Kotlin Set

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In this tutorial, we will talk about Kotlin Set with the help of simple examples. Sets are:

  • An unordered collection of elements.
  • Duplicate elements are not allowed in sets.

In Kotlin, we can have both a mutable set and an immutable set.

Kotlin Immutable Set

The immutable set also known as read-only set support only read-only access to the set.

Immutable set are created using setOf() and setOf<T>() functions.

Create and initialize Set 

The below example show you how to:

  • initialize Set using setOf() (contains elements of all type), setOf<T>() (contains elements of specific type) functions.
  • create new Set from another Set with the same elements using toSet() function.
val set1 = setOf("Arun", 10, 1.05f, 'a', null, 10)
val set2 = setOf<String>("Arun", "Neha", "Monika")
val set3 = set2.toSet()
println(set1) //[Arun, 10, 1.05, a, null]
println(set2)  //[Arun, Neha, Monika]
println(set3)  //[Arun, Neha, Monika]

Access items from Set 

The below example show you how to:

val languageSet = setOf<String>("Kotlin", "C++", "C", "Java", "Angular", "Kotlin")

println("The size of set is: ${languageSet.size}")
//The size of set is: 5

println("IntRange for set: ${languageSet.indices}")
//IntRange for set: 0..4

println("The size of set using count(): ${languageSet.count()}")
//The size of set using count(): 5

println("Is set empty: ${languageSet.isEmpty()}")
//Is set empty: false

println("Is set not empty: ${languageSet.isNotEmpty()}")
//Is set not empty: true

println("Element at index 1: ${languageSet.elementAt(1)}")
//Element at index 1: C++

println("Element at index 5 or '-1' if index is out of bounds: ${languageSet.elementAtOrElse(5, { -1 })}")
//Element at index 5 or '-1' if index is out of bounds: -1

println("Element at index 6 or 'null' if index is out of bounds: ${languageSet.elementAtOrNull(6)}")
//Element at index 6 or 'null' if index is out of bounds: null

Find elements in Set

The below example show how to:

val languageSet = setOf<String>("Kotlin", "C", "C++", "Java", "Angular", "Kotlin")

println("Does Java exist in set: ${languageSet.contains("Java")}")
//Does Java exist in set: true
println("Does PHP exist in set: ${languageSet.contains("PHP")}")
//Does PHP exist in set: false
println("Does specified set exist in languageSet: ${languageSet.containsAll(setOf("C", "Java"))}")
//Does specified set exist in languageSet: true
println("The index of Kotlin is: ${languageSet.indexOf("Kotlin")}")
//The index of Kotlin is: 0
println("The index of PHP is: ${languageSet.indexOf("PHP")}")
//The index of PHP is: -1
println("The last index of Kotlin is: ${languageSet.lastIndexOf("Kotlin")}")
//The last index of Kotlin is: 0
println("First element in set: ${languageSet.first()}")
//First element in set: Kotlin
println("Last element in set: ${languageSet.last()}")
//Last element in set: Angular
println(languageSet.indexOfFirst { e -> e.startsWith('C') })//1
println(languageSet.indexOfFirst { e -> e.startsWith('P') })//-1
println(languageSet.indexOfLast { e -> e.endsWith('+') })//2

println(languageSet.find { e -> e.startsWith('C') })  // C
println(languageSet.find { e -> e.startsWith('P') })  // null

println(languageSet.findLast { e -> e.endsWith('+') }) // C++

Mathematical functions of Set

The below example show how to use:

