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Re: Unexpected results comparing float to Fraction

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From Rotwang <sg552@hotmail.co.uk>
Newsgroups comp.lang.python
Subject Re: Unexpected results comparing float to Fraction
Date Mon, 29 Jul 2013 19:20:32 +0100
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On 29/07/2013 17:20, Chris Angelico wrote:
> On Mon, Jul 29, 2013 at 5:09 PM, MRAB <python@mrabarnett.plus.com> wrote:
>> I'm surprised that Fraction(1/3) != Fraction(1, 3); after all, floats
>> are approximate anyway, and the float value 1/3 is more likely to be
>> Fraction(1, 3) than Fraction(6004799503160661, 18014398509481984).
>
> At what point should it become Fraction(1, 3)?
>
>>>> Fraction(0.3)
> Fraction(5404319552844595, 18014398509481984)
>>>> Fraction(0.33)
> Fraction(5944751508129055, 18014398509481984)
>>>> Fraction(0.333)
> Fraction(5998794703657501, 18014398509481984)
>>>> Fraction(0.3333333)
> Fraction(6004798902680711, 18014398509481984)
>>>> Fraction(0.3333333333)
> Fraction(6004799502560181, 18014398509481984)
>>>> Fraction(0.3333333333333)
> Fraction(6004799503160061, 18014398509481984)
>>>> Fraction(0.33333333333333333)
> Fraction(6004799503160661, 18014398509481984)
>
> Rounding off like that is a job for a cool library function (one of
> which was mentioned on this list a little while ago, I believe), but
> not IMO for the Fraction constructor.

How about this?

 >>> from fractions import Fraction
 >>> help(Fraction.limit_denominator)
Help on function limit_denominator in module fractions:

limit_denominator(self, max_denominator=1000000)
     Closest Fraction to self with denominator at most max_denominator.

     >>> Fraction('3.141592653589793').limit_denominator(10)
     Fraction(22, 7)
     >>> Fraction('3.141592653589793').limit_denominator(100)
     Fraction(311, 99)
     >>> Fraction(4321, 8765).limit_denominator(10000)
     Fraction(4321, 8765)

 >>> Fraction(1/3).limit_denominator()
Fraction(1, 3)

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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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