String negative indices?

[Steven D'Aprano]

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.

For instance, in floating point maths, it may be useful for negative
numbers that underflow to be distinguished from positive numbers that
underflow. See, for example,
http://www.savrola.com/resources/negative_zero.html

In statistical mechanics, some systems can have negative absolute
temperatures, including negative zero. Counter-intuitively, negative
absolute temperatures aren't colder than absolute zero, but hotter than
any positive temperature. So, strangely enough, a temperature of -0K is
hotter than a infinitely hot temperature!

(Those wacky physicists and their mathematics...)

See http://en.wikipedia.org/wiki/Negative_absolute_temperature for a
description. And in case you think this is just a modern version of angels
dancing on the head of a pin, negative absolute temperatures are essential
for lasers.

In pure mathematics, zero is usually considered unsigned. However, in
"non-standard analysis" using so-called "hyperreal" or "surreal" numbers,
mathematicians use infinitesimals which are [sloppy hand-waving] like
signed zeroes. To put it another way, only slightly less sloppy,
infinitesimals are zero, but not all the same zero.

When doing calculus with complex numbers, it is very important to
distinguish which direction you are taking your limits in, and so
lim z -> 0+0i is not necessarily the same as lim z -> 0-0i.


-- 
Steven