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Re: a.index(float('nan')) fails

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On Thu, Oct 25, 2012 at 9:04 PM, Terry Reedy <tjreedy@udel.edu> wrote:
> On 10/25/2012 9:46 PM, mamboknave@gmail.com wrote:
>>>>>
>>>>> a = [float('nan'), 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>>>> a
>>
>> [nan, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>>>>
>>>>> a.index(float('nan'))
>>
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> ValueError: list.index(x): x not in list
>>
>> That means, the function .index() cannot detect nan values.
>> It happens on both Python 2.6 and Python 3.1
>>
>> Is this a bug? Or I am not using .index() correctly?
>
>
> It is a consequence of the following, which some people (but not all)
> believe is mandated by the IEEE standard.
>
>>>> nan = float('nan')
>>>> nan is nan
> True

It should be noted, for the record, that "nan is nan" returning True
has nothing to do with the concept of numbers or the IEEE standard and
is purely a consequence that Python runs on hardware with memory
addresses and such.

>>>> nan == nan
> False

Here, equality, IS about number and this is appropriate and conforms
to the IEEE standard.

>>>> nanlist = [nan]
>>>> nan in nanlist
> True
>>>> nanlist.index(nan)
> 0

Here you just see an phenomenon with the python object/reference
model, which, being as it is, has nothing to do with numbers.  This is
an area which, potentially could be changed in Python without
violating the IEEE standard whatsoever.

Mark

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a.index(float('nan')) fails mamboknave@gmail.com - 2012-10-25 18:46 -0700
  Re: a.index(float('nan')) fails Terry Reedy <tjreedy@udel.edu> - 2012-10-25 22:04 -0400
    Re: a.index(float('nan')) fails Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-26 02:44 +0000
      Re: a.index(float('nan')) fails Terry Reedy <tjreedy@udel.edu> - 2012-10-26 03:54 -0400
        Re: a.index(float('nan')) fails Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-26 15:26 +0000
          Re: a.index(float('nan')) fails Terry Reedy <tjreedy@udel.edu> - 2012-10-26 14:49 -0400
    Re: a.index(float('nan')) fails Nobody <nobody@nowhere.com> - 2012-10-27 17:40 +0100
  Re: a.index(float('nan')) fails Cameron Simpson <cs@zip.com.au> - 2012-10-26 13:15 +1100
    Re: a.index(float('nan')) fails mamboknave@gmail.com - 2012-10-25 19:22 -0700
    Re: a.index(float('nan')) fails mamboknave@gmail.com - 2012-10-25 19:22 -0700
  Re: a.index(float('nan')) fails MRAB <python@mrabarnett.plus.com> - 2012-10-26 03:19 +0100
  Re: a.index(float('nan')) fails Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-26 02:33 +0000
    Re: a.index(float('nan')) fails Ethan Furman <ethan@stoneleaf.us> - 2012-10-28 06:07 -0700
  Re: a.index(float('nan')) fails Terry Reedy <tjreedy@udel.edu> - 2012-10-26 04:00 -0400
    Re: a.index(float('nan')) fails Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-26 16:23 +0000
      Re: a.index(float('nan')) fails MRAB <python@mrabarnett.plus.com> - 2012-10-26 17:43 +0100
      Re: a.index(float('nan')) fails Chris Angelico <rosuav@gmail.com> - 2012-10-27 03:45 +1100
        Re: a.index(float('nan')) fails Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-26 18:40 +0000
          Re: a.index(float('nan')) fails Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-10-26 15:17 -0400
          Re: a.index(float('nan')) fails Chris Angelico <rosuav@gmail.com> - 2012-10-27 12:29 +1100
      Re: a.index(float('nan')) fails Terry Reedy <tjreedy@udel.edu> - 2012-10-26 14:58 -0400
      Re: a.index(float('nan')) fails Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-10-27 00:48 -0400
        Re: a.index(float('nan')) fails Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-10-27 08:56 +0200
          Re: a.index(float('nan')) fails Nobody <nobody@nowhere.com> - 2012-10-27 17:45 +0100
          Re: a.index(float('nan')) fails Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-10-27 13:07 -0400
  Re: a.index(float('nan')) fails Mark Adam <dreamingforward@gmail.com> - 2012-10-27 13:33 -0500

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