Re: Bug in floating point multiplication

From	Peter Otten <__peter__@web.de>
Subject	Re: Bug in floating point multiplication
Date	2015-07-05 09:38 +0200
Organization	None
References	<55955048$0$1662$c3e8da3$5496439d@news.astraweb.com> <201507042254.t64Ms3qW031538@theraft.openend.se>
Newsgroups	comp.lang.python
Message-ID	<mailman.302.1436081928.3674.python-list@python.org> (permalink)

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Laura Creighton wrote:

> In a message of Fri, 03 Jul 2015 00:52:55 +1000, "Steven D'Aprano" writes:
>>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
> 
> PyPy says:
> Python 2.7.9 (2.5.1+dfsg-1, Mar 27 2015, 19:45:43)
> [PyPy 2.5.1 with GCC 4.9.2] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>>> x = 1 - 1/2**53
>>>>> assert x == 0.9999999999999999
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AssertionError
>>>>> for i in range(1, 1000000):
> ....     if int(i*x) == i:
> ....             print(i); break
> ....
> ....
> 1
>>>>> 
> 
> So the loop terminates, but there is an Assertion Error.  Did you want
> that?

No. Steven's environment implies

from __future__ import division

which he forgot to mention in the original post:

>>>> x = 1-1/2**53
>>>> x
1
>>>> from __future__ import division
>>>> x = 1-1/2**53
>>>> x
0.9999999999999999

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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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