How do I shuffle an array in Swift?

Question

.shuffle() and .shuffled() are part of Swift

---

Original historic question:

How do I randomize or shuffle the elements within an array in Swift? For example, if my array consists of 52 playing cards, I want to shuffle the array in order to shuffle the deck.

Answer 1

This answer details how to shuffle with a fast and uniform algorithm (Fisher-Yates) in Swift 4.2+ and how to add the same feature in the various previous versions of Swift. The naming and behavior for each Swift version matches the mutating and nonmutating sorting methods for that version.

Swift 4.2+

shuffle and shuffled are native starting Swift 4.2. Example usage:

let x = [1, 2, 3].shuffled()
// x == [2, 3, 1]

let fiveStrings = stride(from: 0, through: 100, by: 5).map(String.init).shuffled()
// fiveStrings == ["20", "45", "70", "30", ...]

var numbers = [1, 2, 3, 4]
numbers.shuffle()
// numbers == [3, 2, 1, 4]

Swift 4.0 and 4.1

These extensions add a shuffle() method to any mutable collection (arrays and unsafe mutable buffers) and a shuffled() method to any sequence:

extension MutableCollection {
    /// Shuffles the contents of this collection.
    mutating func shuffle() {
        let c = count
        guard c > 1 else { return }

        for (firstUnshuffled, unshuffledCount) in zip(indices, stride(from: c, to: 1, by: -1)) {
            // Change `Int` in the next line to `IndexDistance` in < Swift 4.1
            let d: Int = numericCast(arc4random_uniform(numericCast(unshuffledCount)))
            let i = index(firstUnshuffled, offsetBy: d)
            swapAt(firstUnshuffled, i)
        }
    }
}

extension Sequence {
    /// Returns an array with the contents of this sequence, shuffled.
    func shuffled() -> [Element] {
        var result = Array(self)
        result.shuffle()
        return result
    }
}

Same usage as in Swift 4.2 examples above.

---

Swift 3

These extensions add a shuffle() method to any mutable collection and a shuffled() method to any sequence:

extension MutableCollection where Indices.Iterator.Element == Index {
    /// Shuffles the contents of this collection.
    mutating func shuffle() {
        let c = count
        guard c > 1 else { return }

        for (firstUnshuffled , unshuffledCount) in zip(indices, stride(from: c, to: 1, by: -1)) {
            // Change `Int` in the next line to `IndexDistance` in < Swift 3.2
            let d: Int = numericCast(arc4random_uniform(numericCast(unshuffledCount)))
            guard d != 0 else { continue }
            let i = index(firstUnshuffled, offsetBy: d)
            self.swapAt(firstUnshuffled, i)
        }
    }
}

extension Sequence {
    /// Returns an array with the contents of this sequence, shuffled.
    func shuffled() -> [Iterator.Element] {
        var result = Array(self)
        result.shuffle()
        return result
    }
}

Same usage as in Swift 4.2 examples above.

---

Swift 2

(obsolete language: you can't use Swift 2.x to publish on iTunes Connect starting July 2018)

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

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

Usage:

[1, 2, 3].shuffle()
// [2, 3, 1]

let fiveStrings = 0.stride(through: 100, by: 5).map(String.init).shuffle()
// ["20", "45", "70", "30", ...]

var numbers = [1, 2, 3, 4]
numbers.shuffleInPlace()
// [3, 2, 1, 4]

---

Swift 1.2

(obsolete language: you can't use Swift 1.x to publish on iTunes Connect starting July 2018)

shuffle as a mutating array method

This extension will let you shuffle a mutable Array instance in place:

extension Array {
    mutating func shuffle() {
        if count < 2 { return }
        for i in 0..<(count - 1) {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            swap(&self[i], &self[j])
        }
    }
}
var numbers = [1, 2, 3, 4, 5, 6, 7, 8]
numbers.shuffle()                     // e.g., numbers == [6, 1, 8, 3, 2, 4, 7, 5]

shuffled as a non-mutating array method

This extension will let you retrieve a shuffled copy of an Array instance:

extension Array {
    func shuffled() -> [T] {
        if count < 2 { return self }
        var list = self
        for i in 0..<(list.count - 1) {
            let j = Int(arc4random_uniform(UInt32(list.count - i))) + i
            swap(&list[i], &list[j])
        }
        return list
    }
}
let numbers = [1, 2, 3, 4, 5, 6, 7, 8]
let mixedup = numbers.shuffled()     // e.g., mixedup == [6, 1, 8, 3, 2, 4, 7, 5]

Answer 2

Edit: As noted in other answers, Swift 4.2 finally adds random number generation to the standard library, complete with array shuffling.

