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Re: NaN comparisons - Call For Anecdotes

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On Wed, Jul 9, 2014 at 10:53 AM, Steven D'Aprano
> Cast your 64-bit float into a 64-bit int. Or, if it's a C single rather
> than a double, cast the 32-bit float into a 32-bit int. Now you can
> compare them for equality without carrying about NANs, and without losing
> data. Later, when you're ready to start doing some numeric work on them,
> you cast back to floats. That's the idea I had in mind. Perhaps it
> doesn't match your use-case.

Unfortunately, it's not that simple:

>>> import math, struct
>>> def float_to_int(x):
...   return struct.unpack('L', struct.pack('d', x))[0]
...
>>> 0.0 == -0.0
True
>>> float_to_int(0.0) == float_to_int(-0.0)
False
>>> nan1 = struct.unpack('d', b'\x00'*6+b'\xf1\x7f')[0]
>>> math.isnan(nan1)
True
>>> nan2 = struct.unpack('d', b'\x00'*6+b'\xf2\x7f')[0]
>>> math.isnan(nan2)
True
>>> float_to_int(nan1) == float_to_int(nan1)
True
>>> float_to_int(nan2) == float_to_int(nan2)
True
>>> float_to_int(nan1) == float_to_int(nan2)
False

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Thread

NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 16:53 +0200
  Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-08 17:13 +0000
    Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-09 03:21 +1000
    Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 21:02 +0200
      Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 00:10 +0000
        Re: NaN comparisons - Call For Anecdotes Terry Reedy <tjreedy@udel.edu> - 2014-07-09 00:57 -0400
          Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve@pearwood.info> - 2014-07-09 06:43 +0000
            Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-09 16:52 +1000
        Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 17:08 +0200
          Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 16:53 +0000
            Re: NaN comparisons - Call For Anecdotes Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-09 11:26 -0600
            Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 19:44 +0200
        Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-10 01:13 +1000
        Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 18:24 +0200
    Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 21:25 +0200
  Re: NaN comparisons - Call For Anecdotes Rustom Mody <rustompmody@gmail.com> - 2014-07-09 20:07 -0700

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