List comprehension vs loop in Python - Hands-On Comparison

List comprehension vs loop

📖 Scenario: You work in a small bakery. Every day, you record the number of cookies sold in a list. You want to find out how many cookies you sold each day after giving a 10% discount on the price per cookie.

🎯 Goal: Build a program that calculates the discounted cookie prices using both a for loop and a list comprehension. Compare the results.

📋 What You'll Learn

Create a list called cookie_prices with exact values: 2.0, 2.5, 3.0, 2.75, 3.25

Create a variable called discount_rate and set it to 0.10

Use a for loop with variable price to create a list called discounted_prices_loop with prices after discount

Use a list comprehension with variable price to create a list called discounted_prices_comp with prices after discount

Print both discounted_prices_loop and discounted_prices_comp

💡 Why This Matters

🌍 Real World

Calculating discounts on product prices is common in stores and online shops to attract customers.

💼 Career

Understanding loops and list comprehensions helps in data processing tasks, which are common in programming jobs.

1

Create the list of cookie prices

Create a list called cookie_prices with these exact values: 2.0, 2.5, 3.0, 2.75, 3.25

Python

# Create the list cookie_prices with the exact values
# Your code here

Hint

Use square brackets [] to create a list with the given numbers separated by commas.

2

Set the discount rate

Create a variable called discount_rate and set it to 0.10

Python

cookie_prices = [2.0, 2.5, 3.0, 2.75, 3.25]
# Create the variable discount_rate and set it to 0.10
# Your code here

Hint

Use the equals sign = to assign the value 0.10 to the variable discount_rate.

3

Calculate discounted prices using a for loop and a list comprehension

Use a for loop with variable price to create a list called discounted_prices_loop with prices after discount. Then use a list comprehension with variable price to create a list called discounted_prices_comp with prices after discount.

Python

cookie_prices = [2.0, 2.5, 3.0, 2.75, 3.25]
discount_rate = 0.10
# Use a for loop with price to create discounted_prices_loop
# Use a list comprehension with price to create discounted_prices_comp
# Your code here

Hint

In the for loop, multiply each price by (1 - discount_rate) and add it to discounted_prices_loop. In the list comprehension, do the same calculation inside square brackets.

4

Print the discounted prices lists

Print the list discounted_prices_loop and then print the list discounted_prices_comp

Python

cookie_prices = [2.0, 2.5, 3.0, 2.75, 3.25]
discount_rate = 0.10

discounted_prices_loop = []
for price in cookie_prices:
    discounted_price = price * (1 - discount_rate)
    discounted_prices_loop.append(discounted_price)

discounted_prices_comp = [price * (1 - discount_rate) for price in cookie_prices]

# Print discounted_prices_loop
# Print discounted_prices_comp
# Your code here

Hint

Use two print statements, one for each list.

Practice

1. What is the main advantage of using list comprehension over a traditional for loop in Python?

easy

A. It runs slower than a loop

B. It creates lists in fewer lines and is more readable

C. It cannot be used to create lists

D. It requires more code to write

Solution

1. Step 1: Understand list comprehension purpose

   List comprehension is designed to create lists in a concise and readable way.

2. Step 2: Compare with traditional loops

   Traditional loops require more lines and are less compact than list comprehensions.

3. Final Answer:

   It creates lists in fewer lines and is more readable -> Option B

4. Quick Check:

   List comprehension = concise and readable [OK]

Hint: List comprehension is shorter and clearer than loops [OK]

Common Mistakes:

• Thinking list comprehension is slower
• Believing list comprehension can't create lists
• Confusing loops as always simpler

2. Which of the following is the correct syntax for a list comprehension that creates a list of squares of numbers from 0 to 4?

easy

A. [x**2 for x in range(5)]

B. [for x in range(5) x**2]

C. [x^2 in range(5)]

D. for x in range(5): x**2

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is: [expression for variable in iterable].

2. Step 2: Check each option

   [x**2 for x in range(5)] matches the correct syntax. Others have syntax errors or missing parts.

3. Final Answer:

   [x**2 for x in range(5)] -> Option A

4. Quick Check:

   Correct list comprehension syntax = [x**2 for x in range(5)] [OK]

Hint: Remember: [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after expression
• Using ^ instead of ** for power
• Writing loop without brackets

3. What is the output of the following code?

nums = [1, 2, 3]
squares = []
for n in nums:
    squares.append(n**2)
print(squares)

medium

A. [1, 2, 3]

B. [2, 4, 6]

C. [1, 4, 9]

D. Error

Solution

1. Step 1: Understand the loop operation

   The loop goes through each number in nums and appends its square to squares.

2. Step 2: Calculate squares for each element

   1 squared is 1, 2 squared is 4, 3 squared is 9, so squares = [1, 4, 9].

3. Final Answer:

   [1, 4, 9] -> Option C

4. Quick Check:

   Squares of [1,2,3] = [1,4,9] [OK]

Hint: Loop appends squares, so output is squared list [OK]

Common Mistakes:

• Confusing append with extend
• Expecting original list instead of squares
• Syntax errors in loop

4. Identify the error in this list comprehension:

result = [x*2 for x in range(5)
print(result)

medium

A. print statement is incorrect

B. Using wrong operator * instead of **

C. range(5) should be range[5]

D. Missing closing bracket "]" in list comprehension

Solution

1. Step 1: Check list comprehension syntax

   The list comprehension is missing the closing square bracket "]" at the end.

2. Step 2: Verify other parts

   Operator * is correct for multiplication, range(5) is correct, and print syntax is valid.

3. Final Answer:

   Missing closing bracket "]" in list comprehension -> Option D

4. Quick Check:

   Syntax error due to missing bracket [OK]

Hint: Always close brackets in comprehensions [OK]

Common Mistakes:

• Forgetting closing bracket
• Confusing parentheses and brackets
• Misusing range syntax

5. You want to create a list of even numbers from 0 to 10 using list comprehension. Which code correctly does this?

hard

A. [x for x in range(11) if x % 2 == 0]

B. [x*2 for x in range(5)]

C. [x for x in range(10) if x % 2]

D. [x for x in range(11) if x / 2 == 0]

Solution

1. Step 1: Understand the requirement

   We want even numbers from 0 to 10 inclusive, so numbers divisible by 2.

2. Step 2: Analyze each option

   [x for x in range(11) if x % 2 == 0] uses correct range and condition (x % 2 == 0). [x*2 for x in range(5)] doubles numbers 0-4, giving [0,2,4,6,8]. [x for x in range(10) if x % 2] filters odd numbers (x % 2 is true for odd). [x for x in range(11) if x / 2 == 0] uses division instead of modulo, which is incorrect.

3. Final Answer:

   [x for x in range(11) if x % 2 == 0] -> Option A

4. Quick Check:

   Filter even numbers with modulo == 0 [OK]

Hint: Use modulo % 2 == 0 to filter even numbers [OK]

Common Mistakes:

• Using multiplication instead of filtering
• Using wrong condition for even check
• Confusing division with modulo