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".