Nested lists in Python - Step-by-Step Execution

Concept Flow - Nested lists

Create outer list

↓

Add inner lists as elements

↓

Access outer list element (inner list)

↓

Access element inside inner list

↓

Use nested loops to iterate

↓

Process or print inner elements

We create a list that contains other lists. We access inner lists by indexing the outer list, then access elements inside those inner lists.

Execution Sample

Python

matrix = [[1, 2], [3, 4], [5, 6]]
for i in range(len(matrix)):
    for j in range(len(matrix[i])):
        print(matrix[i][j])

This code creates a nested list (a list of lists) and prints each number inside it using nested loops.

Execution Table

Step	i (outer index)	j (inner index)	Condition i < len(matrix)	Condition j < len(matrix[i])	Action	Output
1	0	0	0 < 3 → True	0 < 2 → True	Print matrix[0][0]	1
2	0	1	0 < 3 → True	1 < 2 → True	Print matrix[0][1]	2
3	0	2	0 < 3 → True	2 < 2 → False	Inner loop ends
4	1	0	1 < 3 → True	0 < 2 → True	Print matrix[1][0]	3
5	1	1	1 < 3 → True	1 < 2 → True	Print matrix[1][1]	4
6	1	2	1 < 3 → True	2 < 2 → False	Inner loop ends
7	2	0	2 < 3 → True	0 < 2 → True	Print matrix[2][0]	5
8	2	1	2 < 3 → True	1 < 2 → True	Print matrix[2][1]	6
9	2	2	2 < 3 → True	2 < 2 → False	Inner loop ends
10	3	-	3 < 3 → False	-	Outer loop ends

💡 Outer loop ends when i reaches 3 because 3 < 3 is False

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	Final
i	-	0	0	1	1	2	2	2	3	3
j	-	0	1	0	1	0	1	2	2	-
Output	-	1	2	3	4	5	6	-	-	-

Key Moments - 3 Insights

Why do we need two loops to access elements in nested lists?

Because the outer list contains inner lists, the first loop picks an inner list, and the second loop goes through elements inside that inner list. See execution_table rows 1-9.

Why does the inner loop stop when j equals 2 for the first inner list?

Because the inner list has length 2, so when j reaches 2, the condition j < len(matrix[i]) becomes False (see row 3).

What happens if we try to access matrix[3][0]?

It causes an error because the outer list has only 3 elements (indices 0,1,2). The outer loop stops before i reaches 3 (see row 10).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'i' at step 7?

A 0

B 1

C 2

D 3

💡 Hint

Check the 'i (outer index)' column at step 7 in the execution_table.

At which step does the inner loop first end for the outer list's first element?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for the first row where 'Condition j < len(matrix[i])' is False in execution_table.

If the inner lists had 3 elements instead of 2, how would the execution table change?

A The outer loop would run fewer times

B There would be more steps with j = 2 printing new values

C The inner loop would end earlier

D The outer loop would never end

💡 Hint

Check how the inner loop depends on the length of inner lists in execution_table rows.

Concept Snapshot

Nested lists are lists inside lists.
Access with two indices: outer and inner.
Use nested loops to visit all elements.
Outer loop picks inner list, inner loop picks element.
Index carefully to avoid errors.

Full Transcript

Nested lists are lists that contain other lists as elements. To access elements inside, we first select an inner list by indexing the outer list, then select an element inside that inner list by indexing again. This requires two indices. To process all elements, we use nested loops: the outer loop goes through each inner list, and the inner loop goes through each element inside that inner list. The loops stop when indices reach the lengths of their respective lists. Trying to access beyond these lengths causes errors. This example code creates a nested list called matrix with three inner lists, each containing two numbers. The nested loops print each number one by one. The execution table shows each step, the values of indices, conditions checked, actions taken, and output printed. The variable tracker shows how the indices and output change over time. Key moments clarify why two loops are needed, why loops stop at certain points, and what happens if we try to access out-of-range elements. The quiz tests understanding of index values at steps, loop ending conditions, and how changing inner list size affects execution. The snapshot summarizes the concept in a few lines for quick review.

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