[Numpy-discussion] nan, sign, and all that

[Robert Kern]

On Thu, Oct 2, 2008 at 02:37, Stéfan van der Walt <stefan at sun.ac.za> wrote:
> Hi Charles,
>
> 2008/10/2 Charles R Harris <charlesr.harris at gmail.com>:
>> In [3]: a = array([NAN, 0, NAN, 1])
>> In [4]: b = array([0, NAN, NAN, 0])
>>
>> In [5]: fmax(a,b)
>> Out[5]: array([  0.,   0.,  NaN,   1.])
>>
>> In [6]: fmin(a,b)
>> Out[6]: array([  0.,   0.,  NaN,   0.])
>
> These are great, many thanks!
>
> My only gripe is that they have the same NaN-handling as amin and
> friends, which I consider to be broken.

No, these follow well-defined C99 semantics of the fmin() and fmax()
functions in libm. If exactly one of the arguments is a NaN, the
non-NaN argument is returned. This is *not* the current behavior of
amin() et al., which just do naive comparisons.

> Others also mentioned that
> this should be changed, and I think David C wrote a patch for it (but
> I am not informed as to the speed implications).
>
> If I had to choose, this would be my preferred output:
>
> In [5]: fmax(a,b)
> Out[5]: array([  NaN,   NaN,  NaN,   1.])

Chuck proposes letting minimum() and maximum() have that behavior.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco