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Summary - Maps and Sets

Combine Set and Map to write a phone book.

Summary: phone book with a set and a map

In this summary you will build a phone book. You use a MutableSet<String> to guarantee names are unique, and a MutableMap<String, String> to bind each name to a number.

val names = mutableSetOf("Ava", "Liam")
val phones = mutableMapOf("Ava" to "5551", "Liam" to "5552")
println(names)
println(phones)

The set drops duplicate names; the map binds a name to a number for quick lookup. Together they form a unique-key lookup table.

Practice Build and print the phone book

Create a mutableSetOf("Ava", "Liam") and a mutableMapOf("Ava" to "5551", "Liam" to "5552"). Print the set first with println(names), then the map with println(phones).

Starter code

Main.kt
fun main() {
    // Phone book: hold unique names in a set, numbers in a map
    // Add two people and print the set first, then the map
}

Sample solution

Main.kt
fun main() {
    val names = mutableSetOf("Ava", "Liam")
    val phones = mutableMapOf("Ava" to "5551", "Liam" to "5552")
    println(names)
    println(phones)
}

Expected output

[Ava, Liam]
{Ava=5551, Liam=5552}

Practice Look up a number with get

Create a mutableMapOf("Ava" to "5551", "Liam" to "5552") and print Ava's number with println(phones.get("Ava")).

Starter code

Main.kt
fun main() {
    val phones = mutableMapOf("Ava" to "5551", "Liam" to "5552")
    // Get "Ava"'s number with get and print it
}

Sample solution

Main.kt
fun main() {
    val phones = mutableMapOf("Ava" to "5551", "Liam" to "5552")
    println(phones.get("Ava"))
}

Expected output

5551

Questions and answers

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

1 Which pair suits a phone book?

  • mutableSetOf<String> + mutableMapOf<String, String> Correct answer
  • listOf + listOf
  • setOf + setOf
  • mutableListOf + mutableMapOf (only list)

Explanation: Set keeps unique names, map holds number lookup.

2 Fill in the blank: add a pair to the book

Question 
phones.____("Nora", "5553")
Answer 
phones.put("Nora", "5553")

Explanation: phones.put("Nora", "5553") adds a new person to the book.

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