Negative Indices

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One clever notion in Python is the idea of a “negative index” that allows you to index into a string starting at the end. To understand this, it helps to refer to the normal indices of a string.

Suppose we had the following string and slice:

s = 'hello world'
print(s[2:len(s) - 2])

This is the same idea of slices that we saw in the last slide. To understand why this prints llo wor , we can think of the picture showing the indices of the str as shown in the following image ( Hint: Think of where 2 and len(s)-2 fall on this picture ).

Asking Python to go ” n before the end” is such a common task, they provide another scheme for indexing that uses negative numbers! The idea is you start at the last character ( 'd' ) being at index -1 (since it is at index len(s) - 1 in our indexing scheme), the second to last ( 'l' ) being -2 , etc. Pictorially, the negative indices are shown in the following image.

This means you can get the same output as above by referring to index -2 instead of len(s) - 2 , just like in the code below.

s = 'hello world'
print(s[2:-2])

Notice that the end index is still exclusive, we are just able to use -2 as the end index instead of len(s)-2 .

Pause And Think

Before running the code in the next cell, what do you think it would print?

s = 'hello world'
print(s[10:2:-2])