String negative indices?

[Steven D'Aprano]

On Sat, 24 Jun 2006 05:36:17 -0700, Filip Wasilewski wrote:

> Steven D'Aprano wrote:
>> On Fri, 23 Jun 2006 02:17:39 -0700, Filip Wasilewski wrote:
>>
>> > drtimhill at comcast.net wrote:
>> >
>> >> Logically, I should be able to enter x[-2:-0] to get the last and next to last characters. However, since Python doesn't distinguish between positive and negative zero, this doesn't work. Instead, I have to enter x[-2:].
>> >
>> > Hooray! Logically there is no such thing as positive or negative zero,
>> > or did I miss something in the primary?
>>
>> No, not in the primary, or even in the secondary, but possibly in the
>> tertiary.
>>
>> For many purposes, it doesn't make sense to distinguish +0 from -0. But
>> for other purposes, it does.
> 
> I believe you will agree that this is mostly a matter of notation of
> some fact rather than really trying to sign zero.

No. 

In floating point, +0 *really is* different from -0 -- the two floats
have different bit patterns. Floating point libraries must be specifically
programmed to ignore that difference, making zero treated differently than
other floats. That's -- usually -- a good thing.

In mathematics, well, maybe... certainly in the Real number system, there
is no difference, and +0 and -0 are just two ways of writing the same
thing. In the hyperreals, +0 and -0 are the same, but there are
infinitesimals which are different, and signed. I don't know enough about
the surreals to comment. In matrix maths, there are an infinite number of
different matrices where all the elements are zero -- they are all
distinct, different, zeroes.

And in "the real world", all numbers are an abstraction anyway, so I'm
not too worried about correspondence to reality. If mathematicians find a
use for distinguishing +0 from -0 (as the statistical physicists have
done) they will treat them as "really" different. If they don't, they
won't.


-- 
Steven.