Set operations

Learn set management with add, remove and contains.

Set operations: add, remove, contains

On a mutable set add(x) inserts a new element. If the element already exists it returns false, otherwise true. remove(x) takes the element out and returns true if it was present, false otherwise. contains(x) answers the presence question with a Boolean.

val tags = mutableSetOf("new", "important")
tags.add("urgent")      // true
tags.add("new")         // false (already there)
tags.remove("important")  // true

These Boolean returns are handy when you want to check whether an element was actually added.

Set operations: union, intersect, subtract

Kotlin sets support union, intersect and subtract. These return a new set; the originals are unchanged.

val a = setOf(1, 2, 3)
val b = setOf(2, 3, 4)
a union b       // {1, 2, 3, 4}
a intersect b    // {2, 3}
a subtract b     // {1}

These operations are handy when comparing tag, category or permission sets: which tags are shared, which are only on one side?

Exercise Add, remove and print size on a set

Start with mutableSetOf("a", "b"). Add "c" with add, remove "a" with remove and print the size with println(colors.size).

Show example solution

fun main() {
    val colors = mutableSetOf("a", "b")
    colors.add("c")
    colors.remove("a")
    println(colors.size)
}

Test yourself

Match method to return value

add(x)

true: added / false: already there
remove(x)

true: was there / false: absent
contains(x)

present? Boolean
size

Unique element count
What does set.add("already there") return on a set?
- true (always)
- false (already there)
- Unit
- Throws an exception
Show answer

false (already there) — add returns false when the element is already present; true when newly added.
Fill in the blank: add to the set
```
colors.____("yellow")
```
Show answer
```
colors.add("yellow")
```

Next lesson: Creating maps

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