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. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
