Bug in floating point multiplication

Ian Kelly ian.g.kelly at gmail.com
Thu Jul 2 17:28:42 CEST 2015


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 at 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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