Hrounding error

cooperq at gmail.com cooperq at gmail.com
Wed Jun 18 18:26:04 CEST 2008


On Jun 18, 8:02 am, "Jerry Hill" <malaclyp... at gmail.com> wrote:
> On Wed, Jun 18, 2008 at 1:47 AM,  <coop... at gmail.com> wrote:
> >>>> 234 - 23234.2345
> > -23000.234499999999
>
> > This is not correct by my calculations.
>
> Python floating point operations use the underlying C floating point
> libraries which, in turn, usually rely on the hardware's floating
> point implementations.  This Wikipedia article talks about how those
> values are usually stored and manipulated:http://en.wikipedia.org/wiki/IEEE_floating-point_standard
>
> So, which IEEE double precision floating point value would you like
> instead?  As far as I understand it, these are your two choices:
>
> 'ba490c020f76d6c0' = -23000.234499999999
> 'bb490c020f76d6c0' = -23000.234500000002
>
> Alternatively, investigate the Decimal module, but keep in mind that
> it can have the same sorts of issues, just on different numbers.
> Compare, for instance:
>
> >>> (1.0/3.0) * 3.0
>
> 1.0
>
> >>> from decimal import Decimal
> >>> ( Decimal('1.0')/Decimal('3.0') ) * Decimal('3.0')
>
> Decimal("0.9999999999999999999999999999")
>
>
>
> --
> Jerry

So it seems then that python might not be very good for doing
precision floating point work, because there is a good chance its
floating points will be off by a (very small) amount?  Or is there a
way to get around this and be guaranteed an accurate answer?



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