Split a random value into four that sum up to it

Question

I have one value like 24, and I have four textboxes. How can I dynamically generate four values that add up to 24?

All the values must be integers and can\'t be nega

Answer 1

As a recursive function the algorithm is very nice:

func getRandomValues(amount: Int, total: Int) -> [Int] {
    if amount == 1 { return [total] }
    if amount == total { return Array(count: amount, repeatedValue: 1) }
    let number = Int(arc4random()) % (total - amount + 1) + 1
    return [number] + getRandomValues(amount - 1, total - number)
}

And with safety check:

func getRandomValues(amount: Int, total: Int) -> [Int]? {
    if !(1...total ~= amount) { return nil }
    if amount == 1 { return [total] }
    if amount == total { return Array(count: amount, repeatedValue: 1) }
    let number = Int(arc4random()) % (total - amount + 1) + 1
    return [number] + getRandomValues(amount - 1, total - number)!
}

As @MartinR pointed out the code above is extremely biased. So in order to have a uniform distribution of the output values you should use this piece of code:

func getRandomValues(amount: Int, total: Int) -> [Int] {
    var numberSet = Set()

    // add splitting points to numberSet
    for _ in 1...amount - 1 {
        var number = Int(arc4random()) % (total - 1) + 1
        while numberSet.contains(number) {
            number = Int(arc4random()) % (total - 1) + 1
        }
        numberSet.insert(number)
    }

    // sort numberSet and return the differences between the splitting points
    let sortedArray = (Array(numberSet) + [0, total]).sort()
    return sortedArray.enumerate().flatMap{
        indexElement in
        if indexElement.index == amount { return nil }
        return sortedArray[indexElement.index + 1] - indexElement.element
    }
}