>
> > I find myself needing to calculate the difference between two Counters
> > or multisets or bags.
>
> > I want those items that are unique to each bag.
>
> Tell us about your use cases.  I'm curious how a program would ascribe
> semantic meaning to the result.  The phrase "unique to each bag"
> doesn't quite cover it, perhaps something like "number in either
> source above the minimum held in common".
>
> AFAICT, I've never needed something like this as a primitive.  Even
> the xor operation for regular sets is rarely used.
>
> > I know how to
> > calculate it:
>
> >     >>> b = Counter(a=1, b=2)
> >     >>> c = Counter(a=3, b=1)
> >     >>> diff = (b - c) + (c - b)
> >     >>> diff
> >     Counter({'a': 2, 'b': 1})
>
> That seems simple enough.
> You could also use:
>
>  diff = (b | c) - (b & c)   # max(b,c) - min(b,c)
>
> Raymond

Hi Raymond and others,

Lets say you have two *sets* of integers representing two near-copies
of some system, then a measure of their difference could be calculated
as:

len(X.symmetric_difference(Y)) / (len(X) + len(Y)) * 100 %

If the two collections of integers are allowed duplicates then you
need a Counter/bag/multi-set type and the diff calculation I gave
originally.

Thanks.



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