Nested lists in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested list indexing

What is the output of this Python code?

Python

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
result = matrix[1][2]
print(result)

A 6

B 5

C 4

D IndexError

💡 Hint

Remember that indexing starts at 0 and matrix[1] is the second list.

matrix[1] accesses the second list [4, 5, 6]. Then [2] accesses the third element in that list, which is 6.

❓ Predict Output

intermediate

Length of nested lists

What is the output of this code?

Python

nested = [[1, 2], [3, 4, 5], [6]]
length = len(nested[1])
print(length)

A 1

B 3

C 2

D TypeError

💡 Hint

len() returns the number of elements in the list at index 1.

nested[1] is [3, 4, 5], which has 3 elements.

❓ Predict Output

advanced

Output of nested list comprehension

What is the output of this code?

Python

matrix = [[i * j for j in range(3)] for i in range(3)]
print(matrix)

A [[0, 0, 0], [1, 1, 1], [2, 2, 2]]

B [[0, 1, 2], [0, 1, 2], [0, 1, 2]]

C [[0, 0, 0], [0, 1, 2], [0, 2, 4]]

D SyntaxError

💡 Hint

Each element is i multiplied by j for j in 0 to 2, repeated for i in 0 to 2.

For i=0: [0*0, 0*1, 0*2] = [0,0,0]; i=1: [0,1,2]; i=2: [0,2,4].

❓ Predict Output

advanced

Value after nested list modification

What is the value of nested[0][1] after running this code?

Python

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

A 2

B IndexError

C 4

D 3

💡 Hint

nested[1][0] is assigned to nested[0][1].

nested[1][0] is 3, so nested[0][1] becomes 3.

🧠 Conceptual

expert

Error type from invalid nested list access

What error does this code raise?

Python

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

A IndexError

B TypeError

C AttributeError

D KeyError

💡 Hint

Check if the outer list has an element at index 2.

lst has only indices 0 and 1. Accessing lst[2] causes IndexError.

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