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.

Exercise 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.

Show example solution

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

Test yourself

Which one creates a mutable set?
- mutableSetOf(1, 2, 3)
- setOf(1, 2, 3)
- mutableListOf(1, 2, 3)
- list(1, 2, 3)
Show answer

mutableSetOf(1, 2, 3) — mutableSetOf gives a mutable set; add and remove work.

Fill in the blank: read-only set

val colors = ____("red", "green", "blue")

Show answer

val colors = setOf("red", "green", "blue")

Match set expression to behavior

setOf(...)

Read-only set
mutableSetOf(...)

Mutable set
size

Unique element count
contains(x)

Is x present? Boolean

Next lesson: Set operations

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