Nested lists in Python

Introduction

Nested lists let you store lists inside another list. This helps organize data in groups, like a list of lists.

When you want to store a grid or table of values, like rows and columns.

When you need to group related items together inside a bigger list.

When you want to represent complex data like a list of students, where each student has a list of grades.

When you want to loop through multiple levels of data, like a list of shopping lists.

When you want to build structures like matrices or adjacency lists for graphs.

Syntax

Python

nested_list = [
    ['item1', 'item2', 'item3'],
    ['item4', 'item5', 'item6'],
    ['item7', 'item8', 'item9']
]

Each item inside the main list can itself be a list.

You can access elements using two sets of square brackets: nested_list[row][column]

Examples

This shows an empty nested list with no inner lists.

Python

empty_nested_list = []
print(empty_nested_list)

A nested list with one inner list and one element. Accessing that element prints 42.

Python

single_element_nested_list = [[42]]
print(single_element_nested_list[0][0])

Inner lists can have different lengths. This prints 5 from the second inner list.

Python

mixed_nested_list = [[1, 2], [3, 4, 5], []]
print(mixed_nested_list[1][2])

Accessing the last element (60) in the last inner list.

Python

access_last_element = nested_list = [[10, 20], [30, 40], [50, 60]]
print(nested_list[2][1])

Sample Program

This program creates a nested list like a 3x3 grid, prints it, changes the middle value, and prints it again.

Python

def print_nested_list(nested_list):
    for row_index, inner_list in enumerate(nested_list):
        print(f"Row {row_index}: {inner_list}")

# Create a nested list representing a 3x3 grid
grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print("Original nested list:")
print_nested_list(grid)

# Change the middle element
grid[1][1] = 55

print("\nNested list after changing the middle element:")
print_nested_list(grid)

Important Notes

Accessing elements in nested lists takes two steps: first the outer list, then the inner list.

Time complexity to access an element is O(1) because lists allow direct access by index.

Be careful with empty inner lists; trying to access elements inside them causes errors.

Use nested lists when you need simple multi-level grouping. For more complex data, consider other structures like dictionaries.

Summary

Nested lists store lists inside lists to organize data in multiple levels.

You access elements with two sets of brackets: outer and inner indexes.

They are useful for grids, tables, and grouped data.

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