Nested lists in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested list in Python?

A nested list is a list that contains other lists as its elements. It's like a box inside another box, where each inner box can hold items.

beginner

How do you access an element inside a nested list?

You use multiple square brackets. The first bracket selects the inner list, and the second bracket selects the element inside that inner list. For example, list[0][1] accesses the second item of the first inner list.

intermediate

How can you add an element to an inner list inside a nested list?

First, access the inner list using its index, then use the append() method. For example, nested[1].append(5) adds 5 to the second inner list.

beginner

What does this code output?

nested = [[1, 2], [3, 4]]
print(nested[1][0])

It outputs 3 because nested[1] is the second inner list [3, 4], and nested[1][0] is the first element of that list, which is 3.

beginner

Can nested lists have different lengths for inner lists?

Yes! Inner lists can have different numbers of elements. For example, [[1, 2], [3, 4, 5]] is valid.

What is the output of this code?

lst = [[10, 20], [30, 40]]
print(lst[0][1])

A 20

B 10

C 30

D 40

lst[0] is [10, 20], and lst[0][1] is the second element, which is 20.

How do you add the number 5 to the second inner list in nested = [[1], [2, 3]]?

A nested[0].append(5)

B nested.append(5)

C nested[1][5]

D nested[1].append(5)

nested[1] accesses the second inner list, and append(5) adds 5 to it.

Which of these is a valid nested list?

A [1, 2, 3, 4]

B [[1, 2], [3, 4, 5]]

C [[1, 2], 3, 4]

D [1, [2, 3], 4]

A nested list must have lists inside it. Options B and D have inner lists.

What does nested[2][0] do if nested = [[1], [2, 3]]?

A Returns 2

B Returns 1

C Causes an error because nested has no third inner list

D Returns 3

nested has only two inner lists at indexes 0 and 1, so index 2 is out of range.

How can you print all elements of a nested list nested = [[1, 2], [3, 4]]?

A Use two loops: for inner in nested: for item in inner: print(item)

B Use one loop: for item in nested: print(item)

C Use nested.print()

D Use nested.flatten()

Two loops are needed: one to go through inner lists, one to go through items inside them.

Explain what a nested list is and how you access elements inside it.

Think of a list inside another list like boxes inside boxes.

Describe how to add an element to an inner list within a nested list.

First find the right inner list, then add the item.