Re: clarification about subscript of python string

On 3/16/25 08:42, Alan Gauld via Tutor wrote:
> On 14/03/2025 13:08, GGHSS POLUR bot via Tutor wrote:
>> Respected sir/mam
>>             1. Subscript of python string is positive and negative.
>>             2. Subscript of python string is positive or negative.
>>
>> in above statement which one right. please help me.
> 
> The question is a bit vague.
> 
> Both statements are correct depending on context.
> Python strings do have both positive and negative indexing.
> A single character can be indexed using either a positive or
> negative index or even both at the same time!

Yes, Python uses some syntax tricks to extend the idea of indexing.

A numeric index into an iterable (not limited to strings) give you that 
numberd'th element, remembering that Python's indexing is zero-based.

Negative indexing is special syntax that means "count backwards from the 
end", mainly a convenience so you don't have to compute the length of 
the sequence first, so if:

stuff = "12345'

you can get the next-to-last element by doing

letter = stuff[-2]

instead of needing to write

letter = stuff[len(stuff) - 2]

there's no -0 in negative indexing, -1 means the last element.  And 
there's not *really* negative indexing, it's more a syntax quirk - if 
your iterable has length 3, and you try for -5, that's an "out of range" 
error, just like trying to use a positive index greater than the length 
would do.

===

If the iterable supports the slicing protocol (which all of strings, 
lists and tuples do) you get
"slicing" - a sub-sequence selected by using a : to separate starting 
index and ending index. Here there's another syntax quirk - omitting the 
starting index means "beginning" and omitting the ending index means 
"end".  This is where you can see positive and negative indices at the 
same time, like

substr = stuff[1:-2]


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