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.