"Jim Jewett" <jimjjewett at gmail.com> wrote:
>
> >> For 0: hash(+0.0)==hash(-0.0)==hash(0)=hash(0L)=0
>
> > Unfortunately, that assumes that equality is transitive.
>
> No, but the (transitively closed set of equivalent objects) must have
> the same hash. ...
Er, how do you have a transitive closure for a non-transitive operation?
I really do mean that quite a lot of floating-point bells and whistles
are non-transitive. The only one most people will have come across is
IEEE NaNs, where 'a is b' does not imply 'a == b', but there are a
lot of others (and have been since time immemorial). I don't THINK
that IEEE 754R decimal introduces any, though I am not prepared to
bet on it.
> > let us say that I am implementing a special function and want to
> > distinguish -0.0 and +0.0. Why can't I use a dictionary?
>
> Because they are equal. They aren't identical, but they are equal.
You have missed my point, which is extended floating-points effectively
downgrade the status of the purely numeric comparisons, and therefore
introduce a reasonable requirement for using a tighter match. Note
that I am merely commenting that this needs bearing in mind, and NOT
that anything should be changed.
> > >>> a = float("+0.0")
> > >>> b = float("-0.0")
> > >>> print a, b
> > 0.0 -0.0
>
> With the standard windows distribution, I get just
>
> 0.0 0.0
Watch that space :-) Expect it to change.
Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email: nmm1 at cam.ac.uk
Tel.: +44 1223 334761 Fax: +44 1223 334679
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