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Creating maps

Learn to create maps with mapOf and mutableMapOf.

Creating maps: mapOf and mutableMapOf

A map binds keys to values. mapOf returns a read-only map, while mutableMapOf gives a mutable map. Pairs are written with to: "key" to value.

val ages = mapOf("Ava" to 25, "Liam" to 30)
println(ages["Ava"])  // 25

Keys are unique: if you write the same key twice, the second value overwrites the first. The inferred type looks like Map<String, Int>.

keys, values and entries

You get all keys of a map with keys, all values with values, and key-value pairs with entries. You can iterate them like a list.

val ages = mapOf("Ava" to 25, "Liam" to 30)
ages.keys      // [Ava, Liam]
ages.values    // [25, 30]
ages.entries   // [Ava=25, Liam=30]

entries returns a set of Entry<K, V>. Each entry has .key and .value fields.

Practice Print a two-person age map

Create a mapOf("Ava" to 25, "Liam" to 30) and print it with println.

Starter code

Main.kt
fun main() {
    // Create a map of two people's ages and print it
}

Sample solution

Main.kt
fun main() {
    val ages = mapOf("Ava" to 25, "Liam" to 30)
    println(ages)
}

Expected output

{Ava=25, Liam=30}

Questions and answers

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

1 Fill in the blank: read-only map

Question 
val ages = ____("Ava" to 25, "Liam" to 30)
Answer 
val ages = mapOf("Ava" to 25, "Liam" to 30)

Explanation: mapOf("..." to ...) returns a read-only map.

2 How many elements does mapOf("a" to 1, "a" to 2) have?

  • 2 (both kept)
  • Throws an exception
  • 1 (second a overwrites) Correct answer
  • 0 (error)

Explanation: Keys are unique; the second value overwrites the first.

3 Match map expression to role

Term Meaning
mapOf(...) Read-only map
mutableMapOf(...) Mutable map
to Key-value connector
keys All keys

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