Filter and filterNot in Kotlin

Introduction

Filter and filterNot help you pick items from a list based on a rule. They make it easy to keep or remove items you want.

You want to get only even numbers from a list of numbers.

You want to remove all empty strings from a list of words.

You want to find all students who passed an exam from a list.

You want to exclude all negative numbers from a list.

Syntax

Kotlin

val filteredList = list.filter { item -> condition }
val filteredNotList = list.filterNot { item -> condition }

filter keeps items where the condition is true.

filterNot keeps items where the condition is false.

Examples

This keeps only even numbers from the list.

Kotlin

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

This removes empty strings from the list.

Kotlin

val words = listOf("apple", "", "banana", "", "cherry")
val nonEmpty = words.filterNot { it.isEmpty() }

Sample Program

This program shows how to use filter and filterNot to split a list based on a condition.

Kotlin

fun main() {
    val numbers = listOf(10, 15, 20, 25, 30)
    val filtered = numbers.filter { it > 20 }
    val filteredNot = numbers.filterNot { it > 20 }
    println("Numbers greater than 20: $filtered")
    println("Numbers NOT greater than 20: $filteredNot")
}

Important Notes

Use filter when you want to keep items matching a condition.

Use filterNot when you want to keep items NOT matching a condition.

Both return a new list and do not change the original list.

Summary

filter keeps items where the condition is true.

filterNot keeps items where the condition is false.

They help you easily select or exclude items from collections.

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