Random Numbers in Swift

Written by Reinder de Vries on August 14 2020 in App Development, Swift

Random Numbers in Swift

Let’s take a look at randomness and random numbers in Swift. Swift has a number of built in functions that help you generate random numbers, with a few caveats though!

But first… why would you need random numbers in Swift?

  • In game apps, you use random numbers to simulate dice rolls, or even simulate “chaos” and unpredictability in your games, like random enemy positions or random upgrade spawns
  • In practical iOS development, you use randomness to pick a random item from an array
  • When creating an object, or a new user, you sometimes need a random unique ID

Let’s get started!

  1. Random Numbers in Swift 4.2 and up
  2. Random Numbers Before Swift 4.2
  3. Random Integers with “arc4random_uniform(_:)”
  4. Random Doubles with “drand48()”
  5. An Easy Function for Random Numbers
  6. Picking a Random Element from an Array
  7. Generating A Random String
  8. Further Reading

This article can be used with Swift 5, and prior versions. Simply skip to the appropriate chapter.

Random Number Functions in Swift 4.2 and up

In Swift 4.2. and higher, the way you work with random numbers has changed. Instead of using the imported C function arc4random(), you can now use Swift’s own native functions.

Let’s look at an example:

let number = Int.random(in: 0..<10)

The above example generates a random integer number between 0 and 10. The half-open range operator ..< is used, and the result is a range that runs from 0 to 10, not including 10.

You can also use the closed range operator ... to get a random integer from 0 to 10, including 10. Like this:

let number = Int.random(in: 0...10)

You can use the random(in:) function to get random values for several primitive types, such as Int, Double, Float and even Bool. Like this:

let fraction = Float.random(in: 0..<1)

The above example returns a random floating-point value, i.e. a number with a fraction, between 0 and 1.

And the example below either returns true or false – a random boolean! They’re super useful for randomly testing conditional logic.

let stayOrGo = Bool.random()

What about picking a random element from an array? You can do that like this:

let names = ["Ford", "Zaphod", "Trillian", "Arthur", "Marvin"]
let randomName = names.randomElement()

In the above code you use randomElement() on the names array. You can use this function on any Collection, such as arrays and dictionaries. Keep in mind that the returned random element is an optional.

Can you also use the new random functions in Swift 4.2. to shuffle an array? Yes! Randomizing the order of an array is surprisingly simple:

var names = ["Ford", "Zaphod", "Trillian", "Arthur", "Marvin"]
names.shuffle()
// `names` can now be: ["Zaphod", "Marvin", "Arthur", "Ford", "Trillian"]

The shuffle functions use Swift’s typical naming structure, so shuffle() shuffles the array in-place, mutating the original array, and shuffled() returns a copy of the shuffled array.

You can even shuffle a Sequence, like this:

let sequence = 0 ..< 7
let shuffledSequence = sequence.shuffled()
// `shuffledSequence` can now be: [0, 6, 2, 3, 4, 1, 5]

Simple, right? Much simpler than the pre-4.2 arc4random_uniform(_:) with all that type casting…

Are you using a Swift version lower than 4.2? Then use the random number functions in the chapters below.

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Random Number Functions Before Swift 4.2

Swift has three typical functions for random numbers:

  • arc4random() returns a random number between zero and 232–1
  • arc4random_uniform(_:) returns a random number between zero and the first parameter, minus one.
  • drand48() returns a random Double between 0.0 and 1.0

Both arc4random() and arc4random_uniform(_:) use the UInt32 type instead of the more typical Int.

It’s recommended to use the arc4random_uniform(_:) instead of arc4random(), because the latter suffers from a condition called “modulo bias”. This causes particular numbers to appear more often than others, undermining the “randomness” of the function.

Keep in mind that the random numbers from these functions are semi-random and they’re generated with a mathematical function. Although they appear random, if you repeat the random function often enough you’ll see patterns and repetitions. This is fine for games or shuffling an array, but you shouldn’t use these random functions for cryptography or security purposes.

Fun fact: early computers, and especially computer games, included a file with random numbers. The program would sequentially pick random numbers from this file whenever it needed a random number, instead of calculating a random number on its own!

These days the mechanism isn’t much different, although iOS or Android has a large reserve of randomly generated numbers. Calculating a large set of random numbers before using it is known as seeding.

Random Numbers with “arc4random_uniform(_:)”

The function arc4random_uniform(_:) takes one parameter, the upper bound. It’ll return a random number between 0 and this upper bound, minus 1.

Like this:

arc4random_uniform(42)
// Output: 13

This will return a random number between 0 and 41. The result is of type UInt32, so if you want to work with an ordinary integer in your code, you’ll have to convert or type cast it to Int.

