Bug in floating point multiplication

duncan smith buzzard at invalid.invalid
Thu Jul 2 18:08:25 CEST 2015


On 02/07/15 15:52, Steven D'Aprano 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.
> 
> 
> 

Python 2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2] on linux2
Type "copyright", "credits" or "license()" for more information.
>>> from __future__ import division
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
	if int(i*x) == i:
		print(i); break

		
>>>


Python 3.4.0 (default, Jun 19 2015, 14:20:21)
[GCC 4.8.2] on linux
Type "copyright", "credits" or "license()" for more information.
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
	if int(i*x) == i:
		print(i); break

		
>>>


Ubuntu 14.04

Duncan


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