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

[Michael F. Stemper]

On 05/08/2021 12.35, Jack Brandom wrote:
> The FAQ at
> 
>    https://docs.python.org/3/faq/programming.html#what-s-a-negative-index
> 
> makes me think that I can always replace negative indices with positive
> ones --- even in slices, although the FAQ seems not to say anything
> about slices.
> 
> With slices, it doesn't seem to always work.  For instance, I can
> reverse a "Jack" this way:
> 
>>>> s = "Jack Brandom"
>>>> s[3 : -13 : -1]
> 'kcaJ'
> 
> I have no idea how to replace that -13 with a positive index.  Is it
> possible at all?

I don't think so, because the second number (in this case -13) is the
index before which you stop. For example:

 >>> s
'Jack Brandom'
 >>> s[3:0:-1]
'kca'
 >>> s[3:1:-1]
'kc'
 >>>

However, since you want to go through item 0 in the original string,
you don't need a number there at all:

 >>> s[3::-1]
'kcaJ'
 >>>

Or, it you want to be more explicit, you could separately grab the
substring and then reverse it:

 >>> s[:4][::-1]
'kcaJ'
 >>>

Does any of this help?

-- 
Michael F. Stemper
If it isn't running programs and it isn't fusing atoms, it's just 
bending space.