How to shuffle an array using arc4random_uniform()

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

Modern Swift code will use Swift’s own shuffle() and shuffled() methods, but sometimes you might find code that does it by hand using arc4random_uniform() or similar.

Nate Cook wrote a simple shuffle() extension to arrays that implements the Fisher-Yates shuffle algorithm in Swift. Here's the code:

extension Array {
    mutating func customShuffle() {
        for i in 0 ..< (count - 1) {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            swapAt(i, j)
        }
    }
}