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Creating sets

Learn to create sets with setOf and mutableSetOf.

Creating sets: setOf and mutableSetOf

A set holds unique elements: if you add the same element twice, only one copy is kept. setOf returns a read-only set, while mutableSetOf gives a mutable set.

val fruits = setOf("apple", "banana", "apple")
println(fruits)  // [apple, banana]

The second "apple" is dropped; the set keeps a single apple. Unlike a list, a set has no index concept; elements are unique by content, not by position.

Automatic deduplication

A set stores elements by content, not by insertion order. If an element is already present, a second add is silently dropped. This is handy for deduplicating data in one shot.

val tags = mutableSetOf("new", "important", "new")
println(tags.size)  // 2
println(tags.contains("new"))  // true

size gives the count of unique elements. contains returns a Boolean telling whether the element is in the set; same as with a list.

Practice Print a set with a duplicate

Create a setOf("apple", "banana", "apple") (third element repeats) and print it with println. You will see the duplicate dropped.

Starter code

Main.kt
fun main() {
    // Give three fruit names (repeat one) and print the set
}

Sample solution

Main.kt
fun main() {
    val fruits = setOf("apple", "banana", "apple")
    println(fruits)
}

Expected output

[apple, banana]

Questions and answers

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

1 Which one creates a mutable set?

  • mutableSetOf(1, 2, 3) Correct answer
  • setOf(1, 2, 3)
  • mutableListOf(1, 2, 3)
  • list(1, 2, 3)

Explanation: mutableSetOf gives a mutable set; add and remove work.

2 Fill in the blank: read-only set

Question 
val colors = ____("red", "green", "blue")
Answer 
val colors = setOf("red", "green", "blue")

Explanation: setOf("...") builds a read-only set.

3 Match set expression to behavior

Term Meaning
setOf(...) Read-only set
mutableSetOf(...) Mutable set
size Unique element count
contains(x) Is x present? Boolean

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