[Python-Dev] Why is nan != nan?

[Antoine Pitrou]

Le Thu, 25 Mar 2010 07:19:24 -0700, Curt Hagenlocher a écrit :
> On Thu, Mar 25, 2010 at 7:08 AM, Nick Coghlan <ncoghlan at gmail.com>
> wrote:
> 
>> Jesus Cea wrote:
>> > But IEEE 754 was created by pretty clever guys and sure they had a
>> > reason for define things in the way they are. Probably we are missing
>> > something.
>>
>> Yes, this is where their "implementable in a hardware circuit" focus
>> comes in. They were primarily thinking of a floating point
>> representation where the 32/64 bits are *it* - you can't have "multiple
>> NaNs" because you don't have the bits available to describe them.
>>
> Wait, what? I haven't been paying much attention, but this is backwards.
> There are multiple representations of NaN in the IEEE encoding; that's
> actually part of the problem with saying that NaN = NaN or NaN != NaN.
> If you want to ignore the "payload" in the NaN, then you're not just
> comparing bits any more.

This sounds a bit sophistic, if the (Python) user doesn't have access to 
the payload anyway.


Antoine.