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List methods

Learn list processing with map, filter and sortedWith.

List methods: map, filter, sortedWith

To transform a list, map { } turns each element into a new value. filter { } keeps only those that match a condition. sortedWith(cmp) sorts by a comparator; sortedBy { } is its shortcut.

val nums = listOf(3, 7, 12)
val doubles = nums.map { it * 2 }       // [6, 14, 24]
val big = nums.filter { it > 5 }       // [7, 12]
val sorted = nums.sortedBy { it }      // [3, 7, 12]

These methods do not change the original list; they return a new one. it is the automatic name of the element inside the lambda.

sortedWith and compareBy

sortedWith takes a Comparator. The easiest way is to give a key with compareBy { } and sort by it. To sort in descending order, use reverseOrder().

val people = listOf("Selin", "Ayşe", "Mert")
val byName = people.sortedBy { it }            // [Ayşe, Mert, Selin]
val byLen = people.sortedBy { it.length }      // length ascending
val desc = people.sortedWith(compareByDescending { it })

These methods return a new list; the original is unchanged. To sort by two keys, chain compareBy({ a }, { b }).

Practice Print the doubles with map

Create a listOf(3, 7, 12). Use map { it * 2 } to compute the double of each number and print the new list with println.

Starter code

Main.kt
fun main() {
    val nums = listOf(3, 7, 12)
    // Compute the double of each number with map and print it
}

Sample solution

Main.kt
fun main() {
    val nums = listOf(3, 7, 12)
    val doubles = nums.map { it * 2 }
    println(doubles)
}

Expected output

[6, 14, 24]

Questions and answers

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

1 What does listOf(1, 2, 3).map { it * 2 } return?

  • [1, 2, 3]
  • 6
  • [2, 4, 6] Correct answer
  • [1, 4, 9]

Explanation: map multiplies each element by 2 and returns a new list.

2 Fill in the blank: keep matching items

Question 
nums.____ { it > 10 }
Answer 
nums.filter { it > 10 }

Explanation: nums.filter { it > 10 } keeps only the large elements.

3 Match method to purpose

Term Meaning
map { } Transform each element
filter { } Keep matching items
sortedBy { } Sort by a key
sortedWith(cmp) Sort with a custom comparator

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together