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How to generate fair random numbers using GKShuffledDistribution

Swift version: 5.2

Paul Hudson    @twostraws   

A so-called “fair” random number generator is one that generates each of its possible values in equal amounts and with an even distribution. For example, if you were generating numbers between 1 and 4, you might get 4, 2, 1, 3, but you would never get 4 4 1 4.

GameplayKit has support for fair random number generation using GKShuffledDistribution. First, add an import for the GameplayKit framework:

import GameplayKit

Second, create an instance of GKShuffledDistribution, telling it the lowest and highest values it can generate:

let distribution = GKShuffledDistribution(lowestValue: 1, highestValue: 8)

Finally, call nextInt() on it as needed to generate numbers. You should get all numbers between 1 and 8 at least once before you see any repeated.

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Available from iOS 9.0 – see Hacking with Swift tutorial 35

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