[Python-Dev] Rounding float to int directly ...

[Nick Maclaren]

Aahz <aahz at pythoncraft.com> wrote:
> On Tue, Aug 01, 2006, M.-A. Lemburg wrote:
> >
> > You often have a need for controlled rounding when doing financial
> > calculations or in situations where you want to compare two floats
> > with a given accuracy, e.g. to work around rounding problems ;-)
> >
> > The usual approach is to use full float accuracy throughout the
> > calculation and then apply rounding a certain key places.
> 
> That's what Decimal() is for.

Well, maybe.  There are other approaches, too, and Decimal has its
problems with that.  In particular, when people need precisely
defined decimal rounding, they ALWAYS need fixed-point and not
floating-point.

> (Note that I don't care all that much about round(), but I do think we
> want to canonicalize Decimal() for all purposes in standard Python where
> people care about accuracy.  Additional float features can go into
> NumPy.)

Really?  That sounds like dogma, not science.

Decimal doesn't even help people who care about accuracy.  At most
(and with the reservation mentioned above), it means that you can
can map external decimal formats to internal ones without loss of
precision.  Not a big deal, as there ARE no requirements for doing
that for floating-point, and there are plenty of other solutions for
fixed-point.

People who care about the accuracy of calculations prefer binary,
as it is a more accurate model.  That isn't a big deal, either.


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