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.

Practice

1. What is the main advantage of using list comprehension over a traditional for loop in Python?

easy

A. It runs slower than a loop

B. It creates lists in fewer lines and is more readable

C. It cannot be used to create lists

D. It requires more code to write

Solution

1. Step 1: Understand list comprehension purpose

   List comprehension is designed to create lists in a concise and readable way.

2. Step 2: Compare with traditional loops

   Traditional loops require more lines and are less compact than list comprehensions.

3. Final Answer:

   It creates lists in fewer lines and is more readable -> Option B

4. Quick Check:

   List comprehension = concise and readable [OK]

Hint: List comprehension is shorter and clearer than loops [OK]

Common Mistakes:

• Thinking list comprehension is slower
• Believing list comprehension can't create lists
• Confusing loops as always simpler

2. Which of the following is the correct syntax for a list comprehension that creates a list of squares of numbers from 0 to 4?

easy

A. [x**2 for x in range(5)]

B. [for x in range(5) x**2]

C. [x^2 in range(5)]

D. for x in range(5): x**2

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is: [expression for variable in iterable].

2. Step 2: Check each option

   [x**2 for x in range(5)] matches the correct syntax. Others have syntax errors or missing parts.

3. Final Answer:

   [x**2 for x in range(5)] -> Option A

4. Quick Check:

   Correct list comprehension syntax = [x**2 for x in range(5)] [OK]

Hint: Remember: [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after expression
• Using ^ instead of ** for power
• Writing loop without brackets

3. What is the output of the following code?

nums = [1, 2, 3]
squares = []
for n in nums:
    squares.append(n**2)
print(squares)

medium

A. [1, 2, 3]

B. [2, 4, 6]

C. [1, 4, 9]

D. Error

Solution

1. Step 1: Understand the loop operation

   The loop goes through each number in nums and appends its square to squares.

2. Step 2: Calculate squares for each element

   1 squared is 1, 2 squared is 4, 3 squared is 9, so squares = [1, 4, 9].

3. Final Answer:

   [1, 4, 9] -> Option C

4. Quick Check:

   Squares of [1,2,3] = [1,4,9] [OK]

Hint: Loop appends squares, so output is squared list [OK]

Common Mistakes:

• Confusing append with extend
• Expecting original list instead of squares
• Syntax errors in loop

4. Identify the error in this list comprehension:

result = [x*2 for x in range(5)
print(result)

medium

A. print statement is incorrect

B. Using wrong operator * instead of **

C. range(5) should be range[5]

D. Missing closing bracket "]" in list comprehension

Solution

1. Step 1: Check list comprehension syntax

   The list comprehension is missing the closing square bracket "]" at the end.

2. Step 2: Verify other parts

   Operator * is correct for multiplication, range(5) is correct, and print syntax is valid.

3. Final Answer:

   Missing closing bracket "]" in list comprehension -> Option D

4. Quick Check:

   Syntax error due to missing bracket [OK]

Hint: Always close brackets in comprehensions [OK]

Common Mistakes:

• Forgetting closing bracket
• Confusing parentheses and brackets
• Misusing range syntax

5. You want to create a list of even numbers from 0 to 10 using list comprehension. Which code correctly does this?

hard

A. [x for x in range(11) if x % 2 == 0]

B. [x*2 for x in range(5)]

C. [x for x in range(10) if x % 2]

D. [x for x in range(11) if x / 2 == 0]

Solution

1. Step 1: Understand the requirement

   We want even numbers from 0 to 10 inclusive, so numbers divisible by 2.

2. Step 2: Analyze each option

   [x for x in range(11) if x % 2 == 0] uses correct range and condition (x % 2 == 0). [x*2 for x in range(5)] doubles numbers 0-4, giving [0,2,4,6,8]. [x for x in range(10) if x % 2] filters odd numbers (x % 2 is true for odd). [x for x in range(11) if x / 2 == 0] uses division instead of modulo, which is incorrect.

3. Final Answer:

   [x for x in range(11) if x % 2 == 0] -> Option A

4. Quick Check:

   Filter even numbers with modulo == 0 [OK]

Hint: Use modulo % 2 == 0 to filter even numbers [OK]

Common Mistakes:

• Using multiplication instead of filtering
• Using wrong condition for even check
• Confusing division with modulo