List comprehension with if–else in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of list comprehension with if–else

What is the output of the following Python code?

Python

numbers = [1, 2, 3, 4, 5]
result = [x*2 if x % 2 == 0 else x+1 for x in numbers]
print(result)

A [2, 4, 4, 8, 6]

B [2, 2, 4, 4, 6]

C [2, 4, 4, 8, 5]

D [1, 4, 3, 8, 5]

💡 Hint

Remember that the if–else inside the list comprehension applies to each element before adding it to the list.

For each number, if it is even (x % 2 == 0), multiply by 2; otherwise, add 1. So 1+1=2, 2*2=4, 3+1=4, 4*2=8, 5+1=6.

❓ Predict Output

intermediate

Value of variable after list comprehension

What is the value of output after running this code?

Python

values = [10, 15, 20]
output = ['even' if v % 2 == 0 else 'odd' for v in values]

A ['even', 'even', 'even']

B ['even', 'odd', 'even']

C ['odd', 'odd', 'odd']

D ['odd', 'even', 'odd']

💡 Hint

Check each number if it is divisible by 2.

10 is even, 15 is odd, 20 is even, so the list is ['even', 'odd', 'even'].

❓ Predict Output

advanced

Output of nested list comprehension with if–else

What is the output of this nested list comprehension?

Python

matrix = [[1, 2], [3, 4]]
result = [[x*10 if x % 2 == 0 else x+5 for x in row] for row in matrix]
print(result)

A [[5, 20], [8, 40]]

B [[6, 2], [8, 40]]

C [[6, 20], [8, 4]]

D [[6, 20], [8, 40]]

💡 Hint

Apply the if–else to each element inside each inner list.

For each number: if even, multiply by 10; if odd, add 5. So 1+5=6, 2*10=20, 3+5=8, 4*10=40.

🔧 Debug

advanced

Identify the error in list comprehension with if–else

Which option will cause a SyntaxError when running this code?

Python

nums = [1, 2, 3]
result = [x*2 if x > 1 for x in nums]

A result = [x*2 for x in nums if x > 1]

B result = [x*2 if x > 1 else x for x in nums]

C result = [x*2 if x > 1 for x in nums if x < 3]

D result = [x*2 if x > 1 else x for x in nums if x < 3]

💡 Hint

Check the placement of the if and else keywords in the comprehension.

Option C has two 'if' clauses without an 'else' in the ternary expression, causing SyntaxError.

🧠 Conceptual

expert

Understanding evaluation order in list comprehension with if–else

Consider this code snippet:

result = [f(x) if cond(x) else g(x) for x in data]

Which statement best describes the evaluation order?

A For each x in data, cond(x) is checked first; then either f(x) or g(x) is called accordingly.

B All f(x) are computed first, then all g(x) are computed after.

C The functions f and g are called only once each before the comprehension starts.

D cond(x) is checked after f(x) and g(x) are both called for each x.

💡 Hint

Think about how list comprehensions process each element one by one.

The condition cond(x) is evaluated for each element x; based on that, either f(x) or g(x) is called immediately.

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