How to get random element from a set in Swift?

Question

As of Swift 1.2, Apple introduces Set collection type.

Say, I have a set like:

var set = Set(arrayLiteral: 1, 2, 3, 4, 5)


        

Answer 1

extension Set {
    func randomElement() -> Element? {
        return count == 0 ? nil : self[advance(self.startIndex, Int(arc4random()) % count)]
    }
}

Answer 2

If you want a 'random' element from a Set then you use:

/// A member of the set, or `nil` if the set is empty.
var first: T? { get }

Get the 0th index or the 1,000,000th index makes no difference - they are all an arbitrary object.

But, if you want repeated calls to return a likely different element each time, then first might not fit the bill.

Answer 3

Probably the best approach is advance which walks successor for you:

func randomElementIndex<T>(s: Set<T>) -> T {
    let n = Int(arc4random_uniform(UInt32(s.count)))
    let i = advance(s.startIndex, n)
    return s[i]
}

(EDIT: Heh; noticed you actually updated the question to include this answer before I added it to my answer... well, still a good idea and I learned something too. :D)

You can also walk the set rather than the indices (this was my first thought, but then I remembered advance).

func randomElement<T>(s: Set<T>) -> T {
    let n = Int(arc4random_uniform(UInt32(s.count)))
    for (i, e) in enumerate(s) {
        if i == n { return e }
    }
    fatalError("The above loop must succeed")
}

Answer 4

In swift 3

extension Set {
    public func randomObject() -> Element? {
        let n = Int(arc4random_uniform(UInt32(self.count)))
        let index = self.index(self.startIndex, offsetBy: n)
        return self.count > 0 ? self[index] : nil
    }
}

Answer 5

As per comments above re Swift updates, used a minor change for an extension to Set:

func randomElement() -> Element?
{
    let randomInt = Int(arc4random_uniform(UInt32(self.count)))
    let index = startIndex.advancedBy(randomInt)
    return count == 0 ? nil: self[index]
}

Answer 6

Starting with Swift 4.2, you can use randomElement:

let random = set.randomElement()