NaN comparisons - Call For Anecdotes

Anders J. Munch 2014 at
Tue Jul 8 20:29:52 CEST 2014

Chris Angelico wrote:
> Why *should* all NaNs be equal to each other? You said on the other
> list that NaN==NaN was equivalent to (2+2)==(1+3), but that assumes
> that NaN is a single "thing".

I don't actually care if all NaN bitpatterns are in the same equivalence group 
or if each bitpattern is its own equivalence group. I just want the == 
equivalence relation to be sound.

> For hash keys, float object identity will successfully look them up:

Except you can't expect to rely on object identity in most interesting cases.

 >>> x = float('nan')
 >>> import struct
 >>> y = struct.unpack('<f', struct.pack('<f', x))[0]
 >>> d[x] = "found"
 >>> d[y]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: nan

and also:

 >>> def f(): return float('inf')/float('inf')
 >>> f() == f()
 >>> f() is f()

> But any time you compare floats for equality, you *already* have to
> understand what you're doing (because of rounding and such), so I
> don't see why the special case on NaN is significant, unless as
> mentioned above, you want all NaNs to be equal, which doesn't make
> sense.
Let me conjure up a simple example:

| class Monitor(Thread):
| def run(self):
| old = self.get_current_value()
| while not self.Terminated:
| new = self.get_current_value()
| if new != old:
| print(time.asctime(), "changed to", new)
| old = new
| time.sleep(1)

This is a completely generic change detection algorithm, and not a 
"floating-point algorithm" in any way: It will work on strings, lists, sets, 
anything that get_current_value returns, including non-NaN floats. You don't 
need to know anything about floating-point representation to write or use such 
an algorithm, why should you? It works on tuples, sets, lists, serial port 
handles, module objects, pretty much anything you can imagine -- except NaN floats.

regards, Anders

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