on slices, negative indices, which are the equivalent procedures?

[Hope Rouselle]

Chris Angelico <rosuav at gmail.com> writes:

> On Tue, Aug 10, 2021 at 7:24 AM Jack Brandom <jbrandom at example.com> wrote:
>>
>> Greg Ewing <greg.ewing at canterbury.ac.nz> writes:
>>
>> > On 6/08/21 12:00 pm, Jack Brandom wrote:
>> >> It seems
>> >> that I'd begin at position 3 (that's "k" which I save somewhere), then I
>> >> subtract 1 from 3, getting 2 (that's "c", which I save somewhere), then
>> >> I subtract 1 from 2, getting 1 (that's "a", ...), then I subtract 1 from
>> >> 1, getting 0 (that's J, ...), so I got "kcaJ" but my counter is 0 not
>> >> -13, which was my stopping point.
>> >
>> > You need to first replace any negative or missing indices with
>> > equivalent indices measured from the start of the string.
>> >
>> > When you do that in this example, you end up iterating backwards from 3
>> > and stopping at -1.
>>
>> Yeah, that makes sense now.  But it sucks that the rule for replacing
>> negative indices is sometimes missing index and sometimes positive
>> index.  (That is, we can't always use positive indices.  Sometimes we
>> must use no index at all.  I mean that's how it looks to my eyes.)
>
> Sometimes, the index  you need to use is the value None. You cannot
> use a positive number to indicate the position to the left of zero -
> at least, not if you consider numbers to be on a number line.

True --- I understand that now.  (I think someone else in this thread
had pointed that out too. But thanks for clarifying it.)