List comprehension with if–else in Python - Mini Project: Build & Apply

List comprehension with if-else

📖 Scenario: You are working in a fruit store. You have a list of fruit names. You want to create a new list that shows if each fruit is 'Sweet' or 'Sour'. For this, you will use list comprehension with an if-else condition.

🎯 Goal: Create a list of fruits, then use list comprehension with if-else to label each fruit as 'Sweet' or 'Sour'. Finally, print the new list.

📋 What You'll Learn

Create a list called fruits with these exact values: 'apple', 'lemon', 'banana', 'lime', 'cherry'

Create a list comprehension called taste_labels that labels each fruit as 'Sweet' if the fruit is 'apple', 'banana', or 'cherry', otherwise 'Sour'

Print the taste_labels list

💡 Why This Matters

🌍 Real World

Stores and shops often need to categorize items quickly based on properties like taste, price, or availability. Using list comprehension with if-else helps automate this.

💼 Career

Understanding list comprehension with conditions is useful for data processing, filtering, and labeling tasks in many programming jobs.

1

Create the list of fruits

Create a list called fruits with these exact values: 'apple', 'lemon', 'banana', 'lime', 'cherry'

Python

# Create the list called fruits with the exact fruit names
# Your code here

Hint

Use square brackets [] to create a list and separate items with commas.

2

Set up the sweet fruits list

Create a list called sweet_fruits that contains these exact fruits: 'apple', 'banana', 'cherry'

Python

fruits = ['apple', 'lemon', 'banana', 'lime', 'cherry']
# Create the list called sweet_fruits with the exact fruit names
# Your code here

Hint

This list will help you check if a fruit is sweet or sour.

3

Create the list comprehension with if-else

Create a list comprehension called taste_labels that goes through each fruit in fruits and adds 'Sweet' if the fruit is in sweet_fruits, otherwise adds 'Sour'

Python

fruits = ['apple', 'lemon', 'banana', 'lime', 'cherry']
sweet_fruits = ['apple', 'banana', 'cherry']
# Create the list comprehension taste_labels with if-else
# Your code here

Hint

Use the format: [value_if_true if condition else value_if_false for item in list]

4

Print the taste labels

Write a print statement to display the taste_labels list

Python

fruits = ['apple', 'lemon', 'banana', 'lime', 'cherry']
sweet_fruits = ['apple', 'banana', 'cherry']
taste_labels = ['Sweet' if fruit in sweet_fruits else 'Sour' for fruit in fruits]
# Print the taste_labels list
# Your code here

Hint

Use print(taste_labels) to show the result.

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