[Python-Dev] Caching float(0.0)

Nick Maclaren nmm1 at cus.cam.ac.uk
Wed Oct 4 14:12:06 CEST 2006

On Wed, Oct 04, 2006 at 12:42:04AM -0400, Tim Peters wrote:
> > If C90 doesn't distinguish -0.0 and +0.0, how can Python?
> > Can you give a simple example where the difference between the two
> > is apparent to the Python programmer?
> Perhaps surprsingly, many (well, comparatively many, compared to none
> ....) people have noticed that the platform atan2 cares a lot:

Once upon a time, floating-point was used as an approximation to
mathematical real numbers, and anything which was mathematically
undefined in real arithmetic was regarded as an error in floating-
point.  This allowed a reasonable amount of numeric validation,
because the main remaining discrepancy was that floating-point
has only limited precision and range.

Most of the numerical experts that I know of still favour that
approach, and it is the one standardised by the ISO LIA-1, LIA-2
and LIA-3 standards for floating-point arithmetic.

atan2(0.0,0.0) should be an error.

But C99 differs.  While words do not fail me, they are inappropriate
for this mailing list :-(

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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