Nested list comprehension in Python - Mini Project: Build & Apply

Nested list comprehension

📖 Scenario: You work in a bakery that sells boxes of assorted cookies. Each box contains several types of cookies, and you want to create a simple list showing all cookie types available in all boxes.

🎯 Goal: Build a nested list comprehension to flatten a list of cookie boxes into a single list of cookie types.

📋 What You'll Learn

Create a list of lists called cookie_boxes with exact cookie types

Create a variable called all_cookies using nested list comprehension

Print the all_cookies list

💡 Why This Matters

🌍 Real World

Bakeries and stores often have grouped data like boxes or categories. Flattening such data helps in searching or displaying all items easily.

💼 Career

Understanding nested list comprehension is useful for data processing jobs, where you often work with nested or grouped data structures.

1

Create the cookie boxes data

Create a list called cookie_boxes with these exact lists inside: ["chocolate chip", "oatmeal"], ["peanut butter", "snickerdoodle"], and ["sugar", "gingersnap"].

Python

# Create the list cookie_boxes with the exact cookie types
# Your code here

Hint

Remember, cookie_boxes is a list containing three lists, each with two cookie names as strings.

2

Prepare the variable for all cookies

Create an empty list called all_cookies to hold all cookie types before using list comprehension.

Python

cookie_boxes = [["chocolate chip", "oatmeal"], ["peanut butter", "snickerdoodle"], ["sugar", "gingersnap"]]
# Create an empty list called all_cookies
# Your code here

Hint

Just create a variable named all_cookies and set it to an empty list [].

3

Use nested list comprehension to flatten the list

Use nested list comprehension to fill all_cookies with every cookie from each box in cookie_boxes. Use all_cookies = [cookie for box in cookie_boxes for cookie in box] exactly.

Python

cookie_boxes = [["chocolate chip", "oatmeal"], ["peanut butter", "snickerdoodle"], ["sugar", "gingersnap"]]
all_cookies = []
# Use nested list comprehension to fill all_cookies
# Your code here

Hint

Remember the order: first loop over each box in cookie_boxes, then over each cookie in that box.

4

Print the list of all cookies

Print the all_cookies list using print(all_cookies).

Python

cookie_boxes = [["chocolate chip", "oatmeal"], ["peanut butter", "snickerdoodle"], ["sugar", "gingersnap"]]
all_cookies = [cookie for box in cookie_boxes for cookie in box]
# Print the all_cookies list
# Your code here

Hint

Use the print() function to show the list.

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