List comprehension with if–else in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a list that doubles each number in the range 1 to 5.

Python

result = [x[1]2 for x in range(1, 6)]
print(result)

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

A**

B*

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' will add 2 instead of doubling.

Using '**' will square the number, not double it.

The * operator multiplies each number by 2, doubling it.

2fill in blank

medium

Complete the code to create a list that labels numbers as 'even' or 'odd' using if–else inside a list comprehension.

Python

labels = ['even' if x[1] 2 == 0 else 'odd' for x in range(1, 6)]
print(labels)

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

A%

B+

C//

D**

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' or '**' will not check divisibility.

Using '//' gives the quotient, not the remainder.

The '%' operator gives the remainder. If remainder is 0 when divided by 2, the number is even.

3fill in blank

hard

Fix the error in the list comprehension that should replace negative numbers with 0.

Python

numbers = [-3, 5, -1, 7]
result = [x if x [1] 0 else 0 for x in numbers]
print(result)

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

A==

B<

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will keep negative numbers instead of replacing them.

Using '==' or '!=' will not correctly filter positive numbers.

We keep x if it is greater than 0; otherwise, replace with 0.

4fill in blank

hard

Fill both blanks to create a list that squares even numbers and cubes odd numbers from 1 to 5.

Python

result = [x[1]2 if x [2] 2 == 0 else x**3 for x in range(1, 6)]
print(result)

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

A**

B*

C%

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '**' will multiply but not square.

Using '+' instead of '%' will not check evenness.

Use '**' to square numbers and '%' to check if a number is even.

5fill in blank

hard

Fill both blanks to create a list of strings labeling numbers as 'small', 'medium', or 'large' based on their value.

Python

sizes = ['small' if x [1] 3 else 'medium' if x [2] 6 else 'large' for x in range(1, 10)]
print(sizes)

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

A<=

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' instead of '<' changes the boundary conditions.

Using '>' or '>=' instead of '<' will misclassify numbers.

Use '<' to check if x is less than 3 or 6.

Practice

1. What does the following list comprehension do?
[x if x % 2 == 0 else -x for x in range(5)]

easy

A. Creates a list of numbers from 0 to 4 without changes

B. Creates a list of numbers where even numbers stay the same and odd numbers become negative

C. Creates a list of only negative numbers from 0 to 4

D. Creates a list of only even numbers from 0 to 4

Solution

1. Step 1: Understand the condition in the comprehension

   The condition x % 2 == 0 checks if a number is even.

2. Step 2: Apply the if-else for each number in range(5)

   If even, keep x; if odd, use -x. So 0,2,4 stay same; 1,3 become -1,-3.

3. Final Answer:

   Creates a list of numbers where even numbers stay the same and odd numbers become negative -> Option B

4. Quick Check:

   if-else picks value based on even check = D [OK]

Hint: If-else picks value before for loop [OK]

Common Mistakes:

• Thinking if-else goes after the for loop
• Confusing condition meaning (odd vs even)
• Ignoring the else part

2. Which of these is the correct syntax for a list comprehension with if-else in Python?

easy

A. [x if x > 0 else 0 for x in nums]

B. [x for x in nums if x > 0 else 0]

C. [if x > 0 then x else 0 for x in nums]

D. [x if x > 0 else 0 in nums]

Solution

1. Step 1: Recall correct if-else placement in list comprehension

   The if-else expression must come before the for loop inside the brackets.

2. Step 2: Check each option's syntax

   [x if x > 0 else 0 for x in nums] correctly places x if x > 0 else 0 before for x in nums. Others have syntax errors.

3. Final Answer:

   [x if x > 0 else 0 for x in nums] -> Option A

4. Quick Check:

   if-else before for loop = C [OK]

Hint: If-else must be before for in comprehension [OK]

Common Mistakes:

• Putting if-else after the for loop
• Using 'then' keyword (not in Python)
• Missing for loop entirely

3. What is the output of this code?

nums = [1, 2, 3, 4]
result = [x*2 if x % 2 == 0 else x+1 for x in nums]
print(result)

medium

A. [2, 4, 4, 6]

B. [2, 4, 4, 4]

C. [2, 4, 4, 8]

D. [2, 2, 4, 8]

Solution

1. Step 1: Evaluate each element with condition

   For each number: if even, multiply by 2; if odd, add 1. So 1->2, 2->4, 3->4, 4->8.

2. Step 2: Collect results in list

   The final list is [2, 4, 4, 6].

3. Final Answer:

   [2, 4, 4, 6] -> Option A

4. Quick Check:

   if even *2 else +1 = [2,4,4,6] [OK]

Hint: Check each item with condition carefully [OK]

Common Mistakes:

• Mixing up multiplication and addition
• Applying condition after the loop
• Wrong output list length

4. Find the error in this list comprehension:

values = [10, 15, 20]
new_values = [x if x > 15 else x*2 for x in values if x]

medium

A. The if condition after the for loop is invalid syntax

B. The if-else expression must be inside parentheses

C. No error, code runs fine

D. The else part should come after the for loop

Solution

1. Step 1: Analyze the list comprehension structure

   The if-else expression is before the for loop, and the trailing if filters values.

2. Step 2: Confirm syntax correctness

   Trailing if after for is allowed to filter items; no syntax error here.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   Trailing if filters allowed = B [OK]

Hint: Trailing if filters items, allowed after for [OK]

Common Mistakes:

• Thinking trailing if is invalid
• Expecting else after for loop
• Misplacing parentheses

5. You have a list of temperatures in Celsius: temps = [22, -5, 0, 15, -10]. Use a list comprehension with if-else to create a new list where temperatures below 0 become the string 'Freezing', and others stay as numbers.
Which code does this correctly?

hard

A. [temp for temp in temps if temp >= 0 else 'Freezing']

B. [temp if temp < 0 else 'Freezing' for temp in temps]

C. [temp if temp > 0 else 'Freezing' for temp in temps]

D. ['Freezing' if temp < 0 else temp for temp in temps]

Solution

1. Step 1: Understand the condition and output

   Temperatures below 0 become 'Freezing', others stay numbers.

2. Step 2: Check each option's if-else placement and logic

   ['Freezing' if temp < 0 else temp for temp in temps] correctly uses 'Freezing' if temp < 0 else temp before the for loop. Others have wrong order or logic.

3. Final Answer:

   ['Freezing' if temp < 0 else temp for temp in temps] -> Option D

4. Quick Check:

   if-else before for, correct condition = A [OK]

Hint: Put if-else before for, condition matches output [OK]

Common Mistakes:

• Swapping if and else parts
• Placing if-else after for loop
• Using filter if instead of if-else expression