Nested list comprehension in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested list comprehension with condition

What is the output of the following Python code?

Python

result = [[x * y for y in range(3)] for x in range(3)]
print(result)

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

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

C [[0, 0, 0], [1, 2, 3], [2, 4, 6]]

D [[0, 0, 0], [0, 1, 2], [0, 2, 4]]

💡 Hint

Think about how the outer and inner loops multiply the indices.

The outer loop runs x from 0 to 2. The inner loop runs y from 0 to 2. Each element is x*y, so for x=0 all are 0, for x=1 it's [0,1,2], for x=2 it's [0,2,4].

❓ Predict Output

intermediate

Nested list comprehension with filtering

What is the output of this code snippet?

Python

result = [[x + y for y in range(4) if (x + y) % 2 == 0] for x in range(3)]
print(result)

A [[0, 2], [2, 4], [2, 4]]

B [[0, 1, 2, 3], [1, 2, 3, 4], [2, 3, 4, 5]]

C [[0, 2], [0, 2], [0, 2]]

D [[0, 2], [1, 3], [2, 4]]

💡 Hint

Only include sums that are even numbers.

For each x from 0 to 2, include x+y only if (x+y) even: x=0: y=0(0), y=2(2) → [0,2]; x=1: y=1(2), y=3(4) → [2,4]; x=2: y=0(2), y=2(4) → [2,4].

🔧 Debug

advanced

Identify the error in nested list comprehension

What error does this code raise when run?

Python

result = [[x * y for x in range(3)] for y in range(3)]
print(result[3][0])

A TypeError

B IndexError

C SyntaxError

D KeyError

💡 Hint

Check the indices used to access the list elements.

The outer list has 3 elements indexed 0,1,2. Accessing index 3 causes IndexError.

📝 Syntax

advanced

Which nested list comprehension is syntactically correct?

Choose the option that is a valid nested list comprehension in Python.

A [x + y for x in range(3), y in range(3)]

B [[x + y for x in range(3) if y] for y in range(3)]

C [[x + y for x in range(3)] for y in range(3)]

D [[x + y for x in range(3) for y in range(3)]]

💡 Hint

Remember the syntax for nested list comprehensions uses separate for clauses inside brackets.

Option C has invalid syntax with comma between for clauses. Option C uses correct nested list comprehension syntax for a 2D list. Option C is syntactically valid but adds a filter. Option C uses chained for clauses in one comprehension, creating a list with a single flat sublist.

🚀 Application

expert

Create a multiplication table using nested list comprehension

Which option creates a 5x5 multiplication table (list of lists) where each element is the product of its row and column indices (starting from 1)?

A [[i * j for j in range(1, 6)] for i in range(1, 6)]

B [[i * j for i in range(1, 6)] for j in range(1, 6)]

C [[i + j for j in range(1, 6)] for i in range(1, 6)]

D [[i * j for j in range(5)] for i in range(5)]

💡 Hint

Rows and columns start at 1, so use range(1, 6).

Option A correctly multiplies row index i and column index j from 1 to 5. Option A produces the same table due to commutativity of multiplication. Option A adds instead of multiplies. Option A starts indices from 0.