Lambda with filter() in Python - Mini Project: Build & Apply

Filter Even Numbers Using Lambda

📖 Scenario: Imagine you have a list of numbers representing daily sales counts. You want to find only the even sales numbers to analyze patterns.

🎯 Goal: Build a small program that uses a lambda function with filter() to select even numbers from a list.

📋 What You'll Learn

Create a list called sales with exact numbers: 12, 7, 9, 20, 15, 8

Create a lambda function inside filter() to select even numbers

Store the filtered result as a list called even_sales

Print the even_sales list

💡 Why This Matters

🌍 Real World

Filtering data like sales numbers helps businesses focus on specific patterns, such as even or odd counts, which can be important for inventory or promotions.

💼 Career

Understanding how to use lambda with filter() is useful for data cleaning and processing tasks common in data analysis and software development.

1

Create the sales list

Create a list called sales with these exact numbers: 12, 7, 9, 20, 15, 8.

Python

# Create the sales list with the exact numbers
# Your code here

Hint

Use square brackets [] to create the list and separate numbers with commas.

2

Prepare to filter even numbers

Create a variable called even_sales and assign it the result of filter() using a lambda function that checks if a number is even (divisible by 2) applied to the sales list.

Python

sales = [12, 7, 9, 20, 15, 8]
# Use filter() with a lambda to select even numbers from sales
# Your code here

Hint

The lambda function should take one argument and return True if the number is even.

3

Convert filter object to list

Convert the filter object even_sales to a list by wrapping it with list() and reassign it back to even_sales.

Python

sales = [12, 7, 9, 20, 15, 8]
even_sales = filter(lambda x: x % 2 == 0, sales)
# Convert even_sales to a list
# Your code here

Hint

Use list() to turn the filter result into a list you can print.

4

Print the even sales list

Print the even_sales list to display the filtered even numbers.

Python

sales = [12, 7, 9, 20, 15, 8]
even_sales = filter(lambda x: x % 2 == 0, sales)
even_sales = list(even_sales)
# Print the even_sales list
# Your code here

Hint

Use print(even_sales) to show the result.

Practice

1. What does the following code do?
filter(lambda x: x > 5, [2, 7, 4, 10])

easy

A. Selects numbers greater than 5 from the list

B. Selects numbers less than 5 from the list

C. Returns the sum of numbers greater than 5

D. Sorts the list in ascending order

Solution

1. Step 1: Understand the lambda condition

   The lambda function checks if each number is greater than 5.

2. Step 2: Apply filter with the lambda

   filter() keeps only numbers where the lambda returns True, so numbers > 5.

3. Final Answer:

   Selects numbers greater than 5 from the list -> Option A

4. Quick Check:

   filter with lambda x > 5 = select numbers > 5 [OK]

Hint: filter + lambda picks items where condition is True [OK]

Common Mistakes:

• Thinking filter sums or sorts the list
• Confusing greater than with less than
• Assuming filter changes the original list

2. Which of these is the correct syntax to filter even numbers from a list nums using lambda and filter?

easy

A. filter(lambda x: x % 2 == 0, nums)

B. filter(lambda x: x // 2 == 0, nums)

C. filter(lambda x: x % 2, nums)

D. filter(lambda x: x == 2, nums)

Solution

1. Step 1: Identify condition for even numbers

   Even numbers have zero remainder when divided by 2, so x % 2 == 0.

2. Step 2: Check filter syntax

   filter(lambda x: x % 2 == 0, nums) correctly filters even numbers.

3. Final Answer:

   filter(lambda x: x % 2 == 0, nums) -> Option A

4. Quick Check:

   Even check = x % 2 == 0 [OK]

Hint: Use x % 2 == 0 to check even numbers [OK]

Common Mistakes:

• Using floor division (//) instead of modulo (%)
• Using x % 2 without comparison (filters odd numbers)
• Checking equality to 2 instead of remainder zero

3. What is the output of this code?

nums = [1, 3, 6, 8, 11]
result = list(filter(lambda x: x < 7, nums))
print(result)

medium

A. [7, 8, 11]

B. [6, 8, 11]

C. [1, 3, 6, 8]

D. [1, 3, 6]

Solution

1. Step 1: Understand the filter condition

   The lambda keeps numbers less than 7.

2. Step 2: Apply condition to each list item

   1 < 7 (keep), 3 < 7 (keep), 6 < 7 (keep), 8 < 7 (no), 11 < 7 (no).

3. Final Answer:

   [1, 3, 6] -> Option D

4. Quick Check:

   Filter x < 7 = [1, 3, 6] [OK]

Hint: Filter keeps items where lambda returns True [OK]

Common Mistakes:

• Including numbers not meeting condition
• Confusing less than with greater than
• Not converting filter object to list before printing

4. Find the error in this code snippet:

nums = [10, 15, 20]
filtered = filter(lambda x: x % 2 = 0, nums)
print(list(filtered))

medium

A. Incorrect list conversion syntax

B. Missing parentheses after filter

C. Using '=' instead of '==' in lambda condition

D. Lambda function missing argument

Solution

1. Step 1: Check lambda condition syntax

   The condition uses '=' which is assignment, not comparison.

2. Step 2: Correct operator usage

   It should be '==' to compare if x % 2 equals 0.

3. Final Answer:

   Using '=' instead of '==' in lambda condition -> Option C

4. Quick Check:

   Comparison needs '==' not '=' [OK]

Hint: Use '==' for comparison inside lambda [OK]

Common Mistakes:

• Using '=' instead of '==' in conditions
• Forgetting to convert filter result to list
• Missing lambda argument

5. You have a list of words: words = ['apple', '', 'banana', ' ', 'cherry', None]. Which code correctly filters out empty strings, strings with only spaces, and None values using lambda and filter?

hard

A. list(filter(lambda x: x == '', words))

B. list(filter(lambda x: x and x.strip(), words))

C. list(filter(lambda x: x is None, words))

D. list(filter(lambda x: x.strip() == '', words))

Solution

1. Step 1: Understand filtering criteria

   We want to remove empty strings (''), strings with only spaces (' '), and None values.

2. Step 2: Analyze each option

   list(filter(lambda x: x and x.strip(), words)) checks x is truthy (not None or empty) and x.strip() removes spaces; if non-empty after strip, it keeps the word.

3. Final Answer:

   list(filter(lambda x: x and x.strip(), words)) -> Option B

4. Quick Check:

   Filter removes empty, spaces, None with x and x.strip() [OK]

Hint: Use x and x.strip() to remove empty/space/None [OK]

Common Mistakes:

• Filtering only empty strings but not spaces or None
• Checking equality to None incorrectly
• Calling strip() on None causing error