  • max() : returns the largest element or `null` if there are no elements.
  • min() : returns the smallest element or `null` if there are no elements.
  • sum() : returns the sum of all elements in the set.
  • average() : returns an average value of elements in the set.
  • union : returns a set containing all distinct elements from both sets.
  • intersect : returns a set containing all common elements of both sets.
  • subtract : remove all the elements of specified set from the given set (if element exist).
val numSet = setOf<Int>(1, 2, 3)

println("Max element in set: ${numSet.max()}")
//Max element in set: 3
println("Min element in set: ${numSet.min()}")
//Min element in set: 1
println("Sum of elements in set: ${numSet.sum()}")
//Sum of elements in set: 6
println("Average of elements in set: ${numSet.average()}")
//Average of elements in set: 2.0
println(numSet union setOf(4, 5))
//[1, 2, 3, 4, 5]
println(setOf(6) union numSet)
//[6, 1, 2, 3]
println(numSet intersect setOf(2, 5)) //[2]
println(numSet subtract setOf(2, 3)) //[1]

Kotlin Mutable Set

MutableSet inherits Set and supports read/write access, you can add, update or remove items.

MutableSet are created using mutableSetOf() and mutableSetOf()<T>() functions.

Create and initialize mutableSet

The below example show you how to:

  • initialize MutableSet using mutableSetOf() (contains elements of all type), mutableSetOf()<T>() (contains elements of specific type).
  • create new MutableSet from another MutableSet with the same elements using toMutableSet() function.
fun main() {
    val set1 = mutableSetOf("Arun", 10, 1.05f, 'a', null, 10)
    val set2 = mutableSetOf<String>("Arun", "Neha", "Monika")
    val set3 = set2.toMutableSet()
    println(set1)  //[Arun, 10, 1.05, a, null]
    println(set2)  //[Arun, Neha, Monika]
    println(set3)  //[Arun, Neha, Monika]
}

MutableSet specific functions

The below example show you how to use:

  • add(element): adds the specified element to the set. Returns true if the element has been added, false if the element is already contained in the set.
  • addAll():  adds all of the elements of the specified set to this set.
  • remove(element): removes element from set.
  • removeAll(): removes all of this set’s elements that are also contained in the specified set.
  • retainAll(): retains only the elements defined in this set that are contained in the specified set.
  • clear(): removes all elements of the set.
fun main() {
    val languages = mutableSetOf<String>("Kotlin", "Java", "Kotlin")
    println(languages)  //[Kotlin, Java]
    languages.add("Angular")
    println(languages)  //[Kotlin, Java, Angular]
    languages.addAll(setOf("C++", "C"))
    println(languages)  //[Kotlin, Java, Angular, C++, C]
    languages.remove("Java")
    println(languages)  //[Kotlin, Angular, C++, C]
    languages.removeAll(setOf("Angular"))
    println(languages)  //[Kotlin, C++, C]
    languages.retainAll(setOf("C++", "C"))
    println(languages)  //[C++, C]
    languages.clear()  //[]
    println(languages)
}

Iterate over Set in Kotlin

The examples show you how to iterate over a Set using:

  • forEach() method.
  • simple for loop
  • for loop with item index
  • forEachIndexed() that gives us index and value in each iteration.
  • Iterator and a while loop.
fun main() {

    val numSet = setOf(1, 5, 7, 8)

    print("print list using forEach method: ")
    numSet.forEach { e -> print("$e, ") }
    println()

    print("print list using for loop: ")
    for (word in numSet) {
        print("$word, ")
    }
    println()

    print("for loop with item index: ")
    for (i in 0 until numSet.size) {
        print("${numSet.elementAt(i)}, ")
    }
    println()
   
    println("print list using forEachIndexed() : ")
    numSet.forEachIndexed({ i, e -> println("$i - $e") })

    print("print list using Iterator: ")
    val it: Iterator<Int> = numSet.iterator()
    while (it.hasNext()) {
        val e = it.next()
        print("$e, ")
    }
    println()   
 }

Output:

print list using forEach method: 1, 5, 7, 8, 
print list using for loop: 1, 5, 7, 8, 
for loop with item index: 1, 5, 7, 8, 
print list using forEachIndexed(): 
0 - 1
1 - 5
2 - 7
3 - 8
print list using Iterator: 1, 5, 7, 8,

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