Generate a Swift array of nonrepeating random numbers

Question

I\'d like to generate multiple different random numbers in Swift. Here is the procedure.

1. Set up an empty array
2. Generate a random number

3. Ch

Answer 1

The construct you’re looking for with your approach might be something like:

var temp: [Int] = []
while temp.count < 4 {
    var randomNumber: Int
    do {
        randomNumber = randomInt(1, 5)
    } while contains(temp, randomNumber)
    temp.append(randomNumber)
}

This will be fine for tiny ranges like yours, but for larger ranges it will be very slow, because for the last few numbers you are waiting for the random number to hit precisely the remaining handful of possibilities. I just tried generating from a range of 200 in a playground and it took 9 seconds.

If you want a random selection of numbers with guaranteed coverage over a range, you could generate it like by taking that range and shuffling it, like this:

func shuffle(source: S) -> [S.Generator.Element] {
    var rangen = GeneratorOf { arc4random() }
    let a = Array(Zip2(rangen, source))
    return a.sorted { $0.0 < $1.0 }.map { $0.1 }
}

let min = 1, max = 5
shuffle(min...max)

If you want a selection of n non-repeating random numbers from a range 0.., there’s a particularly pretty algorithm to do this that generates an ascending sequence of random numbers from that range:

func randomGeneratorOf(#n: Int, #from: Int) -> GeneratorOf {

    var select = UInt32(n)
    var remaining = UInt32(from)
    var i = 0

    return GeneratorOf {
        while i < from {
            if arc4random_uniform(remaining) < select {
                --select
                --remaining
                return i++
            }
            else {
                --remaining
                ++i
            }
        }
        return nil
    }
}

Which you could use like so:

let engines = [
    "Duck","Emily","Gordon","Henry", "Mavis",
    "Belle","James","Edward","Thomas","Toby"
]

let picks = Array(randomGeneratorOf(n: 3, from: engines.count))

for engine in PermutationGenerator(elements: engines, indices: picks) {
    println(engine)
}