[Numpy-discussion] Different results from repeated calculation

Charles R Harris charlesr.harris at gmail.com
Mon Jan 29 13:02:50 CST 2007


On 1/29/07, Keith Goodman <kwgoodman at gmail.com> wrote:
>
> On 1/27/07, Charles R Harris <charlesr.harris at gmail.com> wrote:
>
> >  Oddly, the relative error is always the same:
> >
> > 98 z different 2.0494565872e-16
> > 99 z different 2.0494565872e-16
> >
> > Which is nearly the same as the double precision 2.2204460492503131e-16,
> the
> > difference being due to the fact that the precision is defined relative
> to
> > 1, and the error in the computation are in a number relatively larger
> (more
> > bits set, but not yet 2).
>
> Fernando (Ubuntu) and I (Debian) get a different result
> (8.47032947254e-22) than you (Fedora) do (2.0494565872e-16).



That's odd, the LSB bit of the  double precision mantissa is only about
2.2e-16, so  you can't *get* differences as  small as 8.4e-22 without about
70 bit mantissa's. Hmmm, and extended double precision only has 63 bit
mantissa's. Are you sure you are computing the error correctly?

Chuck
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