Raymond,
I think you made a typographical error in your Counter.update example.
>>> from collections import Counter
>>> c = Counter(a=4, b=2, c=0, d=-2)
>>> d = Counter(a=1, b=-5, c=-2, d=6)
>>> c.update(d)
>>> c
Counter({'a': 5, 'd': 4, 'c': -2, 'b': -3})
Pair programming ;-)
On Tue, Nov 24, 2015 at 1:02 AM Raymond Hettinger <
raymond.hettin...@gmail.com> wrote:
>
> > On Nov 23, 2015, at 10:43 AM, Vlastimil Brom <vlastimil.b...@gmail.com>
> wrote:
> >
> >> Is there any particular reason counters drop negative values when you
> add
> >> them together? I definitely expected them to act like ints do when you
> add
> >> negatives, and had to subclass it to get what I think is the obvious
> >> behavior.
> >> _______________________________________________
> >> Python-Dev mailing list
> > ...
> > Hi,
> > this is probably more appropriate for the general python list rathere
> > then this developers' maillist, however, as I asked a similar question
> > some time ago, I got some detailed explanations for the the current
> > design decissions from the original developer; cf.:
> > https://mail.python.org/pipermail/python-list/2010-March/570618.html
> >
> > (I didn't check possible changes in Counter since that version (3.1 at
> > that time).)
>
> In Python3.2, Counter grew a subtract() method:
>
> >>> c = Counter(a=4, b=2, c=0, d=-2)
> >>> d = Counter(a=1, b=2, c=3, d=4)
> >>> c.subtract(d)
> >>> c
> Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6})
>
> The update() method has been around since the beginning:
>
> >>> from collections import Counter
> >>> c = Counter(a=4, b=2, c=0, d=-2)
> >>> d = Counter(a=1, b=-5, c=-2, d=6)
> >>> c.update(d)
> >>> d
> Counter({'d': 6, 'a': 1, 'c': -2, 'b': -5})
>
>
> So, you have two ways of doing counter math:
>
> 1. Normal integer arithmetic using update() and subtract() does straight
> addition and subtraction, either starting with or ending-up with negative
> values.
>
> 2. Saturating arithmetic using the operators: + - & | excludes
> non-positive results. This supports bag-like behavior (c.f. smalltalk) and
> multiset operations (https://en.wikipedia.org/wiki/Multiset).
>
>
> Raymond
>
>
>
>
>
>
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