Filter and filterNot in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the filter function do in Kotlin?

The filter function returns a list containing only the elements that satisfy a given condition (predicate).

beginner

What is the difference between filter and filterNot in Kotlin?

filter keeps elements that match the condition, while filterNot keeps elements that do NOT match the condition.

beginner

How would you use filterNot to remove all even numbers from a list?

Use filterNot { it % 2 == 0 } to keep only odd numbers by removing even numbers.

beginner

Given val numbers = listOf(1, 2, 3, 4, 5), what is the result of numbers.filter { it > 3 }?

The result is [4, 5] because only numbers greater than 3 are kept.

intermediate

Can filter and filterNot be used with any Kotlin collection?

Yes, both functions can be used with any Kotlin collection like lists, sets, and sequences.

What does filterNot { it % 2 == 0 } do to a list of integers?

A Keeps only odd numbers

B Keeps only even numbers

C Removes odd numbers

D Removes all numbers

It removes elements where the condition is true (even numbers), so only odd numbers remain.

Which function keeps elements that satisfy the condition?

A filterNot

B map

C reduce

D filter

filter keeps elements matching the condition.

What will listOf(1,2,3).filter { it > 5 } return?

A [] (empty list)

B Error

C [5]

D [1, 2, 3]

No elements are greater than 5, so the result is an empty list.

If you want to remove elements that are less than 10, which function and condition would you use?

A filter { it < 10 }

B filterNot { it < 10 }

C filter { it > 10 }

D filterNot { it > 10 }

filterNot removes elements where the condition is true, so it removes elements less than 10.

Are filter and filterNot functions destructive (do they change the original list)?

A Yes, they modify the original list

B Only filter modifies the list

C No, they return a new list

D Only filterNot modifies the list

Both return new lists and do not change the original collection.

Explain how filter and filterNot work in Kotlin with an example.

Think about keeping or removing elements based on a condition.

Describe a real-life situation where you might use filter and filterNot in a Kotlin program.

Imagine sorting or selecting items from a list based on a rule.