List comprehension with condition in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of list comprehension filtering even numbers

What is the output of this Python code?

nums = [1, 2, 3, 4, 5]
even_nums = [x for x in nums if x % 2 == 0]
print(even_nums)

Python

nums = [1, 2, 3, 4, 5]
even_nums = [x for x in nums if x % 2 == 0]
print(even_nums)

A [2, 4]

B [1, 3, 5]

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

D []

💡 Hint

Remember, the condition filters only numbers divisible by 2.

The list comprehension includes only numbers where x % 2 == 0, which are 2 and 4.

❓ Predict Output

intermediate

List comprehension with else clause output

What is the output of this code?

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

Python

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

A [0, 2, 0]

B [1, 2, 3]

C [1, 0, 3]

D [2]

💡 Hint

The else part replaces odd numbers with 0.

For each number, if it is even, keep it; otherwise, replace with 0. So 1 and 3 become 0, 2 stays 2.

❓ Predict Output

advanced

Output of nested list comprehension with condition

What does this code print?

matrix = [[1, 2], [3, 4], [5, 6]]
result = [x for row in matrix for x in row if x % 2 == 0]
print(result)

Python

matrix = [[1, 2], [3, 4], [5, 6]]
result = [x for row in matrix for x in row if x % 2 == 0]
print(result)

A [1, 2, 3, 4, 5, 6]

B [1, 3, 5]

C [2, 4, 6]

D [2, 4]

💡 Hint

It flattens the matrix and keeps only even numbers.

The comprehension goes through each row, then each number, and keeps only even numbers: 2, 4, 6.

❓ Predict Output

advanced

Output of list comprehension with multiple conditions

What is printed by this code?

nums = range(10)
result = [x*x for x in nums if x % 2 == 0 and x > 5]
print(result)

Python

nums = range(10)
result = [x*x for x in nums if x % 2 == 0 and x > 5]
print(result)

A [25, 36, 49, 64, 81]

B [0, 4, 16, 36, 64]

C [6, 8]

D [36, 64]

💡 Hint

Filter numbers that are even and greater than 5, then square them.

Only 6 and 8 are even and greater than 5. Their squares are 36 and 64.

🧠 Conceptual

expert

Understanding evaluation order in list comprehension with condition

Consider this code:

values = [0, 1, 2, 3]
result = [10 / x for x in values if x != 0]
print(result)

Which statement is true about this code?

A It raises a ZeroDivisionError because 0 is in values.

B It prints [10.0, 5.0, 3.3333333333333335] without error.

C It prints [10, 5, 3] because division returns integers.

D It raises a SyntaxError due to the condition placement.

💡 Hint

The condition filters out zero before division happens.

The condition x != 0 excludes zero, so division by zero never happens. Division returns floats.

Practice

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

easy

A. Creates a list of odd numbers from 0 to 4

B. Creates a list of even numbers from 0 to 4

C. Creates a list of numbers from 1 to 5

D. Creates a list of numbers divisible by 3

Solution

1. Step 1: Understand the range and condition

   The range(5) generates numbers 0, 1, 2, 3, 4. The condition x % 2 == 0 selects numbers divisible by 2 (even numbers).

2. Step 2: Apply the condition to each number

   From 0 to 4, the even numbers are 0, 2, and 4. These are included in the new list.

3. Final Answer:

   [0, 2, 4] -> Option B

4. Quick Check:

   Even numbers from 0 to 4 = [0, 2, 4] [OK]

Hint: Look for the condition after 'if' to filter items [OK]

Common Mistakes:

• Confusing even and odd numbers
• Ignoring the condition and taking all numbers
• Misunderstanding the range function

2. Which of the following is the correct syntax for a list comprehension that selects numbers greater than 10 from list nums?

easy

A. [x for x > 10 in nums]

B. [x if x > 10 for x in nums]

C. [if x > 10 for x in nums]

D. [x for x in nums if x > 10]

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is [expression for item in iterable if condition]. Here, expression and item are both x, iterable is nums, and condition is x > 10.

2. Step 2: Check each option

   [x for x in nums if x > 10] matches the correct syntax. The other options have misplaced or missing parts.

3. Final Answer:

   [x for x in nums if x > 10] -> Option D

4. Quick Check:

   Correct list comprehension syntax = [x for x in nums if x > 10] [OK]

Hint: Remember: for comes before if in list comprehension [OK]

Common Mistakes:

• Placing 'if' before 'for'
• Omitting 'for' keyword
• Using incorrect order of clauses

3. What is the output of this code?

nums = [3, 7, 12, 5, 20]
result = [n*2 for n in nums if n > 6]
print(result)

medium

A. [14, 24, 40]

B. [3, 7, 12, 5, 20]

C. [6, 14, 24, 10, 40]

D. [12, 24, 40]

Solution

1. Step 1: Identify numbers greater than 6

   From the list, numbers greater than 6 are 7, 12, and 20.

2. Step 2: Multiply each selected number by 2

   7*2=14, 12*2=24, 20*2=40. These form the new list.

3. Final Answer:

   [14, 24, 40] -> Option A

4. Quick Check:

   Filter >6 and double = [14, 24, 40] [OK]

Hint: Apply condition first, then transform each item [OK]

Common Mistakes:

• Multiplying all numbers, ignoring condition
• Including numbers less than or equal to 6
• Confusing multiplication with addition

4. Find the error in this list comprehension:

values = [1, 2, 3, 4]
new_values = [x for x in values if x > 2 else 0]

medium

A. SyntaxError due to incorrect use of else in list comprehension

B. No error, code runs fine

C. TypeError because of wrong condition

D. NameError because x is undefined

Solution

1. Step 1: Understand list comprehension with condition

   In list comprehension, 'if' can be used to filter items, but 'else' must be part of a conditional expression, not after 'if' alone.

2. Step 2: Identify the syntax error

   The code uses 'if x > 2 else 0' incorrectly. The correct way is to use a conditional expression inside the expression part, like [x if x > 2 else 0 for x in values].

3. Final Answer:

   SyntaxError due to incorrect use of else in list comprehension -> Option A

4. Quick Check:

   Else must be inside expression, not after if clause [OK]

Hint: Use if-else inside expression, not after if clause [OK]

Common Mistakes:

• Placing else after if in list comprehension filter
• Confusing filter condition with conditional expression
• Ignoring Python syntax rules for comprehensions

5. You have a list of strings: words = ['apple', '', 'banana', 'cherry', '', 'date']. Which list comprehension creates a new list with only non-empty strings in uppercase?

hard

A. [w for w in words if w.upper()]

B. [w.upper() if w else w for w in words]

C. [w.upper() for w in words if w]

D. [w.upper() for w in words if w == '']

Solution

1. Step 1: Understand filtering non-empty strings

   In Python, empty strings are false in boolean context. Using if w filters out empty strings.

2. Step 2: Convert filtered strings to uppercase

   For each non-empty string w, w.upper() converts it to uppercase.

3. Final Answer:

   [w.upper() for w in words if w] -> Option C

4. Quick Check:

   Filter non-empty and uppercase = [w.upper() for w in words if w] [OK]

Hint: Use 'if w' to filter non-empty strings in list comprehension [OK]

Common Mistakes:

• Filtering empty strings incorrectly
• Using if-else incorrectly inside comprehension
• Not converting strings to uppercase