[Numpy-discussion] float('inf') // 1 = ?

[Sebastian Berg]

On Do, 2014-09-18 at 19:13 +0200, Sebastian Berg wrote:
> On Thu, 2014-09-18 at 18:55 +0200, Petr Viktorin wrote:
> > Sorry for the lack of subject before.
> > 
> > On Thu, Sep 18, 2014 at 6:48 PM, Benjamin Root <ben.root at ou.edu> wrote:
> > > My vote is that NumPy is correct here. I see no reason why
> > >>>> float('inf') / 1
> > > and
> > >>>> float('inf') // 1
> > >
> 
> Didn't read python ideas I have to admit (at least not enough to see
> many arguments). My biggest argument is that numpy does exactly this:
> 
> >>> import math
> >>> math.floor(float('inf')/1)
> 
> Maybe I am naive thinking that floordivide is basically a divide+floor
> operation, but arguing for NaN seems half arsed to me on first sight.
> Either it is `inf` and the equivalence Benjamin Root noted holds or you
> make it an error. NaN is not reasonable for an error return *especially*
> not for the standard lib (it *might* be for numpy).
> 

Ok, sorry, that was too quick. Since we already have a "special" value
like Inf, it is OK to not give an error in the standard lib, since
Inf-Inf is also NaN, etc.
However, I read the arguments as "Inf is not an integral number", and I
frankly don't understand that at all. Infinity isn't a real number as
well!?

If you represent the result as an IEEE float (integral or not) you can
use Inf as a valid result IMO. Arguably all of the limits:

floor(float) -> Inf
float // 1 -> Inf

exist for float -> Inf and can be represented. Also IEEE seems to define
the floor operation like this and I don't see a reason why to violate
`a//b == floor(a/b)`. As long as the result is represented as an IEEE
floating point, which knows Infinity, I argue that it is correct. If it
is not an IEEE floating point, it is an error in any case.

- Sebastian


> - Sebastian
> 
> > > should return different results.
> > 
> > I recommend reading the python-ideas thread; there are some arguments
> > for both sides.
> > 
> > > On Thu, Sep 18, 2014 at 12:31 PM, Petr Viktorin <encukou at gmail.com> wrote:
> > >>
> > >> Hello,
> > >> Over at Python-ideas, there is a thread [0] about the following
> > >> discrepancy:
> > >>
> > >> >>> numpy.array(float('inf')) // 1
> > >> inf
> > >> >>> float('inf') // 1
> > >> nan
> > >>
> > >> There are reasons for either result, but I believe it would be very
> > >> nice if either Python or Numpy changed, so they would give the same
> > >> value.
> > >> If any of you have reasons to defend Numpy's (or Python's) choice, or
> > >> otherwise want to chime in, please post there.
> > >>
> > >> Thanks,
> > >> Petr Viktorin
> > >>
> > >>
> > >> [0]
> > >> https://mail.python.org/pipermail/python-ideas/2014-September/029365.html
> > >> _______________________________________________
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> > >
> > >
> > >
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> 
> 
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