Nested lists in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the first element of the first nested list.

Python

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

Drag options to blanks, or click blank then click option'

A[1]

B[0]

C[[0]]

D(0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets.

Using the wrong index like [1] instead of [0].

To get the first nested list, use [0]. Then [0] accesses its first element.

2fill in blank

medium

Complete the code to print the length of the second nested list.

Python

nested = [[5, 6, 7], [8, 9]]
print(len(nested[1]))

Drag options to blanks, or click blank then click option'

A[1]

B[0]

C[2]

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 0 instead of 1.

Trying to get length of the whole nested list instead of the inner list.

The second nested list is at index 1. Use len(nested[1]) to get its length.

3fill in blank

hard

Fix the error in the code to correctly access the number 10 inside the nested list.

Python

numbers = [[4, 5], [6, 7, 8], [9, 10]]
print(numbers[1][1])

Drag options to blanks, or click blank then click option'

A[2]

B[3]

C[1]

D(2)

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 which is out of range.

Using parentheses instead of square brackets.

The number 10 is in the third nested list, which is at index 2. Then index [1] accesses 10.

4fill in blank

hard

Fill both blanks to create a nested list with two inner lists, each containing numbers 1 to 3 and 4 to 6.

Python

nested = [[1], [2]]
print(nested)

Drag options to blanks, or click blank then click option'

A[1, 2, 3]

B[4, 5, 6]

C[7, 8, 9]

D[0, 1, 2]

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong number ranges.

Putting numbers directly without lists.

The first inner list should be [1, 2, 3] and the second [4, 5, 6] to match the instruction.

5fill in blank

hard

Fill all three blanks to create a nested list comprehension that squares numbers 1 to 3 and 4 to 6 separately.

Python

nested_squares = [[x[1]2 for x in range([2], 4)], [x[1]2 for x in range(4, [3])] ]
print(nested_squares)

Drag options to blanks, or click blank then click option'

A**

B1

C4

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication * instead of power **.

Wrong range start or end values.

The operator for power is **. The first range starts at 1, the second ends before 7 to include 6.

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