However, the GKRandom / GKRandomDistribution suite in GameplayKit can still be useful with the new RandomNumberGenerator protocol — if you add extensions to the GameplayKit RNGs to conform to the new standard library protocol, you can easily get:

• sendable RNGs (that can reproduce a "random" sequence when needed for testing)
• RNGs that sacrifice robustness for speed
• RNGs that produce non-uniform distributions

...and still make use of the nice new "native" random APIs in Swift.

The rest of this answer concerns such RNGs and/or their use in older Swift compilers.

---

There are some good answers here already, as well as some good illustrations of why writing your own shuffle can be error-prone if you're not careful.

In iOS 9, macOS 10.11, and tvOS 9 (or later), you don't have to write your own. There's an efficient, correct implementation of Fisher-Yates in GameplayKit (which, despite the name, is not just for games).

If you just want a unique shuffle:

let shuffled = GKRandomSource.sharedRandom().arrayByShufflingObjects(in: array)

If you want to be able to replicate a shuffle or series of shuffles, choose and seed a specific random source; e.g.

let lcg = GKLinearCongruentialRandomSource(seed: mySeedValue)
let shuffled = lcg.arrayByShufflingObjects(in: array)

In iOS 10 / macOS 10.12 / tvOS 10, there's also a convenience syntax for shuffling via an extension on NSArray. Of course, that's a little cumbersome when you're using a Swift Array (and it loses its element type on coming back to Swift):

let shuffled1 = (array as NSArray).shuffled(using: random) // -> [Any]
let shuffled2 = (array as NSArray).shuffled() // use default random source

But it's pretty easy to make a type-preserving Swift wrapper for it:

extension Array {
    func shuffled(using source: GKRandomSource) -> [Element] {
        return (self as NSArray).shuffled(using: source) as! [Element]
    }
    func shuffled() -> [Element] {
        return (self as NSArray).shuffled() as! [Element]
    }
}
let shuffled3 = array.shuffled(using: random)
let shuffled4 = array.shuffled()

Answer 3

In Swift 2.0, GameplayKit may come to the rescue! (supported by iOS9 or later)

import GameplayKit

func shuffle() {
    array = GKRandomSource.sharedRandom().arrayByShufflingObjectsInArray(array)
}

Answer 4

Here's something possibly a little shorter:

sorted(a) {_, _ in arc4random() % 2 == 0}

Answer 5

As of swift 4.2 there are two handy functions:

// shuffles the array in place
myArray.shuffle()

and

// generates a new array with shuffled elements of the old array
let newArray = myArray.shuffled()

Answer 6

Taking Nate's algorithm I wanted to see how this would look with Swift 2 and protocol extensions.

This is what I came up with.

extension MutableCollectionType where Self.Index == Int {
    mutating func shuffleInPlace() {
        let c = self.count
        for i in 0..<(c - 1) {
            let j = Int(arc4random_uniform(UInt32(c - i))) + i
            swap(&self[i], &self[j])
        }
    }
}

extension MutableCollectionType where Self.Index == Int {
    func shuffle() -> Self {
        var r = self
        let c = self.count
        for i in 0..<(c - 1) {
            let j = Int(arc4random_uniform(UInt32(c - i))) + i
            swap(&r[i], &r[j])
        }
        return r
    }
}

Now, any MutableCollectionType can use these methods given it uses Int as an Index

Answer 7

In my case, I had some problems of swapping objects in Array. Then I scratched my head and went for reinventing the wheel.

// swift 3.0 ready
extension Array {

    func shuffled() -> [Element] {
        var results = [Element]()
        var indexes = (0 ..< count).map { $0 }
        while indexes.count > 0 {
            let indexOfIndexes = Int(arc4random_uniform(UInt32(indexes.count)))
            let index = indexes[indexOfIndexes]
            results.append(self[index])
            indexes.remove(at: indexOfIndexes)
        }
        return results
    }

}

Answer 8

This is a version of Nate's implementation of the Fisher-Yates shuffle for Swift 4 (Xcode 9).

extension MutableCollection {
    /// Shuffle the elements of `self` in-place.
    mutating func shuffle() {
        for i in indices.dropLast() {
            let diff = distance(from: i, to: endIndex)
            let j = index(i, offsetBy: numericCast(arc4random_uniform(numericCast(diff))))
            swapAt(i, j)
        }
    }
}

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

The changes are:

• The constraint Indices.Iterator.Element == Index is now part of the Collection protocol, and need not be imposed on the extension anymore.
• Exchanging elements must done by calling swapAt() on the collection, compare SE-0173 Add MutableCollection.swapAt(_:_:).
• Element is an alias for Iterator.Element.

