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Re: Unexpected results comparing float to Fraction

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From Ian Kelly <ian.g.kelly@gmail.com>
Date Mon, 29 Jul 2013 10:40:52 -0600
Subject Re: Unexpected results comparing float to Fraction
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On Mon, Jul 29, 2013 at 10:20 AM, Chris Angelico <rosuav@gmail.com> 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)?

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.

We do a similar rounding when formatting floats to strings, but in
that case one only has to worry about divisors that are powers of 10.
I imagine it's going to take more time to find the correct fraction
when any pair of relatively prime integers can be a candidate
numerator and denominator.  Additionally, the string rounding only
occurs when the float is being formatted for display; we certainly
don't do it as the result of numeric operations where it could result
in loss of precision.

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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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