[Numpy-discussion] Bug in logaddexp2.reduce

[Anne Archibald]

On 31 March 2010 16:21, Charles R Harris <charlesr.harris at gmail.com> wrote:
>
>
> On Wed, Mar 31, 2010 at 11:38 AM, T J <tjhnson at gmail.com> wrote:
>>
>> On Wed, Mar 31, 2010 at 10:30 AM, T J <tjhnson at gmail.com> wrote:
>> > Hi,
>> >
>> > I'm getting some strange behavior with logaddexp2.reduce:
>> >
>> > from itertools import permutations
>> > import numpy as np
>> > x = np.array([-53.584962500721154, -1.5849625007211563,
>> > -0.5849625007211563])
>> > for p in permutations([0,1,2]):
>> >    print p, np.logaddexp2.reduce(x[list(p)])
>> >
>> > Essentially, the result depends on the order of the array...and we get
>> > nans in the "bad" orders.  Likely, this also affects logaddexp.
>> >
>>
>> Sorry, forgot version information:
>>
>> $ python -c "import numpy;print numpy.__version__"
>> 1.5.0.dev8106
>> __
>
> Looks like roundoff error.

No. The whole point of logaddexp and logaddexp2 is that they function
correctly - in particular, avoid unnecessary underflow and overflow -
in this sort of situation. This is a genuine problem.

I see it using the stock numpy in Ubuntu karmic:
In [3]: np.logaddexp2(-0.5849625007211563, -53.584962500721154)
Out[3]: nan

In [4]: np.logaddexp2(-53.584962500721154, -0.5849625007211563)
Out[4]: nan

In [5]: np.log2(2**(-53.584962500721154) + 2**(-0.5849625007211563))
Out[5]: -0.58496250072115608

In [6]: numpy.__version__
Out[6]: '1.3.0'

In [8]: !uname -a
Linux octopus 2.6.31-20-generic #58-Ubuntu SMP Fri Mar 12 05:23:09 UTC
2010 i686 GNU/Linux

(the machine is a 32-bit Pentium M laptop.)

I do not see a similar problem with logaddexp.

Anne

> Chuck
>
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