Chris Angelico <ros...@gmail.com> writes: > On Tue, Aug 10, 2021 at 7:24 AM Jack Brandom <jbran...@example.com> wrote: >> >> Greg Ewing <greg.ew...@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.) -- https://mail.python.org/mailman/listinfo/python-list
