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Map operations

Learn map management with put, get and remove.

Map operations: put, get, remove

On a mutable map put(k, v) adds a new pair or updates an existing key. get(k) returns the value as V?; if the key is missing it returns null. remove(k) takes the key out.

val ages = mutableMapOf("Ava" to 25)
ages.put("Liam", 30)
ages["Nora"] = 28
ages.get("Ava")   // 25
ages.get("missing")  // null

The ages[k] = v syntax does the same job as put; it is just shorter to write.

getOrElse and getOrDefault

Sometimes you want a default value instead of null when a key is missing. getOrElse(k) { default } produces the default with a lambda; getOrDefault(k, default) returns a fixed value.

val ages = mapOf("Ava" to 25)
ages.getOrElse("missing") { 0 }    // 0
ages.getOrDefault("missing", 0)    // 0

These helpers fold the later null check into one line and make the code more readable.

Practice Put a pair and print a value

Start with mutableMapOf("Ava" to 25, "Liam" to 30). Add a new pair with put("Nora", 28), then print Ava's age with println(ages.get("Ava")).

Starter code

Main.kt
fun main() {
    val ages = mutableMapOf("Ava" to 25, "Liam" to 30)
    // Add "Nora" 28 (put), get "Ava"'s age and print it
}

Sample solution

Main.kt
fun main() {
    val ages = mutableMapOf("Ava" to 25, "Liam" to 30)
    ages.put("Nora", 28)
    println(ages.get("Ava"))
}

Expected output

25

Questions and answers

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

1 What does map.get("missing") return?

  • -1
  • null Correct answer
  • Throws an exception
  • Unit

Explanation: On a map, get returns null when the key is missing; no exception.

2 Match method to purpose

Term Meaning
put(k, v) Add/update a key-value pair
get(k) V? (may be null)
remove(k) Remove a key
keys All keys

3 Fill in the blank: put a pair

Question 
ages.____("Nora", 28)
Answer 
ages.put("Nora", 28)

Explanation: ages.put("Nora", 28) adds or updates the pair.

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