Answer 9

Swift 4 Shuffle the elements of an array in a for loop where i is the mixing ratio

var cards = [Int]() //Some Array
let i = 4 // is the mixing ratio
func shuffleCards() {
    for _ in 0 ..< cards.count * i {
        let card = cards.remove(at: Int(arc4random_uniform(UInt32(cards.count))))
        cards.insert(card, at: Int(arc4random_uniform(UInt32(cards.count))))
    }
}

Or with extension Int

func shuffleCards() {
    for _ in 0 ..< cards.count * i {
        let card = cards.remove(at: cards.count.arc4random)
        cards.insert(card, at: cards.count.arc4random)
    }
}
extension Int {
    var arc4random: Int {
        if self > 0 {
            print("Arc for random positiv self \(Int(arc4random_uniform(UInt32(self))))")
        return Int(arc4random_uniform(UInt32(self)))
        } else if self < 0 {
            print("Arc for random negotiv self \(-Int(arc4random_uniform(UInt32(abs(self)))))")
            return -Int(arc4random_uniform(UInt32(abs(self))))
        } else {
            print("Arc for random equal 0")
            return 0
        }
    }
}

Answer 10

This is what I use:

func newShuffledArray(array:NSArray) -> NSArray {
    var mutableArray = array.mutableCopy() as! NSMutableArray
    var count = mutableArray.count
    if count>1 {
        for var i=count-1;i>0;--i{
            mutableArray.exchangeObjectAtIndex(i, withObjectAtIndex: Int(arc4random_uniform(UInt32(i+1))))
        }
    }
    return mutableArray as NSArray
}

Answer 11

With Swift 3, if you want to shuffle an array in place or get a new shuffled array from an array, AnyIterator can help you. The idea is to create an array of indices from your array, to shuffle those indices with an AnyIterator instance and swap(_:_:) function and to map each element of this AnyIterator instance with the array's corresponding element.

---

The following Playground code shows how it works:

import Darwin // required for arc4random_uniform

let array = ["Jock", "Ellie", "Sue Ellen", "Bobby", "JR", "Pamela"]
var indexArray = Array(array.indices)
var index = indexArray.endIndex

let indexIterator: AnyIterator<Int> = AnyIterator {
    guard let nextIndex = indexArray.index(index, offsetBy: -1, limitedBy: indexArray.startIndex)
        else { return nil }

    index = nextIndex
    let randomIndex = Int(arc4random_uniform(UInt32(index)))
    if randomIndex != index {
        swap(&indexArray[randomIndex], &indexArray[index])
    }

    return indexArray[index]
}

let newArray = indexIterator.map { array[$0] }
print(newArray) // may print: ["Jock", "Ellie", "Sue Ellen", "JR", "Pamela", "Bobby"]

---

You can refactor the previous code and create a shuffled() function inside an Array extension in order to get a new shuffled array from an array:

import Darwin // required for arc4random_uniform

extension Array {

    func shuffled() -> Array<Element> {
        var indexArray = Array<Int>(indices)        
        var index = indexArray.endIndex

        let indexIterator = AnyIterator<Int> {
            guard let nextIndex = indexArray.index(index, offsetBy: -1, limitedBy: indexArray.startIndex)
                else { return nil }

            index = nextIndex                
            let randomIndex = Int(arc4random_uniform(UInt32(index)))
            if randomIndex != index {
                swap(&indexArray[randomIndex], &indexArray[index])
            }

            return indexArray[index]
        }

        return indexIterator.map { self[$0] }
    }

}

Usage:

let array = ["Jock", "Ellie", "Sue Ellen", "Bobby", "JR", "Pamela"]
let newArray = array.shuffled()
print(newArray) // may print: ["Bobby", "Pamela", "Jock", "Ellie", "JR", "Sue Ellen"]

let emptyArray = [String]()
let newEmptyArray = emptyArray.shuffled()
print(newEmptyArray) // prints: []

