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map and filter

Learn collection transformation with map and filter.

map and filter

map applies a transformation to every element of a list and returns a new list. Example: numbers.map { it * 2 } doubles each number. The original list is unchanged.

filter keeps the elements that satisfy a condition. Example: numbers.filter { it > 5 } takes only numbers greater than five. Both functions take a lambda; it refers to the current element. They can be chained: transform with map first, then narrow with filter.

filter and conditions

filter takes a lambda and collects the elements for which the lambda returns true. The condition must always return a Boolean. Example: numbers.filter { it % 2 == 0 } selects even numbers.

On an empty list filter returns an empty list without error. filterNotNull takes the non-null elements. When map and filter are used together, transformation happens first then selection; order matters because each step produces a new list.

Practice Transform with map and filter

Define val numbers = listOf(1, 2, 3, 4, 5). First double each number with map { it * 2 }, then keep those greater than five with filter { it > 5 } and print the result with println.

Starter code

Main.kt
fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    // Double with map, keep greater than five with filter, print with println
}

Sample solution

Main.kt
fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    val result = numbers.map { it * 2 }.filter { it > 5 }
    println(result)
}

Expected output

[6, 8, 10]

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does numbers.map { it * 2 } return?

  • The first element
  • Elements matching a condition
  • A single total value
  • A new list with each element doubled Correct answer

Explanation: map applies a transformation to each element and returns a new list.

2 Complete the map call

Question 
numbers.____ { it * 2 }
Answer 
numbers.map { it * 2 }

Explanation: map doubles each element.

3 Match the map and filter expressions

Term Meaning
map Transforms
filter Selects
{ it * 2 } Double
{ it > 5 } Greater than five

All lessons in Intermediate

1 Classes in depth

  1. Primary constructor
  2. Secondary constructor
  3. Properties
  4. lateinit and lazy
  5. companion object
  6. Summary - Classes

2 Inheritance

  1. open
  2. override
  3. super
  4. abstract
  5. interface
  6. Summary - Inheritance

3 Data class

  1. data class
  2. copy
  3. equals and hashCode
  4. toString
  5. Destructuring
  6. Summary - Data class

4 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Date helpers
  5. Putting it together

5 Sealed class

  1. sealed class
  2. when with sealed
  3. Sealed and data
  4. Sealed and state
  5. Sealed and result
  6. Summary - Sealed class

6 Object declaration

  1. object
  2. companion object
  3. Object expression
  4. companion and interface
  5. object vs class
  6. Summary - Object declaration

7 Extension functions

  1. Extension functions
  2. Extension properties
  3. Nullable extensions
  4. companion extensions
  5. Standard library
  6. Summary - Extension functions

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Generics

  1. Generic class
  2. Generic functions
  3. Variance
  4. Type constraints
  5. Reified
  6. Summary - Generics

10 Higher-order functions

  1. Function types
  2. Lambda
  3. it
  4. inline
  5. Scope functions
  6. Summary - Higher-order functions

11 Project: Library app

  1. Model design
  2. Library class
  3. Checking out
  4. Search and filter
  5. Putting it together

12 Advanced collections

  1. map and filter
  2. fold and reduce
  3. groupBy and partition
  4. sortedBy and sortedWith
  5. associate and flatMap
  6. Summary - Advanced collections

13 Delegation

  1. by
  2. Delegated properties
  3. by map
  4. vetoable
  5. Custom delegates
  6. Summary - Delegation

14 Value class

  1. value class
  2. Type safety
  3. Methods
  4. Generic value class
  5. value class vs data class
  6. Summary - Value class

15 Coroutine basics

  1. What are coroutines?
  2. launch
  3. suspend
  4. runBlocking
  5. async/await
  6. Summary - Coroutine basics

16 Flow basics

  1. Flow
  2. flow builder
  3. collect
  4. map and filter
  5. flowOn
  6. Summary - Flow basics

17 File I/O

  1. File
  2. readText
  3. writeText
  4. readLines
  5. appendText
  6. Summary - File I/O

18 Packages and modules

  1. package
  2. import
  3. internal
  4. Gradle
  5. kotlinx
  6. Summary - Packages

19 Testing

  1. JUnit
  2. @Test
  3. Assertions
  4. @BeforeEach
  5. MockK
  6. Summary - Testing

20 Intermediate final

  1. Project design
  2. OOP model
  3. Collections
  4. File I/O
  5. Putting it together