Nested Lists

A nested list is a list that contains other lists! Oftentimes, we’ll visualize a 2D nested list using a grid.

seating_chart = [
    ["Mike", "John", "Julio", "Elizabeth"],
    ["Angela", "Sophia", "Randy", "Maria"],
    ["Raul", "Pauline", "Rachel", "Monica"]
]

Practice: Chained indexing¶

To access individual elements, we can chain indexing operations one after the other. What would Python display?

seating_chart[1]
seating_chart[1][1]
seating_chart[2][3]
seating_chart[2][3][4]

Iteration¶

How do you iterate through a nested list to print out each name one at a time? One way is to loop over each inner list, one inner list at a time.

for row in seating_chart:
    for student in row:
        print(student)

Another way is to loop over the indices of the outer list and then the indices of each inner list.

for ri in range(len(seating_chart)):
    for ci in range(len(seating_chart[ri])):
        print(seating_chart[ri][ci])

When would you prefer to use one approach over the other?

Example: Even more nested lists¶

We can nest lists as deeply as we need to.

letters = [
    [["a", "b"], ["c", "d"], ["e", "f"]],
    [["g", "h"], ["i", "j"], ["k", "l"]]
]

letters[0]
letters[0][2]
letters[1][2][0]

Practice: Average temperature¶

First, write a helper function average that returns the average of a list of values.

[23, 24, 25, 27, 26] # Average: 25.0

Then, write a function weekly_averages that takes a nested list of temperatures for each week and prints the average temperature each week.

temperatures = [
    [23, 24, 25, 27, 26], # Average: 25.0
    [0, -1, 1, -3, -2],   # Average: -1.0
    [9, 10, 11, 12, 11]   # Average: 10.6
]

How does Python Tutor pass each inner list from weekly_averages to average?

Practice: Form study groups¶

Write a function form_groups that takes a list and an int maximum size_limit and returns a nested list where the given list elements are divided into groups of at most the size_limit. For example:

students = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

form_groups(students, 5)
# [[0, 1, 2, 3, 4], [5, 6, 7, 8, 9]]

form_groups(students, 3)
# [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]]

Then, write a docstring and doctests (documentation tests) for both of these examples.