Set

Introduction

Set is an unordered collection of elements that does not allow duplicates. It is an immutable collection, which means that its size and individual elements cannot be changed after the set is initialized.

Let's assume we want to record information about the customers of a café at the end of the working day. Let's try to do it with the help of MutableList:

val visitors = mutableListOf<String>("Vlad", "Vanya", "Liza")
println(visitors) // output: [Vlad, Vanya, Liza]

Is MutableList really the best option for the task? First of all, anyone can change its contents:

visitors[2] = "Nina"
println(visitors) // output: [Vlad, Vanya, Nina]

Second, you could accidentally make a mistake and record a visitor twice:

val visitors = mutableListOf<String>("Vlad", "Vanya", "Liza", "Vanya")
println(visitors) // output: [Vlad, Vanya, Liza, Vanya]

Set can help us to solve this problem since Set is immutable, and it does not allow duplicate elements:

val visitors = setOf<String>("Vlad", "Vanya", "Liza", "Liza")
println(visitors) // output: [Vlad, Vanya, Liza]

We tried to create a Set with two identical elements, but since Set does not support duplicates, we ended up with a set of unique elements only, which is exactly what we need! And of course, Set is immutable, so no one will be able to mess with the contents.

Initialization

Set is a generic type. As you saw from the previous examples, you can initialize it with the help of setOf<E>, where E is the type of elements contained in the set:

val languages = setOf<String>("English", "Russian", "Italian")

The type can also be derived from the context:

val languages = setOf("English", "Russian", "Italian")

If you need to initialize an empty set, you can use the method emptySet:

val numbers = emptySet<Int>()
println(numbers) // output: []

Another way of creating a set is to call a builder function buildSet():

val letters = setOf<Char>('b', 'c')

val set = buildSet<Char> {
    add('a')
    addAll(letters)
    add('d')
}
println(set) // output: [a, b, c, d]

Methods and properties

Let's see how the methods you already know work in Set.

First, let's see how to use isEmpty and size:

val visitors = setOf("Andrew", "Mike")

println("How many people visited our cafe today? ${visitors.size}") // 2
println("Was our cafe empty today? It's ${visitors.isEmpty()}") // Was our cafe empty today? It's false

indexOf(element) and contains work in the following way:

val visitors = setOf("Paula", "Tanya", "Julia")

println("Is it true that Tanya came? It's ${visitors.contains("Tanya")}") // Is it true that Tanya came? It's true
println("And what is her index? ${visitors.indexOf("Tanya")}" ) // And her index is 1

If you want to know who was your first or last customer of the day, that is, which element is in the first/last position, you can use the methods first() and last(). These methods are not very useful since the set is an unordered collection.

val students = setOf("Bob", "Larry")
println(students.first()) // Bob
println(students.last()) // Larry

Use joinToString() to convert a Set to string, for example:

val visitors = setOf("Paula", "Tanya", "Julia")

val joinToString = visitors.joinToString()

println(joinToString) // Paula, Tanya, Julia

If you want to check that all elements from a specific collection are contained in a Set, use the containsAll(elements) method:

val studentsOfAGroup = setOf("Bob", "Larry", "Vlad")
val studentsInClass = setOf("Vlad")

println("Are all the students in the group in class today? It's ${studentsInClass.containsAll(studentsOfAGroup)}") 
// Are all the students in the group in class today? It's false

If you want to add two sets together, simply use the "+" operator, or "-" operator if you want to subtract one from the other. Note that after adding or subtracting, you will get a new set:

val productsList1 = setOf("Banana", "Lime", "Strawberry")
val productsList2 = setOf("Strawberry")

val finalProductsList1 = productsList1 + productsList2
println(finalProductsList1) // [Banana, Lime, Strawberry]

val finalProductsList2 = productsList1 - productsList2
println(finalProductsList2) // [Banana, Lime]

Got a MutableList and want to convert it to Set? Not a big deal, use toSet()method:

val groceries = mutableListOf("Pen", "Pineapple", "Apple", "Super Pen", "Apple", "Pen")
println(groceries.toSet()) // [Pen, Pineapple, Apple, Super Pen]

Iterating through elements

You can iterate through the elements in the Set with the help of the for loop. Let's see an example:

val visitors = setOf("Vlad", "Liza", "Vanya", "Nina")

for (visitor in visitors) {
    println("Hello $visitor!")
}

// output:
// Hello Vlad!
// Hello Liza!
// Hello Vanya!
// Hello Nina!

Conclusion

Now you know what a Set is and how it differs from a MutableList. Use Set when you don't want the content to be changed and want to keep the unique elements only. You know how to initialize a set, check if it's empty, see if it contains a certain element, find out the index of an element, convert the set to a string, and iterate through the Set with the help of for loop.

Ready to test yourself? Let's move on to the practice problems!