newbie string question

[Peter Hansen]

Don Low wrote:
> 
> >    0   1   2   3   4   5    <-- subscript index
> >  +---+---+---+---+---+---+
> > "| P | y | t | h | o | n |"
> >  +---+---+---+---+---+---+
> >  0   1   2   3   4   5   6  <-- slice index
> >
> So if I understand, pystr[3] refers to the subscript index, but pystr[3:]
> refers to the slice index. pystr[2:5] means slice at slice index 2 and 5,
> not start at subscript index 2 and slice at slice index 5.
> 
> I just want to understand this once and for all.

Exactly right.  And as Fernando reminded us, negative indexes
can be used as well, but with similar interpretation:

   -6  -5  -4  -3  -2  -1    <-- negative subscript index
  +---+---+---+---+---+---+
 "| P | y | t | h | o | n |"
  +---+---+---+---+---+---+
 -6  -5  -4  -3  -2  -1   :  <-- negative slice index

so pystr[-5:-2] is "yth", pystr[-4:] is "thon",
and pystr[-5] is just "y".

You can start to confuse yourself again, maybe :-), by
combining positive and negative slice indices:

   pystr[-5:4]

The two diagrams above make it clear what happens tho. ;-)

-Peter