Re: Bug in floating point multiplication

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From	Paul Rubin <no.email@nospam.invalid>
Newsgroups	comp.lang.python
Subject	Re: Bug in floating point multiplication
Date	Thu, 02 Jul 2015 08:26:06 -0700
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Steven D'Aprano <steve@pearwood.info> writes:
> x = 1 - 1/2**53
> assert x == 0.9999999999999999

In Python 2.x I don't see how that assert can possibly succeed, since
x is the integer 1.  But I tested it anyway on 2.7.5 under Fedora 19
and it threw an assertion error.

I changed it to say 1 - 1/2.0**53 and then the loop runs to completion.

sys.version is:
2.7.5 (default, Nov  3 2014, 14:33:39) \n[GCC 4.8.3 20140911 (Red Hat 4.8.3-7)]

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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