String slicing with reverse indexing

Question

I am investigating the slicing of a string with the following code:

s='abcdefgh'
print(len(s))
print(s[-1:-9:-1])
print(s[-8])

whose output is:

8
hgfedcba
a

Why when printing reverse we need to slice till -9 while the last character i.e 'a' at -8?

Does this answer your question? [Understanding slice notation](https://stackoverflow.com/questions/509211/understanding-slice-notation) — baduker, Sep 21 '20 at 12:46

score 1 · Answer 1 · answered Sep 21 '20 at 12:46

That is simply because how indexing works in Python.

s[-1: -9: -1] has 8 characters just like s or s[0: 9: 1] has 8 characters. The last index is always ignored. This is done so that things like range(n) have, like the call suggests, n terms although it goes from 0 to n-1.

It is clearer if you forget numbers altogether and just look at this object: s[0: len(s): +1]. Reverse the sign of the indexes and substract -1 to get the opposite string s[-0-1: -len(s)-1: -1].

score 0 · Answer 2 · answered Sep 21 '20 at 12:47

In python, slicing can be used to get the sub-section of the Object.

len(S) - return the length of the string.
S[index] - get the character at a particular index from the start. positive index.
S[-index] - get the character at a particular index from the end. negative index.
S[start:end] - slice the string from a start index to an end position not index.
S[start:] - slice the string from a start index to the last position.
S[:end] - slice the string from 0 indexes to the end position.
S[:] - get the whole string like copy the string.
S[i:j:k] - slice the string with step. default k is +1. like s[2:10:2]
s[::-1] - reverse the string.

Slicing does not include the `end` position (ex. `s="abcd"; s[0:4]` does not include `s[4]`) — Gino Mempin, Sep 21 '20 at 13:10
In my answer context, position start from 1 and index starts from 0. — Sachin Kumar, Sep 21 '20 at 13:18

String slicing with reverse indexing

2 Answers2