List comprehension vs loop in Python - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of list comprehension vs loop

What is the output of the following code snippet?

Python

result = []
for i in range(3):
    result.append(i * 2)
print(result)

A [0, 2, 4]

B [0, 1, 2]

C [2, 4, 6]

D [0, 4, 8]

💡 Hint

Think about what happens when you multiply each number in the range by 2.

The loop multiplies each number 0,1,2 by 2 and appends it to the list, resulting in [0, 2, 4].

❓ Predict Output

intermediate

Equivalent list comprehension output

Which option shows the output of this list comprehension?

result = [x**2 for x in range(4)]
print(result)

A [1, 4, 9, 16]

B [0, 1, 4, 9]

C [0, 2, 4, 6]

D [1, 2, 3, 4]

💡 Hint

Remember that x**2 means x squared.

The list comprehension squares each number from 0 to 3, resulting in [0, 1, 4, 9].

❓ Predict Output

advanced

Output difference between loop and list comprehension with condition

What is the output of this code?

result = []
for x in range(5):
    if x % 2 == 0:
        result.append(x + 1)
print(result)

A [0, 2, 4]

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

C [1, 3, 5]

D [2, 4, 6]

💡 Hint

Check which numbers are even and what happens when you add 1.

Only even numbers 0,2,4 are selected and 1 is added to each, resulting in [1, 3, 5].

❓ Predict Output

advanced

List comprehension with condition output

What is the output of this list comprehension?

result = [x + 1 for x in range(5) if x % 2 == 0]
print(result)

A [1, 3, 5]

B [2, 4, 6]

C [0, 2, 4]

D [1, 2, 3, 4, 5]

💡 Hint

The condition filters even numbers, then adds 1 to each.

The list comprehension selects even numbers 0,2,4 and adds 1, resulting in [1, 3, 5].

❓ Predict Output

expert

Comparing performance: loop vs list comprehension

Which option correctly describes the output of this timing code?

import time
start = time.time()
result = []
for i in range(1000000):
    result.append(i)
end = time.time()
print(round(end - start, 2))

and

start = time.time()
result = [i for i in range(1000000)]
end = time.time()
print(round(end - start, 2))

A Both take exactly the same time.

B Loop is faster than list comprehension.

C Both raise a runtime error due to memory.

D List comprehension is generally faster than the loop.

💡 Hint

Think about how Python optimizes list comprehensions.

List comprehensions are optimized in Python and usually run faster than equivalent loops.