String Indexing and Slicing

Note

Source: Indexing table, subscript notation, and exercise adapted from the C# edition (basicstringops/stringindexing.rst). Negative indices, slicing, and step notation are Python-specific features drawn from the SE4ML Python chapter (chapter_python.rst, lines 2512–2552).

Strings are sequences of characters. Python counts positions starting at 0, so the indices of the characters in "coding" are:

Index	0	1	2	3	4	5
Character	c	o	d	i	n	g

There are 6 characters and the last index is 5 — one less than the length.

Warning

The last valid index is len(s) - 1, not len(s). Accessing s[6] on a six-character string raises an IndexError.

Subscript Notation

Use square brackets to access a single character:

>>> s = "coding"
>>> s[0]
'c'
>>> s[2]
'd'
>>> s[5]
'g'

Unlike C#, Python has no separate char type — the result is always a one-character string.

The subscript can be any expression that evaluates to an integer:

>>> n = 3
>>> s[n - 1]
'd'

Negative Indices

Python allows negative indices that count from the right end:

>>> s = "coding"
>>> s[-1]     # last character
'g'
>>> s[-2]     # second from last
'n'
>>> s[-6]     # same as s[0]
'c'

This is a convenience not available in C#. s[-1] is equivalent to s[len(s) - 1].

Slicing

A slice extracts a substring using the notation s[start:stop]. The result includes characters from index start up to, but not including, index stop:

>>> s = "coding"
>>> s[1:4]
'odi'
>>> s[0:3]
'cod'

Omitting either end uses the beginning or end of the string:

>>> s[:3]     # from the start
'cod'
>>> s[3:]     # to the end
'ing'
>>> s[:]      # whole string (copy)
'coding'

Slices work with negative indices too:

>>> s[-3:]    # last three characters
'ing'
>>> s[:-2]    # everything except the last two
'codi'

Step in Slices

An optional third number specifies a step:

>>> s = "abcdefgh"
>>> s[::2]       # every other character
'aceg'
>>> s[::-1]      # reverse the string
'hgfedcba'

Reversing with [::-1] is an idiomatic Python trick worth remembering.

Indexing Exercise

Predict what each line prints, then verify in the REPL:

s = "fragment"
k = 3
print(s[1])
print(s[k])
print(s[2 * k - 2])
print(s[-1])
print(s[2:5])
print(s[::-1])