9.2 Sorting and reversing lists

Learning objectives

By the end of this section you should be able to

• Understand the concept of sorting.
• Use built-in sort() and reverse() methods.

Sorting

Ordering elements in a sequence is often useful. Sorting is the task of arranging elements in a sequence in ascending or descending order.

Sorting can work on numerical or non-numerical data. When ordering text, dictionary order is used. Ex: "bat" comes before "cat" because "b" comes before "c".

Using

sort()

and

reverse()

Python provides methods for arranging elements in a list.

• The sort() method arranges the elements of a list in ascending order. For strings, ASCII values are used and uppercase characters come before lowercase characters, leading to unexpected results. Ex: "A" is ordered before "a" in ascending order but so is "G"; thus, "Gail" comes before "apple".
• The reverse() method reverses the elements in a list.

    # Setup a list of numbers
    num_list = [38, 92, 23, 16]
    print(num_list)

    # Sort the list
    num_list.sort()
    print(num_list)

    # Setup a list of words
    dance_list = ["Stepping", "Ballet", "Salsa", "Kathak", "Hopak", "Flamenco", "Dabke"]

    # Reverse the list
    dance_list.reverse()
    print(dance_list)

    # Sort the list
    dance_list.sort()
    print(dance_list)

    [38, 92, 23, 16]
    [16, 23, 38, 92]
    ["Dabke", "Flamenco", "Hopak", "Kathak", "Salsa", "Ballet", "Stepping"]
    ["Ballet", "Dabke", "Flamenco", "Hopak", "Kathak", "Salsa", "Stepping"]

board_games = ["go", "chess", "scrabble", "checkers"]