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.

Example 1 Sorting and reversing lists

# 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)

The above code's output is:


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

Concepts in Practice

sort and reverse methods

Use the following list for the questions below.

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

Try It

Sorting and reversing

Complete the program below to arrange and print the numbers in ascending and descending order.

num_list = [3, 32, -23, 699, 75, -101, 22, 45, 94, -3, 2]

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.