Lambda with filter() 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, 6]
even_numbers = list(filter(lambda x: x [1] 2 == 0, numbers))
print(even_numbers)

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

A**

B//

C%

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer division '//' instead of modulo '%'.

Using exponent '**' or addition '+' operators incorrectly.

The modulo operator % returns the remainder. Using x % 2 == 0 checks if a number is even.

2fill in blank

medium

Complete the code to filter words longer than 4 characters.

Python

words = ['apple', 'bat', 'carrot', 'dog']
long_words = list(filter(lambda word: len(word) [1] 4, words))
print(long_words)

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

A<

B>

C==

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' which filters shorter words.

Using '==' which filters words exactly 4 characters long.

We want words longer than 4 characters, so we use len(word) > 4.

3fill in blank

hard

Fix the error in the lambda to filter numbers less than 10.

Python

nums = [5, 12, 7, 20, 3]
small_nums = list(filter(lambda n: n [1] 10, nums))
print(small_nums)

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

A<=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which filters numbers greater than 10.

Using '<=' which includes 10, but we want strictly less.

To filter numbers less than 10, use n < 10.

4fill in blank

hard

Fill both blanks to filter words starting with letter 'a' and having length less than 5.

Python

words = ['apple', 'ant', 'bat', 'arc', 'dog']
filtered = list(filter(lambda w: w[1] 'a' and len(w) [2] 5, words))
print(filtered)

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

Astartswith(

B==

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' to compare the whole word instead of checking start.

Using '>' instead of '<' for length comparison.

Use w.startswith('a') to check the first letter and len(w) < 5 for length less than 5.

5fill in blank

hard

Fill both blanks to create a dictionary of words and their lengths, filtering words longer than 3 characters.

Python

words = ['sun', 'moon', 'star', 'sky']
result = {w:: len(w) [1] w in words if len(w) [2] 3}
print(result)

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

Bfor

C>

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Missing colon ':' between key and value.

Using incorrect loop keyword.

Using '<' instead of '>' for filtering.

The dictionary comprehension syntax is {key: value for item in iterable if condition}. Here, w: len(w), for w in words, and len(w) > 3.

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