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.