filter() function in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this filter() example?

Consider the following Python code using filter(). What will be printed?

Python

numbers = [1, 2, 3, 4, 5, 6]
result = list(filter(lambda x: x % 2 == 0, numbers))
print(result)

A [1, 3, 5]

B [1, 2, 3, 4, 5, 6]

C [2, 4, 6]

D []

💡 Hint

filter() keeps items where the function returns True.

The lambda function returns True for even numbers only, so filter() keeps 2, 4, and 6.

❓ Predict Output

intermediate

What does this filter() call return?

What is the output of this code snippet?

Python

words = ['apple', 'banana', 'cherry', 'date']
result = list(filter(lambda w: len(w) > 5, words))
print(result)

A ['apple', 'banana', 'cherry']

B ['banana', 'cherry']

C ['date']

D ['apple', 'date']

💡 Hint

filter() keeps words longer than 5 letters.

Only 'banana' and 'cherry' have length greater than 5.

❓ Predict Output

advanced

What is the output of this filter() with None as function?

What will this code print?

Python

items = [0, 1, False, True, '', 'Hello', None, [], [1, 2]]
result = list(filter(None, items))
print(result)

A []

B [0, 1, False, True, '', 'Hello', None, [], [1, 2]]

C [False, True, 'Hello', None, [1, 2]]

D [1, True, 'Hello', [1, 2]]

💡 Hint

filter(None, iterable) removes all falsey values.

filter(None, ...) removes items that are false when converted to bool, keeping only truthy values.

❓ Predict Output

advanced

What is the output of this chained filter() example?

What will this code print?

Python

data = range(10)
result = list(filter(lambda x: x % 2 == 0, filter(lambda x: x > 4, data)))
print(result)

A [6, 8]

B [4, 6, 8]

C [0, 2, 4]

D [5, 6, 7, 8, 9]

💡 Hint

First filter keeps numbers > 4, second filter keeps even numbers.

Numbers > 4 are 5,6,7,8,9. Among these, even numbers are 6 and 8.

🧠 Conceptual

expert

Which option causes a TypeError when used with filter()?

Which of these options will cause a TypeError when passed as the function argument to filter()?

A An integer like 5

B None

C A function that returns True or False

D A lambda expression returning a boolean

💡 Hint

filter() expects a function or None as first argument.

Passing an integer instead of a function or None causes TypeError.

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