Lambda with sorted() in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the list of numbers in ascending order using sorted() with a lambda that returns the number itself.

Python

numbers = [5, 2, 9, 1]
sorted_numbers = sorted(numbers, key=lambda x: [1])
print(sorted_numbers)

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

Alen(x)

Bx

Cx*2

Dx+1

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(x) causes an error because numbers are integers, not sequences.

Using x*2 or x+1 changes the sorting order incorrectly.

The lambda function should return the number itself (x) to sort the list in ascending order.

2fill in blank

medium

Complete the code to sort the list of words by their length using sorted() and a lambda.

Python

words = ['apple', 'fig', 'banana', 'kiwi']
sorted_words = sorted(words, key=lambda w: [1])
print(sorted_words)

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

Aw[0]

Bw.upper()

Clen(w)

Dw[::-1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using w[0] sorts by first letter, not length.

Using w.upper() sorts alphabetically ignoring case.

Using w[::-1] sorts by reversed word, not length.

The lambda should return the length of each word (len(w)) to sort by word length.

3fill in blank

hard

Fix the error in the code to sort a list of tuples by the second element using sorted() and a lambda.

Python

pairs = [(1, 3), (2, 1), (3, 2)]
sorted_pairs = sorted(pairs, key=lambda x: [1])
print(sorted_pairs)

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

Ax[1]

Bx[0]

Cx[2]

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using x[0] sorts by the first element, not the second.

Using x[2] causes an IndexError because tuples have only two elements.

Using x sorts by the whole tuple, which sorts by first element then second.

To sort by the second element of each tuple, the lambda should return x[1].

4fill in blank

hard

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

Python

words = ['cat', 'elephant', 'dog', 'lion']
lengths = {word: [1] for word in words if [2]
print(lengths)

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

Alen(word)

Bword > 3

Clen(word) > 3

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using word > 3 compares a string to a number, causing an error.

Using word alone in the condition includes all words.

Using len(word) without a comparison includes all words.

The dictionary values are the length of each word (len(word)), and the condition filters words longer than 3 characters (len(word) > 3).

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase words as keys and their lengths as values, including only words with length greater than 4.

Python

words = ['tree', 'mountain', 'river', 'sky']
result = [1]: [2] for w in words if [3]
print(result)

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

Aw.upper()

Blen(w)

Clen(w) > 4

Dw.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using w.lower() for keys instead of uppercase.

Not filtering words by length.

Using w instead of w.upper() for keys.

Keys are uppercase words (w.upper()), values are lengths (len(w)), and only words longer than 4 are included (len(w) > 4).

Practice

1. What does the key argument do when used with sorted() and a lambda function in Python?

easy

A. It reverses the order of the sorted list automatically.

B. It changes the original list to a sorted list in place.

C. It tells sorted() how to compare items by extracting a value from each item.

D. It filters out items that do not match the lambda condition.

Solution

1. Step 1: Understand the role of key in sorted()

   The key argument takes a function that extracts a value from each item to use for sorting.

2. Step 2: Understand how lambda works with key

   The lambda function defines this extraction rule inline, telling sorted() what to compare.

3. Final Answer:

   It tells sorted() how to compare items by extracting a value from each item. -> Option C

4. Quick Check:

   key=lambda x: value means compare by value [OK]

Hint: Remember: key=lambda extracts sort value from each item [OK]

Common Mistakes:

• Thinking key changes the original list
• Confusing key with reverse sorting
• Assuming lambda filters items

2. Which of the following is the correct syntax to sort a list of tuples data = [(2, 'b'), (1, 'a'), (3, 'c')] by the first element using sorted() and a lambda?

easy

A. sorted(data, key=lambda x: x[0])

B. sorted(data, lambda x: x[0])

C. sorted(data, key=lambda x: x[1])

D. sorted(data, key=x[0])

Solution

1. Step 1: Identify the correct use of key argument

   The key argument must be named and assigned a function, here a lambda.

2. Step 2: Check lambda syntax and index

   lambda x: x[0] correctly extracts the first element of each tuple for sorting.

3. Final Answer:

   sorted(data, key=lambda x: x[0]) -> Option A

4. Quick Check:

   Correct syntax uses key= and lambda with index [OK]

Hint: Always use key= with lambda for sorting rules [OK]

Common Mistakes:

• Omitting key= argument name
• Using wrong tuple index
• Passing lambda as second positional argument

3. What is the output of this code?

data = ['apple', 'banana', 'cherry']
sorted_list = sorted(data, key=lambda x: len(x))
print(sorted_list)

medium

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

B. ['banana', 'cherry', 'apple']

C. ['cherry', 'banana', 'apple']

D. ['apple', 'cherry', 'banana']

Solution

1. Step 1: Understand sorting by length

   The lambda extracts the length of each string: apple(5), banana(6), cherry(6).

2. Step 2: Sort strings by their length

   Sorted order by length is: apple(5), banana(6), cherry(6). Since banana and cherry have same length, original order among them is preserved (banana before cherry).

3. Final Answer:

   ['apple', 'banana', 'cherry'] -> Option A

4. Quick Check:

   Sort by len(x) = ['apple', 'banana', 'cherry'] [OK]

Hint: Sort by length with key=lambda x: len(x) [OK]

Common Mistakes:

• Assuming alphabetical sort instead of length
• Mixing order of equal length items
• Forgetting sorted returns new list

4. Find the error in this code snippet:

data = [{'name': 'Alice', 'age': 30}, {'name': 'Bob', 'age': 25}]
sorted_data = sorted(data, key=lambda x: x['age'])
print(sorted_data)

medium

A. The code will raise a KeyError because 'age' is missing.

B. There is no error; the code sorts by age correctly.

C. The lambda function should use double quotes for keys.

D. sorted() cannot sort dictionaries.

Solution

1. Step 1: Check dictionary keys and lambda syntax

   Each dictionary has the key 'age', and lambda accesses it correctly with single quotes.

2. Step 2: Confirm sorted() usage on list of dicts

   sorted() can sort list of dictionaries using key function; no error occurs.

3. Final Answer:

   There is no error; the code sorts by age correctly. -> Option B

4. Quick Check:

   Access dict keys with quotes; sorted works on list of dicts [OK]

Hint: Dictionaries can be sorted by key with lambda [OK]

Common Mistakes:

• Thinking single quotes cause error
• Assuming KeyError without missing keys
• Believing sorted() can't handle dicts

5. You have a list of tuples representing products and their prices:

products = [('pen', 1.5), ('notebook', 2.0), ('eraser', 0.5), ('pencil', 1.5)]

How would you sort this list first by price ascending, then by product name alphabetically using sorted() and lambda?

hard

A. sorted(products, key=lambda x: x[1] and x[0])

B. sorted(products, key=lambda x: x[1] or x[0])

C. sorted(products, key=lambda x: x[0], x[1])

D. sorted(products, key=lambda x: (x[1], x[0]))

Solution

1. Step 1: Understand sorting by multiple criteria

   To sort by price then name, use a tuple in key: (price, name).

2. Step 2: Write lambda returning tuple for sorting

   lambda x: (x[1], x[0]) returns price first, then product name.

3. Final Answer:

   sorted(products, key=lambda x: (x[1], x[0])) -> Option D

4. Quick Check:

   Use tuple in key=lambda for multi-level sort [OK]

Hint: Use tuple in lambda key for multi-level sorting [OK]

Common Mistakes:

• Using logical operators instead of tuple
• Passing multiple key arguments
• Reversing order of sort keys