Sets

Set[T] is an unordered collection of unique values. Like Dictionary, the element type must conform to Hashable.

Creating

let tags: Set[String] = ["urgent", "pending", "review"];
let empty: Set[Int] = Set();

Set literals use the same [a, b, c] syntax as arrays — the type annotation is what tells the compiler which to make.

Membership

let tags: Set[String] = ["urgent", "pending", "review"];
tags.contains("urgent");    // true
tags.contains("draft");     // false

Constant-time lookup, since Sets are hash-backed.

Inserting and removing

var tags: Set[String] = [];
tags.insert("urgent");
tags.insert("urgent");    // no-op; already present
tags.remove("urgent");

Inserting an existing value is a silent no-op — that's the point of a set. (insert returns a Bool telling you whether the value was newly added, and remove returns whether it was present.)

Set algebra

let a: Set[Int] = [1, 2, 3];
let b: Set[Int] = [3, 4, 5];

a.union(b);           // {1, 2, 3, 4, 5}
a.intersection(b);    // {3}
a.difference(b);      // {1, 2}
a.isSubset(of: b);    // false

These operations don't mutate; they return a new set.

Iteration

let tags: Set[String] = ["urgent", "pending", "review"];
for tag in tags {
    println(tag);
}

Order is unspecified.

When to reach for Set vs Array

Set when membership testing, deduplication, or set algebra is the primary operation, and order doesn't matter.
Array when order matters, duplicates are allowed, or you need indexed access.

If you find yourself calling array.contains(...) a lot, the data probably wanted to be a Set.