Python Topic 5 Part 2

5.13 String slicing

String slicing basics

Strings are a sequence type, having characters ordered by index from left to right. An index is an integer matching a specific position in a string's sequence of characters. An individual character is read using an index surrounded by brackets. Ex: my_str[5] reads the character at index 5 of the string my_str. Indices start at 0, so index 5 is a reference to the 6th character in the string.

A programmer often needs to read more than one character at a time. Multiple consecutive characters can be read using slice notation. Slice notation has the form my_str[start:end], which creates a new string whose value contains the characters of my_str from indices start to end -1. If my_str is 'Boggle', then my_str[0:3] yields string 'Bog'. Other sequence types like lists and tuples also support slice notation.

Figure 5.13.1: String slicing.

url = 'http://en.wikipedia.org/wiki/Turing'
domain = url[7:23]  # Read 'en.wikipedia.org' from url
print(domain)

en.wikipedia.org

The last character of the slice is one location before the specified end. Consider the string my_str = 'John Doe'. The slice my_str[0:4] includes the element at index 0 (J), 1 (o), 2 (h), and 3 (n), but not 4, thus yielding 'John'. The space character at index 4 is not included. Similarly, my_str[4:7] would yield ' Do', including the space character this time. To retrieve the last character, an end index greater than the length of the string can be used. Ex: my_str[5:8] or my_str[5:10] both yield the string 'Doe'.

Negative numbers can be used to specify an index relative to the end of the string. Ex: If the variable my_str is 'Jane Doe!?', then my_str[0:-2] yields 'Jane Doe' because the -2 refers to the second-to-last character '!', and the character at the end index is not included in the result string.

Table 5.13.1: Common slicing operations.

A list of common slicing operations a programmer might use.
Assume the value of my_str is 'http://en.wikipedia.org/wiki/Nasa/'

The slice stride

Slice notation also provides for a third argument known as the stride. The stride determines how much to increment the index after reading each element. For example, my_str[0:10:2] reads every other element between 0 and 10. The stride defaults to 1 if not specified.

Figure 5.13.3: Slice stride.

numbers = '0123456789'

print(f'All numbers: {numbers[::]}')
print(f'Every even number: {numbers[::2]}')
print(f'Every third number between 1 and 8: {numbers[1:9:3]}')

All numbers: 0123456789
Every even number: 02468
Every third number between 1 and 8: 147