List comprehension with condition in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is list comprehension with condition in Python?

It is a way to create a new list by looping over an existing list and including only items that meet a certain condition.

beginner

How do you write a list comprehension that includes only even numbers from a list nums?

Use:
[x for x in nums if x % 2 == 0]
This means: take each x in nums only if x is even.

beginner

What happens if the condition in a list comprehension is always false?

The result is an empty list because no items meet the condition to be included.

intermediate

Can you use multiple conditions in a list comprehension? How?

Yes, by combining conditions with and or or. Example:
[x for x in nums if x > 0 and x % 2 == 0] selects positive even numbers.

beginner

Rewrite this code using list comprehension with condition:

result = []
for x in nums:
    if x > 10:
        result.append(x * 2)

Using list comprehension:

[x * 2 for x in nums if x > 10]

What does this list comprehension do?
[x for x in range(5) if x != 3]

A Creates a list of numbers from 0 to 4 excluding 3

B Creates a list of numbers from 0 to 3

C Creates a list with only the number 3

D Creates an empty list

It loops over numbers 0 to 4 and includes all except 3.

Which of these is a correct syntax for list comprehension with condition?

A [if x > 0 for x in nums]

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

C [x for x in nums if x > 0]

D [x for if x > 0 in nums]

The condition comes after the loop: for x in nums if condition.

What will be the output of [x*2 for x in [1,2,3] if x > 2]?

A [2, 4, 6]

B [6]

C [4, 6]

D []

Only 3 is greater than 2, so output is [3*2] = [6].

How can you filter out odd numbers from a list nums using list comprehension?

A [x for x in nums if x < 0]

B [x for x in nums if x % 2 == 1]

C [x for x in nums if x > 0]

D [x for x in nums if x % 2 == 0]

Even numbers satisfy x % 2 == 0, so this keeps evens and filters out odds.

What does the condition part in list comprehension control?

A Which items are included in the new list

B How many times the loop runs

C The order of items in the list

D The type of items in the list

The condition decides if an item should be added to the new list.

Explain how to use list comprehension with a condition to create a list of squares of even numbers from another list.

Think about filtering first, then applying the square.

Describe what happens if the condition in a list comprehension never matches any item.

What does an empty filter mean for the output?

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