Filter and filterNot in Kotlin - Mini Project: Build & Apply

Filter and filterNot

📖 Scenario: You are managing a list of products in a store. Some products are on sale, and you want to separate the sale items from the regular items.

🎯 Goal: Build a Kotlin program that uses filter and filterNot to create two lists: one with products on sale and one with products not on sale.

📋 What You'll Learn

Create a list of products with their sale status

Create a variable to hold the sale status to filter by

Use filter to get products on sale

Use filterNot to get products not on sale

Print both filtered lists

💡 Why This Matters

🌍 Real World

Filtering lists is common when you want to separate data based on conditions, like showing only available products or filtering messages.

💼 Career

Understanding how to filter collections is important for data processing, user interface updates, and backend logic in many programming jobs.

1

Create the product list

Create a list called products with these exact entries: "Shoes" to true, "Hat" to false, "Jacket" to true, "Socks" to false.

Kotlin

# Create the list of products with sale status
# Your code here

Hint

Use listOf with pairs like "Name" to Boolean.

2

Set the sale filter

Create a Boolean variable called onSale and set it to true.

Kotlin

val products = listOf("Shoes" to true, "Hat" to false, "Jacket" to true, "Socks" to false)
# Create a Boolean variable called onSale and set it to true
# Your code here

Hint

Just write val onSale = true.

3

Filter products on sale and not on sale

Create a list called saleItems by filtering products where the second value equals onSale. Then create a list called regularItems by filtering products where the second value does NOT equal onSale using filterNot.

Kotlin

val products = listOf("Shoes" to true, "Hat" to false, "Jacket" to true, "Socks" to false)
val onSale = true
# Create saleItems using filter where second equals onSale
# Create regularItems using filterNot where second equals onSale
# Your code here

Hint

Use filter and filterNot with it.second == onSale.

4

Print the filtered lists

Print saleItems and regularItems each on its own line.

Kotlin

val products = listOf("Shoes" to true, "Hat" to false, "Jacket" to true, "Socks" to false)
val onSale = true
val saleItems = products.filter { it.second == onSale }
val regularItems = products.filterNot { it.second == onSale }
# Print saleItems
# Print regularItems
# Your code here

Hint

Use println(saleItems) and println(regularItems).

Practice

1. What does the filter function do in Kotlin collections?

easy

A. Sorts the collection in ascending order

B. Removes all items from the collection

C. Keeps items where the condition is false

D. Keeps items where the condition is true

Solution

1. Step 1: Understand the purpose of filter

   The filter function selects elements that satisfy a given condition (true).

2. Step 2: Compare with other options

   Options B and D describe different operations, and C describes filterNot, not filter.

3. Final Answer:

   Keeps items where the condition is true -> Option D

4. Quick Check:

   filter = keep true items [OK]

Hint: Remember: filter keeps true, filterNot keeps false [OK]

Common Mistakes:

• Confusing filter with filterNot
• Thinking filter removes all items
• Mixing filter with sorting functions

2. Which of the following is the correct syntax to filter even numbers from a list numbers in Kotlin?

easy

A. numbers.filter { it / 2 == 0 }

B. numbers.filterNot { it % 2 == 0 }

C. numbers.filter { it % 2 == 0 }

D. numbers.filter { it % 2 != 0 }

Solution

1. Step 1: Identify the condition for even numbers

   Even numbers satisfy it % 2 == 0.

2. Step 2: Choose correct filter syntax

   filter keeps items where condition is true, so numbers.filter { it % 2 == 0 } keeps even numbers.

3. Final Answer:

   numbers.filter { it % 2 == 0 } -> Option C

4. Quick Check:

   filter with even condition = numbers.filter { it % 2 == 0 } [OK]

Hint: Use filter with condition true to keep desired items [OK]

Common Mistakes:

• Using filterNot instead of filter
• Using division instead of modulo
• Confusing condition signs (== vs !=)

3. What is the output of this Kotlin code?

val nums = listOf(1, 2, 3, 4, 5)
val result = nums.filterNot { it > 3 }
println(result)

medium

A. [1, 2, 3]

B. [4, 5]

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

D. []

Solution

1. Step 1: Understand filterNot condition

   filterNot { it > 3 } keeps items where it > 3 is false, so items ≤ 3.

2. Step 2: Apply condition to list

   Numbers 1, 2, 3 are ≤ 3, so they are kept; 4 and 5 are removed.

3. Final Answer:

   [1, 2, 3] -> Option A

4. Quick Check:

   filterNot > 3 = keep ≤ 3 [OK]

Hint: filterNot keeps items where condition is false [OK]

Common Mistakes:

• Confusing filterNot with filter
• Including numbers greater than 3
• Expecting empty list output

4. Find the error in this Kotlin code that tries to filter out odd numbers:

val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filterNot { it % 2 == 0 }
println(evens)

medium

A. The list is empty after filtering

B. It incorrectly keeps odd numbers instead of even

C. filterNot cannot be used with modulo operator

D. Syntax error in lambda expression

Solution

1. Step 1: Analyze the filterNot condition

   filterNot { it % 2 == 0 } keeps items where it % 2 == 0 is false, so it keeps odd numbers.

2. Step 2: Check intended output

   The code tries to get even numbers but actually keeps odd numbers, so logic is reversed.

3. Final Answer:

   It incorrectly keeps odd numbers instead of even -> Option B

4. Quick Check:

   filterNot with even condition keeps odd [OK]

Hint: filterNot reverses condition; use filter to keep true items [OK]

Common Mistakes:

• Using filterNot instead of filter for positive condition
• Assuming filterNot removes items instead of keeping false
• Misreading modulo condition

5. Given a list val words = listOf("apple", "banana", "", "cherry", ""), which Kotlin expression returns a list of only the non-empty words using filter and filterNot correctly?

hard

A. words.filter { it.isNotEmpty() }

B. words.filterNot { it.isNotEmpty() }

C. words.filter { it.isEmpty() }

D. words.filterNot { it.length > 0 }

Solution

1. Step 1: Identify condition for non-empty strings

   Non-empty strings satisfy it.isNotEmpty().

2. Step 2: Choose correct filter usage

   filter keeps items where condition is true, so words.filter { it.isNotEmpty() } returns only non-empty words.

3. Final Answer:

   words.filter { it.isNotEmpty() } -> Option A

4. Quick Check:

   filter with isNotEmpty = keep non-empty [OK]

Hint: Use filter with positive condition to keep desired items [OK]

Common Mistakes:

• Using filterNot with isNotEmpty to keep empty strings
• Confusing isEmpty and isNotEmpty
• Expecting filterNot to remove empty strings