On Wed, Mar 31, 2010 at 11:03 PM, Anne Archibald <peridot.face...@gmail.com>wrote:
> On 31 March 2010 16:21, Charles R Harris <charlesr.har...@gmail.com> > wrote: > > > > > > On Wed, Mar 31, 2010 at 11:38 AM, T J <tjhn...@gmail.com> wrote: > >> > >> On Wed, Mar 31, 2010 at 10:30 AM, T J <tjhn...@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. > > Well, it did function correctly for me ;) Just some roundoff error. > 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 think this is related to 32 bits somehow. The code for the logaddexp and logaddexp2 functions is almost identical. Maybe the compiler? Chuck
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