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

Using the filter() function to select even numbers

📖 Scenario: Imagine you have a list of numbers representing ages of people in a group. You want to find only the even ages to organize a special event for them.

🎯 Goal: You will build a small program that uses the filter() function to select even numbers from a list of ages.

📋 What You'll Learn

Create a list called ages with the exact values: 12, 17, 20, 25, 30, 33, 40

Create a function called is_even that returns True if a number is even

Use the filter() function with is_even and ages to get only even ages

Convert the filter result to a list called even_ages

Print the even_ages list

💡 Why This Matters

🌍 Real World

Filtering data is common when you want to select only certain items from a list, like customers above a certain age or products in stock.

💼 Career

Understanding filter() helps in data cleaning, processing lists, and working efficiently with collections in many programming jobs.

1

Create the list of ages

Create a list called ages with these exact values: 12, 17, 20, 25, 30, 33, 40

Python

# Create the list called ages with the given numbers
# Your code here

Hint

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

2

Define the function to check even numbers

Define a function called is_even that takes one parameter num and returns True if num % 2 == 0, otherwise returns False

Python

ages = [12, 17, 20, 25, 30, 33, 40]
# Define the function is_even(num) below
# Your code here

Hint

The % operator gives the remainder. Even numbers have remainder 0 when divided by 2.

3

Use filter() to select even ages

Use the filter() function with is_even and ages, then convert the result to a list called even_ages

Python

ages = [12, 17, 20, 25, 30, 33, 40]
def is_even(num):
    return num % 2 == 0
# Use filter() with is_even and ages, convert to list even_ages
# Your code here

Hint

Remember to wrap filter() with list() to get a list.

4

Print the list of even ages

Print the list even_ages using print(even_ages)

Python

ages = [12, 17, 20, 25, 30, 33, 40]
def is_even(num):
    return num % 2 == 0

even_ages = list(filter(is_even, ages))
# Print the even_ages list
# Your code here

Hint

The output should show only the even numbers from the list.

Practice

1. What does the filter() function do in Python?

easy

A. Sorts the items in a list

B. Changes all items in a list to uppercase

C. Selects items from a list that meet a condition

D. Adds all items in a list together

Solution

1. Step 1: Understand the purpose of filter()

   The filter() function takes a function and a list, then keeps only items where the function returns True.

2. Step 2: Compare options with the purpose

   Only Selects items from a list that meet a condition describes selecting items based on a condition, which matches filter()'s job.

3. Final Answer:

   Selects items from a list that meet a condition -> Option C

4. Quick Check:

   filter() selects items [OK]

Hint: Remember: filter keeps items passing the test [OK]

Common Mistakes:

• Thinking filter changes items instead of selecting
• Confusing filter with map or sort
• Assuming filter adds or combines items

2. Which of these is the correct syntax to use filter() to keep even numbers from a list nums?

easy

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

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

C. filter(x % 2 == 0, nums)

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

Solution

1. Step 1: Recall filter() syntax

   The correct syntax is filter(function, iterable), where function tests each item.

2. Step 2: Check each option

   filter(lambda x: x % 2 == 0, nums) uses lambda x: x % 2 == 0 as function and nums as iterable, which is correct.

3. Final Answer:

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

4. Quick Check:

   filter(function, iterable) correct order [OK]

Hint: filter(function, iterable) order matters [OK]

Common Mistakes:

• Swapping function and iterable arguments
• Using expression instead of function
• Missing lambda or function for filtering

3. What is the output of this code?

nums = [1, 2, 3, 4, 5]
even_nums = list(filter(lambda x: x % 2 == 0, nums))
print(even_nums)

medium

A. [1, 3, 5]

B. [2, 4]

C. [1, 2, 3, 4, 5]

D. []

Solution

1. Step 1: Understand the filter condition

   The lambda function keeps numbers where x % 2 == 0, meaning even numbers.

2. Step 2: Apply filter to the list

   From [1, 2, 3, 4, 5], only 2 and 4 are even, so the filtered list is [2, 4].

3. Final Answer:

   [2, 4] -> Option B

4. Quick Check:

   Filter keeps even numbers [OK]

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

Common Mistakes:

• Confusing even and odd numbers
• Forgetting to convert filter to list
• Expecting original list unchanged

4. Find the error in this code snippet:

nums = [10, 15, 20]
result = filter(x % 10 == 0, nums)
print(list(result))

medium

A. Wrong variable name 'nums'

B. No error, code runs fine

C. filter() cannot be converted to list

D. Missing lambda function for filter

Solution

1. Step 1: Check filter function argument

   The first argument to filter() must be a function, but x % 10 == 0 is an expression, not a function.

2. Step 2: Identify fix

   We need to wrap the expression in a lambda: lambda x: x % 10 == 0 to make it a function.

3. Final Answer:

   Missing lambda function for filter -> Option D

4. Quick Check:

   filter needs a function as first argument [OK]

Hint: filter needs a function, use lambda for expressions [OK]

Common Mistakes:

• Passing expression instead of function
• Assuming filter returns list directly
• Ignoring syntax errors in lambda usage

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

hard

A. list(filter(lambda w: w, words))

B. list(filter(lambda w: w == '', words))

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

D. list(filter(lambda w: w != None or w != '', words))

Solution

1. Step 1: Understand filtering out empty and None

   Empty strings and None are 'falsy' in Python, so lambda w: w keeps only truthy values.

2. Step 2: Check each option

   list(filter(lambda w: w, words)) keeps only truthy values, removing empty strings and None. Options B and C keep only empty or None, which is opposite. list(filter(lambda w: w != None or w != '', words)) uses wrong logic and keeps all.

3. Final Answer:

   list(filter(lambda w: w, words)) -> Option A

4. Quick Check:

   filter with lambda w: w removes falsy values [OK]

Hint: Use lambda x: x to remove falsy values [OK]

Common Mistakes:

• Using wrong condition to keep empty or None
• Using 'or' instead of 'and' in condition
• Expecting filter to remove without function