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.

Practice

1. What does the following nested list comprehension do?
[[x * 2 for x in range(3)] for _ in range(2)]

easy

A. Creates a list of two lists, each with numbers from 0 to 3

B. Creates a flat list of numbers doubled from 0 to 5

C. Creates a list of three lists, each with numbers doubled from 0 to 1

D. Creates a list of two lists, each with numbers doubled from 0 to 2

Solution

1. Step 1: Understand inner list comprehension

   The inner part [x * 2 for x in range(3)] creates a list by doubling numbers 0, 1, 2 resulting in [0, 2, 4].

2. Step 2: Understand outer list comprehension

   The outer loop runs twice (for _ in range(2)), repeating the inner list twice, making [[0, 2, 4], [0, 2, 4]].

3. Final Answer:

   Creates a list of two lists, each with numbers doubled from 0 to 2 -> Option D

4. Quick Check:

   Nested loops create repeated doubled lists [OK]

Hint: Inner loop builds list, outer repeats it [OK]

Common Mistakes:

• Confusing outer and inner loops
• Thinking it creates a flat list
• Miscounting range values

2. Which of the following is the correct syntax for a nested list comprehension that creates a 3x3 matrix of zeros?

easy

A. [[0] * 3] * 3

B. [0 for _ in range(3) for _ in range(3)]

C. [[0 for _ in range(3)] for _ in range(3)]

D. [0] * 3 for _ in range(3)

Solution

1. Step 1: Identify correct nested comprehension syntax

   [[0 for _ in range(3)] for _ in range(3)] uses two for loops inside list brackets correctly: outer and inner loops create rows and columns.

2. Step 2: Check other options for syntax errors

   [0 for _ in range(3) for _ in range(3)] is flat comprehension, not nested. [[0] * 3] * 3 uses list multiplication but not comprehension. [0] * 3 for _ in range(3) is invalid syntax.

3. Final Answer:

   [[0 for _ in range(3)] for _ in range(3)] -> Option C

4. Quick Check:

   Nested loops inside brackets = correct syntax [OK]

Hint: Nested loops inside brackets create nested lists [OK]

Common Mistakes:

• Using flat comprehension instead of nested
• Incorrect list multiplication for nested lists
• Syntax errors with missing brackets

3. What is the output of this code?

matrix = [[i + j for j in range(3)] for i in range(2)]
print(matrix)

medium

A. [[0, 1, 2], [1, 2, 3]]

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

C. [[0, 1], [1, 2], [2, 3]]

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

Solution

1. Step 1: Understand inner loop values

   For each i, inner loop adds j (0,1,2) to i. For i=0: [0+0, 0+1, 0+2] = [0,1,2]. For i=1: [1+0, 1+1, 1+2] = [1,2,3].

2. Step 2: Combine results into outer list

   The outer loop runs for i in range(2), so final list is [[0,1,2], [1,2,3]].

3. Final Answer:

   [[0, 1, 2], [1, 2, 3]] -> Option A

4. Quick Check:

   Sum of i and j in nested loops = output [OK]

Hint: Add outer and inner loop vars for each element [OK]

Common Mistakes:

• Mixing i and j values
• Confusing list dimensions
• Incorrect range usage

4. Find the error in this nested list comprehension:

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

medium

A. No error, code runs correctly

B. Inner loop variable x is undefined

C. Outer loop variable y is used before definition

D. Syntax error due to missing brackets

Solution

1. Step 1: Check variable definitions

   Both x and y are defined in their respective loops: x in inner, y in outer.

2. Step 2: Verify syntax and logic

   Syntax is correct with proper brackets. The code multiplies x and y for each pair in ranges 0 to 2.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Variables defined in loops, syntax correct [OK]

Hint: Check variable scopes in nested loops [OK]

Common Mistakes:

• Thinking variables are undefined
• Confusing loop order
• Assuming syntax error without checking

5. Using nested list comprehension, how can you transform this nested list:

data = [[1, 0, 3], [0, 5, 0], [7, 0, 9]]

to replace all zeros with the string 'zero'?

hard

A. [['zero' if row == 0 else x for x in row] for row in data]

B. [['zero' if x == 0 else x for x in row] for row in data]

C. [['zero' if x == 0 else x for row in data] for x in row]

D. [['zero' if x == 0 else x for x in data] for row in data]

Solution

1. Step 1: Use nested loops to access each element

   Outer loop goes through each row, inner loop through each element x in that row.

2. Step 2: Replace zeros with 'zero' using conditional expression

   Use 'zero' if x == 0 else x inside inner loop to transform elements.

3. Step 3: Combine into nested list comprehension

   Putting it together: [[ 'zero' if x == 0 else x for x in row ] for row in data].

4. Final Answer:

   [['zero' if x == 0 else x for x in row] for row in data] -> Option B

5. Quick Check:

   Conditional inside nested comprehension replaces zeros [OK]

Hint: Use if-else inside inner loop for element replacement [OK]

Common Mistakes:

• Mixing loop variables
• Placing condition on wrong variable
• Incorrect loop order or syntax