Hackerrank Comprehensions (List, Dict, Set, and Generator Expressions)

Question 1

What does the following list comprehension produce?

python
[x**2 for x in range(5) if x % 2 == 0]

Accepted Answer

[0, 4, 16]

Answer

range(5) gives 0,1,2,3,4. The condition x%2==0 keeps even numbers: 0,2,4. Squaring them: 0**2=0, 2**2=4, 4**2=16 → [0, 4, 16].

Question 2

Which of the following creates a dictionary mapping each word to its length from a list?

python
words = ['cat', 'elephant', 'ox']

Accepted Answer

{w: len(w) for w in words}

Answer

A dict comprehension uses curly braces with a key:value expression. {w: len(w) for w in words} maps each word string to its integer length, producing {'cat': 3, 'elephant': 8, 'ox': 2}.

Question 3

What is the type and value of the following expression?

python
result = (x*2 for x in range(3))

Accepted Answer

A generator object yielding 0, 2, 4

Answer

Parentheses with a comprehension-style expression create a generator expression, not a tuple. The result is a generator object that lazily yields 0, 2, 4 when iterated. To get a tuple you'd need tuple(x*2 for x in range(3)).

Question 4

What does the following set comprehension produce?

python
{len(w) for w in ['hi', 'bye', 'no', 'yes']}

Accepted Answer

{2, 3}

Answer

The lengths are: 'hi'→2, 'bye'→3, 'no'→2, 'yes'→3. A set comprehension automatically deduplicates, resulting in {2, 3}.

Question 5

How do you flatten a 2D list [[1,2],[3,4],[5,6]] into [1,2,3,4,5,6] using a list comprehension?

Accepted Answer

[x for row in [[1,2],[3,4],[5,6]] for x in row]

Answer

Nested comprehensions iterate outer-to-inner left-to-right: first 'for row in matrix' selects each sublist, then 'for x in row' iterates its elements, yielding a flat list.

(Hackerrank) HackerRank Python Certification Practice Test

(Hackerrank) HackerRank Python Certification Practice Test

Hackerrank Comprehensions (List, Dict, Set, and Generator Expressions)