Sets

Declaring Sets

Sets are unordered collections of unique values. Unique values must be of the same type.

var colors = Set<String>()

You can declare a set with values by using the array literal syntax.

var favoriteColors: Set<String> = ["Red", "Blue", "Green", "Blue"]
// {"Blue", "Green", "Red"}

Modifying values in a set

var favoriteColors: Set = ["Red", "Blue", "Green"]
//favoriteColors = {"Blue", "Green", "Red"}  

You can use the insert(_:) method to add a new item into a set.

favoriteColors.insert("Orange")
//favoriteColors = {"Red", "Green", "Orange", "Blue"}

You can use the remove(_:) method to remove an item from a set. It returns optional containing value that was removed or nil if value was not in the set.

let removedColor = favoriteColors.remove("Red")
//favoriteColors = {"Green", "Orange", "Blue"}
// removedColor = Optional("Red")

let anotherRemovedColor = favoriteColors.remove("Black")
// anotherRemovedColor = nil

Checking whether a set contains a value

var favoriteColors: Set = ["Red", "Blue", "Green"]
//favoriteColors = {"Blue", "Green", "Red"}

You can use the contains(_:) method to check whether a set contains a value. It will return true if the set contains that value.

if favoriteColors.contains("Blue") {
    print("Who doesn't like blue!")
}
// Prints "Who doesn't like blue!"

Performing operations on sets

Common values from both sets:

You can use the intersect(_:) method to create a new set containing all the values common to both sets.

let favoriteColors: Set = ["Red", "Blue", "Green"]
let newColors: Set = ["Purple", "Orange", "Green"]

let intersect = favoriteColors.intersect(newColors) // a AND b
// intersect = {"Green"}

All values from each set:

You can use the union(_:) method to create a new set containing all the unique values from each set.

let union = favoriteColors.union(newColors) // a OR b
// union = {"Red", "Purple", "Green", "Orange", "Blue"}

Notice how the value “Green” only appears once in the new set.

Values that don’t exist in both sets:

You can use the exclusiveOr(_:) method to create a new set containing the unique values from either but not both sets.

let exclusiveOr = favoriteColors.exclusiveOr(newColors) // a XOR b
// exclusiveOr = {"Red", "Purple", "Orange", "Blue"}

Notice how the value “Green” doesn’t appear in the new set, since it was in both sets.

Values that are not in a set:

You can use the subtract(_:) method to create a new set containing values that aren’t in a specific set.

let subtract = favoriteColors.subtract(newColors) // a - (a AND b)
// subtract = {"Blue", "Red"}

Notice how the value “Green” doesn’t appear in the new set, since it was also in the second set.

Adding values of my own type to a Set

In order to define a Set of your own type you need to conform your type to Hashable

struct Starship: Hashable {
    let name: String
    var hashValue: Int { return name.hashValue }
}

func ==(left:Starship, right: Starship) -> Bool {
    return left.name == right.name
}

Now you can create a Set of Starship(s)

let ships : Set<Starship> = [Starship(name:"Enterprise D"), Starship(name:"Voyager"), Starship(name:"Defiant") ]

CountedSet

Swift 3 introduces the CountedSet class (it’s the Swift version of the NSCountedSet Objective-C class).

CountedSet, as suggested by the name, keeps track of how many times a value is present.

let countedSet = CountedSet()
countedSet.add(1)
countedSet.add(1)
countedSet.add(1)
countedSet.add(2)

countedSet.count(for: 1) // 3
countedSet.count(for: 2) // 1