---

As an alternative to the previous code, you can create a shuffle() function inside an Array extension in order to shuffle an array in place:

import Darwin // required for arc4random_uniform

extension Array {

    mutating func shuffle() {
        var indexArray = Array<Int>(indices)
        var index = indexArray.endIndex

        let indexIterator = AnyIterator<Int> {
            guard let nextIndex = indexArray.index(index, offsetBy: -1, limitedBy: indexArray.startIndex)
                else { return nil }

            index = nextIndex                
            let randomIndex = Int(arc4random_uniform(UInt32(index)))
            if randomIndex != index {
                swap(&indexArray[randomIndex], &indexArray[index])
            }

            return indexArray[index]
        }

        self = indexIterator.map { self[$0] }
    }

}

Usage:

var mutatingArray = ["Jock", "Ellie", "Sue Ellen", "Bobby", "JR", "Pamela"]
mutatingArray.shuffle()
print(mutatingArray) // may print ["Sue Ellen", "Pamela", "Jock", "Ellie", "Bobby", "JR"]

Answer 12

Swift 3 solution, following @Nate Cook answer: (work if the index starts with 0, see comments below)

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

extension MutableCollection 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 }
        let countInt = count as! Int

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

Answer 13

This is how its done in a Simplest way.import Gamplaykit to your VC and use the below code. Tested in Xcode 8.

 import GameplayKit

 let array: NSArray = ["Jock", "Ellie", "Sue Ellen", "Bobby", "JR", "Pamela"]

 override func viewDidLoad() {
    super.viewDidLoad()

    print(array.shuffled())  
}

If you want to get a shuffled String from an Array you can use below code..

func suffleString() {

    let ShuffleArray = array.shuffled()

    suffleString.text = ShuffleArray.first as? String

    print(suffleString.text!)

}

Answer 14

In Swift 4.2, there is now a method for both a mutable shuffle and immutable shuffled. You can read more about the random generation and array stuff here.

Answer 15

You can use generic swap function as well and implement mentioned Fisher-Yates:

for idx in 0..<arr.count {
  let rnd = Int(arc4random_uniform(UInt32(idx)))
  if rnd != idx {
    swap(&arr[idx], &arr[rnd])
  }
}

or less verbose:

for idx in 0..<steps.count {
  swap(&steps[idx], &steps[Int(arc4random_uniform(UInt32(idx)))])
}

Answer 16

works!!. organisms is the array to shuffle.

extension Array
{
    /** Randomizes the order of an array's elements. */
    mutating func shuffle()
    {
        for _ in 0..<10
        {
            sort { (_,_) in arc4random() < arc4random() }
        }
    }
}

var organisms = [
    "ant",  "bacteria", "cougar",
    "dog",  "elephant", "firefly",
    "goat", "hedgehog", "iguana"]

print("Original: \(organisms)")

organisms.shuffle()

print("Shuffled: \(organisms)")

Answer 17

Working Array Extension (mutating & non-mutating)

Swift 4.1 / Xcode 9

The top answer is deprecated, so I took it upon myself to create my own extension to shuffle an array in the newest version of Swift, Swift 4.1 (Xcode 9):

extension Array {

// Non-mutating shuffle
    var shuffled : Array {
        let totalCount : Int = self.count
        var shuffledArray : Array = []
        var count : Int = totalCount
        var tempArray : Array = self
        for _ in 0..<totalCount {
            let randomIndex : Int = Int(arc4random_uniform(UInt32(count)))
            let randomElement : Element = tempArray.remove(at: randomIndex)
            shuffledArray.append(randomElement)
            count -= 1
        }
        return shuffledArray
    }

// Mutating shuffle
    mutating func shuffle() {
        let totalCount : Int = self.count
        var shuffledArray : Array = []
        var count : Int = totalCount
        var tempArray : Array = self
        for _ in 0..<totalCount {
            let randomIndex : Int = Int(arc4random_uniform(UInt32(count)))
            let randomElement : Element = tempArray.remove(at: randomIndex)
            shuffledArray.append(randomElement)
            count -= 1
        }
        self = shuffledArray
    }
}

---

Call Non-Mutating Shuffle [Array] -> [Array]:

let array = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]

print(array.shuffled)

This prints array in a random order.

