[Numpy-discussion] What is the sign of nan?

[Robert Kern]

On Mon, Sep 29, 2008 at 18:10, Charles R Harris
<charlesr.harris at gmail.com> wrote:
>
> On Mon, Sep 29, 2008 at 4:54 PM, Charles R Harris
> <charlesr.harris at gmail.com> wrote:
>>
>> On Mon, Sep 29, 2008 at 4:40 PM, Charles R Harris
>> <charlesr.harris at gmail.com> wrote:
>>>
>>> On Mon, Sep 29, 2008 at 4:28 PM, Robert Kern <robert.kern at gmail.com>
>>> wrote:
>>>>
>>>> On Mon, Sep 29, 2008 at 17:13, Charles R Harris
>>>> <charlesr.harris at gmail.com> wrote:
>>>> >
>>>> > On Mon, Sep 29, 2008 at 3:52 PM, Charles R Harris
>>>> > <charlesr.harris at gmail.com> wrote:
>>>> >>
>>>> >> Hi All,
>>>> >>
>>>> >> I've been cleaning up the ufunc loops and the sign function currently
>>>> >> doesn't have a defined behavior for nans. This makes the results
>>>> >> depend on
>>>> >> the order/type of comparisons in the code, which looks fragile to me.
>>>> >> So
>>>> >> what should it return? I vote for nan but am open for suggestions.
>>>> >
>>>> > And while we're at it, lets decide how to treat max/min when nans are
>>>> > involved. Or should we just say the behavior is undefined.
>>>>
>>>> When feasible, I would like float(s)->float functions to return NaN
>>>> when given a NaN as an argument. At least as the main versions of the
>>>> function. Specific NaN-ignoring functions can also be introduced, but
>>>> as separate functions. I don't know what exactly to do about
>>>> float->int functions (e.g. argmin). I also don't know how these should
>>>> interact with the current seterr() state.
>>>
>>> So the proposition is, sign, max, min return nan when any of the
>>> arguments is nan.
>>>
>>> +1
>>
>> I also propose that all logical operators involving nan return false,
>> i.e., ==, !=, <, <=, >, >=, and, or, xor, not.
>>
>
> Currently this is so except for !=. On my machine nan != nan is true. Looks
> like it is being computed in C as !(nan == nan). Hmm, anyone know of a C
> standard on this?

C99 Annex F:

  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf

In particular:

"""
F.8.3 Relational operators
  x != x -> false       The statement x != x is true if x is a NaN.
"""

-- 
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