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Re: Unexpected results comparing float to Fraction

References <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com> <51F693B4.9000201@mrabarnett.plus.com> <CAPTjJmr=73v56-V69xnrDNsSV0SYDcOv5z6yG8+AtkjX2=XgFA@mail.gmail.com> <CALwzidm-Vvn5ZyoRp0j_dcO0oMQQmJHWgESCF4NdSLwg4N=Tfw@mail.gmail.com> <51F6A093.5000803@mrabarnett.plus.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date 2013-07-29 11:35 -0600
Subject Re: Unexpected results comparing float to Fraction
Newsgroups comp.lang.python
Message-ID <mailman.5250.1375119362.3114.python-list@python.org> (permalink)

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On Mon, Jul 29, 2013 at 11:04 AM, MRAB <python@mrabarnett.plus.com> wrote:
> On 29/07/2013 17:40, Ian Kelly wrote:
>> At the point where the float is exactly equal to the value you get
>> from the floating-point division 1/3.  If it's some other float then
>> the user didn't get there by entering 1/3, so it's not worth trying to
>> pretend that they did.
>>
> I thought that you're not meant to check for equality when using floats.

Equality checking is useful for floats when there is exactly one value
that you want to test for as in this case, as opposed to a range of
approximate values that are considered to be equal within rounding
error.  There is exactly one float that the expression 0.1 evaluates
to, and although it's not equal to the decimal 0.1, it is the only
float that we want to format as "0.1".  The immediately preceding and
following floats are 0.09999999999999999 and 0.10000000000000002, and
they are formatted as such because they're not equal to the float that
you get from 0.1.

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Thread

Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 15:43 +0000
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 09:50 -0600
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 17:09 +0100
  Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-07-29 17:20 +0100
    Re: Unexpected results comparing float to Fraction Rotwang <sg552@hotmail.co.uk> - 2013-07-29 19:20 +0100
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 19:32 +0000
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 17:48 +0100
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 17:27 +0000
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 10:40 -0600
    Re: Unexpected results comparing float to Fraction Rotwang <sg552@hotmail.co.uk> - 2013-07-29 19:16 +0100
      Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 13:33 -0600
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 18:04 +0100
    Re: Unexpected results comparing float to Fraction Grant Edwards <invalid@invalid.invalid> - 2013-07-29 18:46 +0000
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 20:19 +0000
  Re: Unexpected results comparing float to Fraction Terry Reedy <tjreedy@udel.edu> - 2013-07-29 13:08 -0400
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 17:29 +0000
      Re: Unexpected results comparing float to Fraction Terry Reedy <tjreedy@udel.edu> - 2013-07-29 16:48 -0400
  Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-07-29 18:14 +0100
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 11:35 -0600
  Re: Unexpected results comparing float to Fraction Serhiy Storchaka <storchaka@gmail.com> - 2013-07-29 22:34 +0300
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 14:35 -0600
  Unexpected results comparing float to Fraction Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-07-30 14:32 +0100
  Re: Unexpected results comparing float to Fraction Tony the Tiger <tony@tiger.invalid> - 2013-07-31 15:23 -0500
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-01 06:20 +0000
      Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-08-01 07:32 +0100
      Re: Unexpected results comparing float to Fraction Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-08-01 10:44 +0100
      Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-08-01 10:48 +0100

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