[Numpy-discussion] Bug in logaddexp2.reduce

[Charles R Harris]

On Wed, Mar 31, 2010 at 11:46 PM, Anne Archibald
<peridot.faceted at gmail.com>wrote:

> On 1 April 2010 01:40, Charles R Harris <charlesr.harris at gmail.com> wrote:
> >
> >
> > On Wed, Mar 31, 2010 at 11:25 PM, <josef.pktd at gmail.com> wrote:
> >>
> >> On Thu, Apr 1, 2010 at 1:22 AM,  <josef.pktd at gmail.com> wrote:
> >> > On Thu, Apr 1, 2010 at 1:17 AM, Charles R Harris
> >> > <charlesr.harris at gmail.com> wrote:
> >> >>
> >> >>
> >> >> On Wed, Mar 31, 2010 at 6:08 PM, <josef.pktd at gmail.com> wrote:
> >> >>>
> >> >>> On Wed, Mar 31, 2010 at 7:37 PM, Warren Weckesser
> >> >>> <warren.weckesser at enthought.com> wrote:
> >> >>> > T J wrote:
> >> >>> >> On Wed, Mar 31, 2010 at 1:21 PM, Charles R Harris
> >> >>> >> <charlesr.harris at gmail.com> wrote:
> >> >>> >>
> >> >>> >>> Looks like roundoff error.
> >> >>> >>>
> >> >>> >>>
> >> >>> >>
> >> >>> >> So this is "expected" behavior?
> >> >>> >>
> >> >>> >> In [1]: np.logaddexp2(-1.5849625007211563, -53.584962500721154)
> >> >>> >> Out[1]: -1.5849625007211561
> >> >>> >>
> >> >>> >> In [2]: np.logaddexp2(-0.5849625007211563, -53.584962500721154)
> >> >>> >> Out[2]: nan
> >> >>> >>
> >> >>> >
> >> >>> > Is any able to reproduce this?  I don't get 'nan' in either 1.4.0
> or
> >> >>> > 2.0.0.dev8313 (32 bit Mac OSX).  In an earlier email T J reported
> >> >>> > using
> >> >>> > 1.5.0.dev8106.
> >> >>>
> >> >>>
> >> >>>
> >> >>> >>> np.logaddexp2(-0.5849625007211563, -53.584962500721154)
> >> >>> nan
> >> >>> >>> np.logaddexp2(-1.5849625007211563, -53.584962500721154)
> >> >>> -1.5849625007211561
> >> >>>
> >> >>> >>> np.version.version
> >> >>> '1.4.0'
> >> >>>
> >> >>> WindowsXP 32
> >> >>>
> >> >>
> >> >> What compiler? Mingw?
> >> >
> >> > yes, mingw 3.4.5. , official binaries release 1.4.0 by David
> >>
> >> sse2 Pentium M
> >>
> >
> > Can you try the exp2/log2 functions with the problem data and see if
> > something goes wrong?
>
> Works fine for me.
>
> If it helps clarify things, the difference between the two problem
> input values is exactly 53 (and that's what logaddexp2 does an exp2
> of); so I can provide a simpler example:
>
> In [23]: np.logaddexp2(0, -53)
> Out[23]: nan
>
> Of course, for me it fails in both orders.
>
>
Ah, that's progress then ;) The effective number of bits in a double is 53
(52 + implicit bit). That shouldn't cause problems but it sure looks
suspicious.

Chuck
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