Re: float("nan") in set or as key

References	(6 earlier) <pan.2011.06.01.20.40.18.453000@nowhere.com> <4de75dd5$0$29983$c3e8da3$5496439d@news.astraweb.com> <pan.2011.06.02.20.47.01.344000@nowhere.com> <isasfm$inl$1@reader1.panix.com> <pan.2011.06.03.23.29.04.984000@nowhere.com>
Date	2011-06-04 09:51 +1000
Subject	Re: float("nan") in set or as key
From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.2444.1307145069.9059.python-list@python.org> (permalink)

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On Sat, Jun 4, 2011 at 9:29 AM, Nobody <nobody@nowhere.com> wrote:
> Floats are supposed to approximate reals. They're also a Python
> data type, and should make some effort to fit in with the rest of
> the language.
>

That's what I thought a week ago. But that's not really true. Floats
are supposed to hold non-integral values, but the data type is "IEEE
754 floating point", not "real number". There's several ways to store
real numbers, and not one of them is (a) perfectly accurate, or (b)
plausibly fast to calculate. Using rationals (fractions) with infinite
range leads to exponential performance costs, and still doesn't
properly handle irrationals like pi. And if you cap the denominator to
a power of 2 and cap the length of the mantissa, err I mean numerator,
then you have IEEE 754 floating point. Python offers you a way to
store and manipulate floating point numbers, not real numbers.

Chris Angelico

Thread

float("nan") in set or as key MRAB <python@mrabarnett.plus.com> - 2011-05-29 00:41 +0100
  Re: float("nan") in set or as key Erik Max Francis <max@alcyone.com> - 2011-05-28 17:16 -0700
  Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 00:26 +0000
  Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-05-29 13:04 +1200
    Re: float("nan") in set or as key John Nagle <nagle@animats.com> - 2011-05-28 23:12 -0700
      Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 10:29 +0000
        Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-05-29 22:19 +0100
          Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 23:31 +0000
            Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-01 21:41 +0100
              Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-02 09:54 +0000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-02 13:05 +0000
                Re: float("nan") in set or as key Robert Kern <robert.kern@gmail.com> - 2011-06-02 12:04 -0500
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-02 21:47 +0100
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-03 14:52 +0000
                Re: float("nan") in set or as key Chris Torek <nospam@torek.net> - 2011-06-03 17:52 +0000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-06 13:54 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-04 00:29 +0100
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-04 09:51 +1000
                Re: float("nan") in set or as key rusi <rustompmody@gmail.com> - 2011-06-04 00:52 -0700
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-04 20:29 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-05 07:21 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-05 19:15 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-06 00:55 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-06 23:14 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-06 23:44 +0000
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-07 11:00 +1000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-06 14:03 +0000
                Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-03 11:17 +1200
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-03 04:23 +0000
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-03 14:35 +1000
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-03 05:59 +0000
                Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-04 12:14 +1200
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-04 02:21 +0000
                Re: float("nan") in set or as key Erik Max Francis <max@alcyone.com> - 2011-06-05 00:27 -0700
          Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-01 21:01 +0000
      Re: float("nan") in set or as key Chris Torek <nospam@torek.net> - 2011-05-30 00:02 +0000
  Re: float("nan") in set or as key Raymond Hettinger <python@rcn.com> - 2011-05-29 21:49 -0700

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