[Numpy-discussion] Different results from repeated calculation

Keith Goodman kwgoodman at gmail.com
Mon Jan 29 13:33:13 CST 2007


On 1/29/07, Keith Goodman <kwgoodman at gmail.com> wrote:
> On 1/29/07, Charles R Harris <charlesr.harris at gmail.com> wrote:
>
> > 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?
>
> That is odd.
>
> 8.4e-22 is just the output of the test script: abs(z - z0).max(). That
> abs is from python.

By playing around with x and y I can get all sorts of values for abs(z
- z0).max(). I can get down to the e-23 range and to 2.2e-16. I've
also seen e-18 and e-22.


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