Message 246059 - Python tracker

Message246059

Author	tim.peters
Recipients	Serge Anuchin, mark.dickinson, rhettinger, steven.daprano, tim.peters
Date	2015-07-02.04:05:44
SpamBayes Score	-1.0
Marked as misclassified	Yes
Message-id	<1435809945.58.0.656251197111.issue24546@psf.upfronthosting.co.za>
In-reply-to

Content
> random() may return 1.0 exactly That shouldn't be possible. Although the code does assume C doubles have at least 53 bits of mantissa precision (in which case it does arithmetic that's exact in at least 53 bits - cannot round up to 1.0; but _could_ round up if the platform C double has less than 53 bits of precision). > py> x = 0.9999999999999999 > py> for i in range(1, 1000000): > ... if int(i*x) == i: > ... print i > ... break > ... > 2049 Very surprising! Which platform & Python is that? The loop runs to completion on my box: Python 2.7.9 (default, Dec 10 2014, 12:28:03) [MSC v.1500 64 bit (AMD64)] on win32

Content

> random() may return 1.0 exactly

That shouldn't be possible.  Although the code does assume C doubles have at least 53 bits of mantissa precision (in which case it does arithmetic that's exact in at least 53 bits - cannot round up to 1.0; but _could_ round up if the platform C double has less than 53 bits of precision).

> py> x = 0.9999999999999999
> py> for i in range(1, 1000000):
> ...     if int(i*x) == i:
> ...             print i
> ...             break
> ...
> 2049

Very surprising!  Which platform & Python is that?  The loop runs to completion on my box:

Python 2.7.9 (default, Dec 10 2014, 12:28:03) [MSC v.1500 64 bit (AMD64)] on win32

History
Date	User	Action	Args
2015-07-02 04:05:45	tim.peters	set	recipients: + tim.peters, rhettinger, mark.dickinson, steven.daprano, Serge Anuchin
2015-07-02 04:05:45	tim.peters	set	messageid: <1435809945.58.0.656251197111.issue24546@psf.upfronthosting.co.za>
2015-07-02 04:05:45	tim.peters	link	issue24546 messages
2015-07-02 04:05:44	tim.peters	create