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Re: Bug in floating point multiplication

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From Ian Kelly <ian.g.kelly@gmail.com>
Date Thu, 2 Jul 2015 09:28:42 -0600
Subject Re: Bug in floating point multiplication
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Xref csiph.com comp.lang.python:93433

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On my Chromebook, using Python 2.7.6 from the Ubuntu Trusty
distribution, I get AssertionError, and x == 1.

In Python 3.4.0 on the same system, the code runs to completion. Both
Pythons appear to be 64-bit builds.

On my Mint 17.1 desktop (which should be using the same packages), I
get the same results.

On Thu, Jul 2, 2015 at 8:52 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
>
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break
>
>
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
>
> See also http://bugs.python.org/issue24546 for more details.
>
>
>
> --
> Steven
>
> --
> https://mail.python.org/mailman/listinfo/python-list

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Thread

Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 00:52 +1000
  Re: Bug in floating point multiplication Michael Poeltl <michael.poeltl@univie.ac.at> - 2015-07-02 17:10 +0200
  Re: Bug in floating point multiplication Chris Angelico <rosuav@gmail.com> - 2015-07-03 01:21 +1000
  Re: Bug in floating point multiplication Robin Becker <robin@reportlab.com> - 2015-07-02 16:24 +0100
  Re: Bug in floating point multiplication Paul Rubin <no.email@nospam.invalid> - 2015-07-02 08:26 -0700
    Re: Bug in floating point multiplication Chris Angelico <rosuav@gmail.com> - 2015-07-03 01:34 +1000
      Re: Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 01:41 +1000
    Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:38 -0600
  Re: Bug in floating point multiplication Vincent Vande Vyvre <vincent.vande.vyvre@telenet.be> - 2015-07-02 17:15 +0200
  Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:28 -0600
  Re: Bug in floating point multiplication Robin Becker <robin@reportlab.com> - 2015-07-02 16:29 +0100
  Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:32 -0600
  Re: Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 01:42 +1000
  Re: Bug in floating point multiplication duncan smith <buzzard@invalid.invalid> - 2015-07-02 17:08 +0100
  Re: Bug in floating point multiplication MRAB <python@mrabarnett.plus.com> - 2015-07-02 17:20 +0100
  Re: Bug in floating point multiplication Tim Chase <python.list@tim.thechases.com> - 2015-07-02 11:59 -0500
  Re: Bug in floating point multiplication Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-07-02 19:49 +0200
    Re: Bug in floating point multiplication Ned Deily <nad@acm.org> - 2015-07-02 22:07 -0700
      Re: Bug in floating point multiplication Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-07-03 18:27 +0200
  Re: Bug in floating point multiplication Laurent Pointal <laurent.pointal@free.fr> - 2015-07-02 23:12 +0200
  Re: Bug in floating point multiplication Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-07-03 16:13 +0100
  Re: Bug in floating point multiplication Laura Creighton <lac@openend.se> - 2015-07-05 00:54 +0200
  Re: Bug in floating point multiplication Peter Otten <__peter__@web.de> - 2015-07-05 09:38 +0200

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