---

Call Mutating Shuffle [Array] = [Array]:

var array = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]

array.shuffle() 
// The array has now been mutated and contains all of its initial 
// values, but in a randomized shuffled order

print(array) 

This prints array in its current order, which has already been randomly shuffled.

---

Hopes this works for everybody, if you have any questions, suggestions, or comments, feel free to ask!

Answer 18

In SWIFT 4

func createShuffledSequenceOfNumbers(max:UInt)->[UInt] {

    var array:[UInt]! = []
    var myArray:[UInt]! = []
    for i in 1...max {
        myArray.append(i)
    }
    for i in 1...max {
        array.append(i)
    }
    var tempArray:[Int]! = []
    for index in 0...(myArray.count - 1) {

        var isNotFinded:Bool = true
        while(isNotFinded){

            let randomNumber = arc4random_uniform(UInt32(myArray.count))
            let randomIndex = Int(randomNumber)

            if(!tempArray.contains(randomIndex)){
                tempArray.append(randomIndex)

                array[randomIndex] = myArray[index]
                isNotFinded = false
            }
        }
    }

    return array
}

Answer 19

If you want to use simple Swift For loop function use this ->

var arrayItems = ["A1", "B2", "C3", "D4", "E5", "F6", "G7", "H8", "X9", "Y10", "Z11"]
var shuffledArray = [String]()

for i in 0..<arrayItems.count
{
    let randomObject = Int(arc4random_uniform(UInt32(items.count)))

    shuffledArray.append(items[randomObject])

    items.remove(at: randomObject)
}

print(shuffledArray)

---

Swift Array suffle using extension ->

extension Array {
    // Order Randomize
    mutating func shuffle() {
        for _ in 0..<count {
            sort { (_,_) in arc4random() < arc4random() }
        }
    }
}

Answer 20

This is how to shuffle one array with a seed in Swift 3.0.

extension MutableCollection where Indices.Iterator.Element == Index {
    mutating func shuffle() {
        let c = count
        guard c > 1 else { return }


        for (firstUnshuffled , unshuffledCount) in zip(indices, stride(from: c, to: 1, by: -1)) {
            srand48(seedNumber)
            let number:Int = numericCast(unshuffledCount)
            let r = floor(drand48() * Double(number))

            let d: IndexDistance = numericCast(Int(r))
            guard d != 0 else { continue }
            let i = index(firstUnshuffled, offsetBy: d)
            swap(&self[firstUnshuffled], &self[i])
        }
    }
}

Answer 21

let shuffl = GKRandomSource.sharedRandom().arrayByShufflingObjects(in: arrayObject)

Answer 22

This is what I use:

import GameplayKit

extension Collection {
    func shuffled() -> [Iterator.Element] {
        let shuffledArray = (self as? NSArray)?.shuffled()
        let outputArray = shuffledArray as? [Iterator.Element]
        return outputArray ?? []
    }
    mutating func shuffle() {
        if let selfShuffled = self.shuffled() as? Self {
            self = selfShuffled
        }
    }
}

// Usage example:

var numbers = [1,2,3,4,5]
numbers.shuffle()

print(numbers) // output example: [2, 3, 5, 4, 1]

print([10, "hi", 9.0].shuffled()) // output example: [hi, 10, 9]

Answer 23

Simple Example:

extension Array {
    mutating func shuffled() {
        for _ in self {
            // generate random indexes that will be swapped
            var (a, b) = (Int(arc4random_uniform(UInt32(self.count - 1))), Int(arc4random_uniform(UInt32(self.count - 1))))
            if a == b { // if the same indexes are generated swap the first and last
                a = 0
                b = self.count - 1
            }
            swap(&self[a], &self[b])
        }
    }
}

var array = [1,2,3,4,5,6,7,8,9,10]
array.shuffled()
print(array) // [9, 8, 3, 5, 7, 6, 4, 2, 1, 10]

Answer 24

Here's some code that runs in playground. You won't need to import Darwin in an actual Xcode project.

import darwin

var a = [1,2,3,4,5,6,7]

func shuffle<ItemType>(item1: ItemType, item2: ItemType) -> Bool {
    return drand48() > 0.5
}

sort(a, shuffle)

println(a)

Answer 25

It stop at "swap(&self[i], &self[j])" when I upgrade the xCode version to 7.4 beta.
fatal error: swapping a location with itself is not supported

I found the reason that i = j (function of swap will exploded)

So I add a condition as below

if (i != j){
    swap(&list[i], &list[j])
}

YA! It's OK for me.