need clarification on -0

[Steven D'Aprano]

On Sat, 28 Nov 2009 15:14:31 -0800, Mark Dickinson wrote:

>> Actually, there ARE computers where you might not see this result.
>> Virtually all of the processors on which Python runs use two's
>> complement arithmetic.  In two's complement, there is no separate value
>> called -0.  0 and -0 have the same bit representation.
>>
>> In one's complement, -0 and 0 have different representations.
> 
> While that's true, I think the implementation of Python is such that the
> Python objects -0 and 0 should always be indistinguishable even on
> machines where the underlying architecture represents integers using
> ones' complement or sign-magnitude.

I don't think that really has any bearing on the Original Poster's 
question -- presumably on such machines, Python should treat both -0 and 
+0 as false in a boolean context and generate the same result.

When it comes to integers, I'm not aware of any mathematical or 
programming system which treats -0 and +0 as distinct entities, even if 
they have different internal representations. But the same doesn't apply 
for floats, where the IEEE standard requires that -0.0 and +0.0 be 
distinct and distinguishable (although it also requires that they compare 
as equal).



-- 
Steven