Randomize two arrays the same way Swift

Question

I know there is a new shuffle method with iOS 9 but I am wondering if there is anyway to shuffle two arrays the same way?

For example

[1,2,3,4] and [a,b,c,d]
shuffle
[3,4,1,2] and [c,d,a,b]

Answer 1

Using the shuffle() method from How do I shuffle an array in Swift? and the ideas from How can I sort multiple arrays based on the sorted order of another array you can shuffle the array indices and then re-order both (or more) arrays accordingly:

let a = [1, 2, 3, 4]
let b = ["a", "b", "c", "d"]

var shuffled_indices = a.indices.shuffle()

let shuffled_a = Array(PermutationGenerator(elements: a, indices: shuffled_indices))
let shuffled_b = Array(PermutationGenerator(elements: b, indices: shuffled_indices))

print(shuffled_a) // [3, 1, 2, 4]
print(shuffled_b) // ["c", "a", "b", "d"]

---

Update for Swift 3 (Xcode 8): PermutationGenerator does not exist in Swift 3 anymore. Using the shuffled() method from Shuffle array swift 3 the same can be achieved with

var shuffled_indices = a.indices.shuffled()

let shuffled_a = shuffled_indices.map { a[$0] }
let shuffled_b = shuffled_indices.map { b[$0] }

Answer 2

Use a dictionary to store the values temporarily, shuffle the keys and then rebuild the other array by extracting the values from the dictionary.

Answer 3

I'm unaware of any built-in shuffle mechanism in Swift 2.0. Assuming this doesn't exist, I borrowed some code from here.

extension CollectionType where Index == Int {
    /// Return a copy of `self` with its elements shuffled
    func shuffle() -> [Generator.Element] {
        var list = Array(self)
        list.shuffleInPlace()
        return list
    }
}

extension MutableCollectionType where Index == Int {
    /// Shuffle the elements of `self` in-place.
    mutating func shuffleInPlace() {
        // empty and single-element collections don't shuffle
        if count < 2 { return }

        for i in 0..<count - 1 {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            guard i != j else { continue }
            swap(&self[i], &self[j])
        }
    }
}

let shuffleOrder = [0,1,2,3]
let shuffled = shuffleOrder.shuffle()


var newArray1 = [String]()
var newArray2 = [String]()

let array1 = ["a", "b", "c", "d"]
let array2 = ["w", "x", "y", "z"]


shuffled.forEach() { index in
    newArray1.append(array1[index])
    newArray2.append(array2[index])
}

This solves the problem in a really straight forward way. It creates an array, shuffleOrder, that just has an index for each possible index in the starting arrays. It then shuffles these indices to create a random sampling order. Finally, it constructs two new arrays, based off of the starting arrays, sampling them with the shuffled values. While this doesn't mutate the original 2 arrays in place, it would be simple to modify this to do so.

Answer 4

Based upon Martin R's original answer, you could approach the problem using GameKit.

The answer is written in Swift4:

var arrayA = [1, 2, 3, 4]
var arrayB = ["a", "b", "c", "d"]

//Get The Indices Of The 1st Array 
var shuffledIndices: [Int] = Array(arrayA.indices)
print("Shuffled Indices = \(shuffledIndices)")

//Shuffle These Using GameKit
shuffledIndices = GKRandomSource.sharedRandom().arrayByShufflingObjects(in: shuffledIndices) as! [Int]

//Map The Objects To The Shuffled Indices
arrayA = shuffledIndices.map { arrayA[$0] }
arrayB = shuffledIndices.map { arrayB[$0] }

//Log The Results
print("""
Array A = \(arrayA)
Array B = \(arrayB)
""")

Hope it helps ^_________^.