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.

Practice

1. What is a nested list in Python?

easy

A. A list with only one element

B. A list that contains other lists as its elements

C. A list that cannot be changed

D. A list that stores only numbers

Solution

1. Step 1: Understand list elements

   A list can hold many types of elements, including other lists.

2. Step 2: Define nested list

   A nested list is a list where some elements are themselves lists.

3. Final Answer:

   A list that contains other lists as its elements -> Option B

4. Quick Check:

   Nested list = list inside list [OK]

Hint: Think of a list inside another list [OK]

Common Mistakes:

• Confusing nested list with single-level list
• Thinking nested list is immutable
• Assuming nested list holds only numbers

2. Which of the following is the correct syntax to create a nested list with two inner lists?

easy

A. [[1, 2], [3, 4]]

B. [1, 2, 3, 4]

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

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

Solution

1. Step 1: Identify nested list syntax

   A nested list has lists inside the main list, each enclosed in brackets.

2. Step 2: Check options

   [[1, 2], [3, 4]] has two inner lists: [1, 2] and [3, 4], correctly nested.

3. Final Answer:

   [[1, 2], [3, 4]] -> Option A

4. Quick Check:

   Two inner lists = double brackets [OK]

Hint: Look for double brackets for nested lists [OK]

Common Mistakes:

• Using single brackets only
• Mixing elements and lists incorrectly
• Missing commas between inner lists

3. What is the output of the following code?

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

medium

A. 3

B. 1

C. 4

D. 2

Solution

1. Step 1: Access outer list element

   matrix[1] accesses the second inner list: [3, 4].

2. Step 2: Access inner list element

   matrix[1][0] accesses the first element of [3, 4], which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   matrix[1][0] = 3 [OK]

Hint: First index outer list, second index inner list [OK]

Common Mistakes:

• Mixing up indexes (using [0][1] instead)
• Forgetting zero-based indexing
• Confusing inner and outer list positions

4. Find the error in this code snippet:

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

medium

A. No error, output is 3

B. SyntaxError due to missing brackets

C. TypeError because data is not a list

D. IndexError because data[2] does not exist

Solution

1. Step 1: Check list length

   data has two elements: indexes 0 and 1 only.

2. Step 2: Accessing invalid index

   data[2] tries to access a third element which does not exist, causing IndexError.

3. Final Answer:

   IndexError because data[2] does not exist -> Option D

4. Quick Check:

   Index out of range = IndexError [OK]

Hint: Check list length before accessing index [OK]

Common Mistakes:

• Assuming index 2 exists
• Confusing syntax error with runtime error
• Ignoring zero-based indexing

5. Given a nested list nums = [[1, 2, 3], [4, 5], [6]], which code correctly prints all elements in order?

hard

A. print(nums[0][0], nums[1][1], nums[2][0])

B. for i in range(len(nums)):
print(nums[i])

C. for sublist in nums:
for num in sublist:
print(num)

D. for num in nums:
print(num)

Solution

1. Step 1: Understand nested iteration

   To print all elements, loop over each inner list, then each element inside it.

2. Step 2: Analyze options

   for sublist in nums: for num in sublist: print(num) uses two loops: outer for sublist, inner for num, printing each number.

3. Final Answer:

   for sublist in nums: for num in sublist: print(num) -> Option C

4. Quick Check:

   Nested loops print all elements [OK]

Hint: Use nested loops to access nested list elements [OK]

Common Mistakes:

• Printing inner lists instead of elements
• Using single loop only
• Hardcoding indexes without loops