Filter and filterNot in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of filter vs filterNot on a list

What is the output of the following Kotlin code?

Kotlin

val numbers = listOf(1, 2, 3, 4, 5)
val filtered = numbers.filter { it % 2 == 0 }
val filteredNot = numbers.filterNot { it % 2 == 0 }
println("filtered: $filtered")
println("filteredNot: $filteredNot")

A

filtered: []
filteredNot: [1, 2, 3, 4, 5]

B

filtered: [2, 4]
filteredNot: [1, 3, 5]

C

filtered: [1, 2, 3, 4, 5]
filteredNot: []

D

filtered: [1, 3, 5]
filteredNot: [2, 4]

💡 Hint

filter keeps elements matching the condition; filterNot keeps those that do not match.

filter returns elements where the condition is true (even numbers), filterNot returns elements where the condition is false (odd numbers).

❓ Predict Output

intermediate

Result size after filter and filterNot

Given the code below, how many elements are in the list 'result' after filtering?

Kotlin

val words = listOf("apple", "banana", "cherry", "date")
val result = words.filterNot { it.length <= 5 }
println(result.size)

A 2

B 3

C 4

D 1

💡 Hint

filterNot keeps elements where the condition is false.

Words with length <= 5 are filtered out; 'banana' and 'cherry' remain, so size is 2.

🔧 Debug

advanced

Why does this filterNot code cause a compilation error?

Identify the error in this Kotlin code snippet and select the correct explanation.

Kotlin

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

A SyntaxError: The lambda expression is incomplete; 'it >' is missing a value to compare.

B TypeError: 'it' is not defined in the lambda expression.

C RuntimeException: The filterNot function cannot be used with comparison operators.

D No error; the code runs and filters out numbers greater than zero.

💡 Hint

Check the lambda expression syntax inside filterNot.

The lambda 'it >' is incomplete and causes a syntax error because it lacks a right-hand operand.

🧠 Conceptual

advanced

Difference between filter and filterNot

Which statement correctly describes the difference between filter and filterNot in Kotlin?

A filter returns elements not matching the predicate; filterNot returns elements matching the predicate.

B filter removes null elements; filterNot removes non-null elements.

C filter and filterNot both return elements matching the predicate but in different order.

D filter returns elements matching the predicate; filterNot returns elements not matching the predicate.

💡 Hint

Think about what happens when the predicate is true or false.

filter keeps elements where the predicate is true; filterNot keeps elements where the predicate is false.

🚀 Application

expert

Filter and filterNot combined usage

What is the output of this Kotlin code snippet?

Kotlin

val data = listOf("cat", "dog", "bird", "fish", "cow")
val filtered = data.filter { it.length > 3 }.filterNot { it.contains('f') }
println(filtered)

A ["bird", "fish"]

B ["fish"]

C ["bird"]

D ["dog", "cat", "cow"]

💡 Hint

First filter keeps words longer than 3 letters, then filterNot removes words containing 'f'.

Words longer than 3 letters are "bird", "fish"; filterNot removes those containing 'f' so "fish" is removed, leaving only "bird".

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