Bug in floating point multiplication

Laura Creighton lac at openend.se
Sun Jul 5 00:54:03 CEST 2015


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?

Laura



More information about the Python-list mailing list