filter() function in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter out even numbers from the list.

Python

numbers = [1, 2, 3, 4, 5]
even_numbers = list(filter([1], numbers))
print(even_numbers)

Drag options to blanks, or click blank then click option'

Alambda x: x % 2 == 0

Blambda x: x * 2

Clambda x: x + 2

Dlambda x: x - 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that changes the number instead of returning True/False.

Forgetting to convert the filter object to a list.

The filter() function needs a function that returns True for even numbers. lambda x: x % 2 == 0 does exactly that.

2fill in blank

medium

Complete the code to filter words longer than 3 characters.

Python

words = ['cat', 'lion', 'dog', 'elephant']
long_words = list(filter([1], words))
print(long_words)

Drag options to blanks, or click blank then click option'

Alambda w: w.startswith('e')

Blambda w: len(w) > 3

Clambda w: w == 'dog'

Dlambda w: len(w) < 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using < 3 instead of > 3.

Filtering by exact word instead of length.

The filter function should keep words with length greater than 3, so lambda w: len(w) > 3 is correct.

3fill in blank

hard

Fix the error in the filter function to keep only positive numbers.

Python

numbers = [-2, 0, 3, 5, -1]
positive = list(filter([1], numbers))
print(positive)

Drag options to blanks, or click blank then click option'

Alambda x: x != 0

Blambda x: x >= 0

Clambda x: x < 0

Dlambda x: x > 0

Attempts:

3 left

💡 Hint

Common Mistakes

Including zero as positive by using >= instead of >.

Filtering negative numbers instead of positive.

To keep only positive numbers (greater than zero), use lambda x: x > 0. Using x >= 0 would include zero, which is not positive.

4fill in blank

hard

Fill both blanks to filter words starting with 'a' and convert them to uppercase.

Python

words = ['apple', 'banana', 'avocado', 'cherry']
filtered = list(filter([1], words))
result = [w.[2]() for w in filtered]
print(result)

Drag options to blanks, or click blank then click option'

Alambda w: w.startswith('a')

Blambda w: w.endswith('a')

Cupper

Dlower

Attempts:

3 left

💡 Hint

Common Mistakes

Using endswith instead of startswith.

Using lower() instead of upper().

The filter keeps words starting with 'a' using lambda w: w.startswith('a'). Then, upper() converts them to uppercase.

5fill in blank

hard

Fill all three blanks to filter numbers divisible by 3, square them, and keep only those greater than 10.

Python

numbers = [1, 3, 4, 6, 9, 12]
filtered = filter([1], numbers)
squared = map(lambda x: x[2]2, filtered)
result = list(filter(lambda x: x [3] 10, squared))
print(result)

Drag options to blanks, or click blank then click option'

Alambda x: x % 3 == 0

B**

C>

Dlambda x: x % 2 == 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong lambda for filtering divisible numbers.

Using multiplication instead of exponentiation for squaring.

Using < instead of > in the last filter.

First, filter numbers divisible by 3 with lambda x: x % 3 == 0. Then square them using ** 2. Finally, keep only squares greater than 10 with > 10.

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