Like this:

let n = Int(arc4random_uniform(42)) 

Constant n is now of type Int, which is easier to work with.

Random Doubles with “drand48()”

What about doubles? As you know, a double is a decimal-point number with double precision. It’s most often used in Swift for number’s that have commas , or fractions.

This is how you generate a random double in Swift, between 0.0 and 1.0:

let d = drand48()
print(d)
// Output: 0.396464773760275

Neat!

Quick note: all computers have trouble representing floating-point numbers and fractions, so logically, the drand48() function works by simply putting a couple of integer numbers after each other…

An Easy Function for Random Numbers

Now, that arc4random_uniform(_:) function is a bit odd to work with. Let’s write a convenience function to help you work with random numbers.

Like this:

func random(_ n:Int) -> Int {
    return Int(arc4random_uniform(UInt32(n)))
}

This function takes an unnamed parameter n of type Int, and returns an integer number between 0 and n-1. As you can see, inside the arc4random_uniform(_:) call, n is converted to UInt32, and the return value of arc4random_uniform(_:) is converted back to Int.

If you’re using Swift 4.2. or greater, you won’t need these convenience functions. Simply use .random() or .randomElement(), as explained in the chapters above.

Next, you can generate a few random numbers like this:

for i in 0...100 {
    print(random(100))
}

This will output a 100 random numbers between 0 and 99.

Swift has a half-open range operator, like this 0..<42. Why don’t you use it to change the convenience function?

Like this:

func random(_ range:Range<Int>) -> Int
{
    return range.lowerBound + Int(arc4random_uniform(UInt32(range.upperBound - range.lowerBound)))
}

Instead of a single input number, this function takes in a Range like 0..<42 and returns a number between 0 and 42, not including 42.

Inside the function the result is first added to the lower bound of the range, ensuring that the random number always starts at that lower bound. Inside the arc4random_uniform(_:) function, the lower bound is subtracted from the upper bound so that the returned result lies neatly between the lower and the upper bound.

This function deliberately uses the half-open range generic type Range, instead of the closed range generic ClosedRange, because it better reflects the range of the resulting numbers from arc4random_uniform(_:).

You can use the random-number-from-range function like this:

for i in 0...100 {
    print(random(1..<42))
}

This will print out a 100 random numbers between 1 and 41.

Picking a Random Element from an Array

How do you get a random number from an array?

As you know the function arc4random_uniform(_:) returns an integer between zero and the upper bound. If we use array.count as the upper bound, the function will return an index number within the bounds of the array!

Like this:

let names = ["Arthur", "Ford", "Zaphod", "Marvin", "Trillian"]

let random = names[Int(arc4random_uniform(UInt32(names.count)))]
print(random)
// Output: Marvin

In this example, a random number between zero and names.count is created, and that’s used to get a value out of the names array with subscript syntax.

You can conveniently turn the example above in an extension for Array, like this:

extension Array 
{
    func random() -> Element {
        return self[Int(arc4random_uniform(UInt32(self.count)))]
    }
}

In the same fashion as before, a random element from the array is returned. In the example above, Element refers to a type alias of the element in the array, and self refers to the current array instance.

You can use it like this:

let names = ["Arthur", "Ford", "Zaphod", "Marvin", "Trillian"]
print(names.random())
// Output: Trillian

What about shuffling an array? Check out this tutorial: Shuffling Arrays in Swift with Fisher-Yates

Generating a Random String

What if you want to generate a random string? You can do that like this:

func randomString(_ n: Int) -> String 
{
    let digits = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
    var result = ""

    for _ in 0..<n {
        result += String(digits.randomElement()!)
    }

    return result
}

In the above example you define a function random(_:) that takes one unnamed parameter n of type Int. The function then defines an alphabet – the characters it’s going to choose from randomly. In the for-loop, a random character from the string is selected and then appended to variable result. This variable is then returned at the end of the function.

Below’s a more concise version of the above randomString(_:) function. It works by mapping a range of integers into random characters from digits, which is then converted back into a String.

func randomString(_ n: Int) -> String {
    let digits = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
    return String(Array(0..<n).map { _ in digits.randomElement()! })
}

You use the function like this:

let password = randomString(24)
print(password)
// Output: qGUjakGkjlRb1sd5jDaj2raW

A few things to keep in mind:

  • Strings in Swift can be a bit tricky… You can read more about them, here: https://oleb.net/blog/2017/11/swift-4-strings/
  • Don’t use these functions to generate random passwords – they aren’t strong enough. Use a password manager like 1Password instead.
  • Be careful when generating random IDs or keys for your database, because it’s easy to create a collision. It’s better to use a unique string generator, like UUID.

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Further Reading

Awesome! We generated a whole bunch of random stuff. Here’s what we discussed:

  • How to generate a random number in Swift
  • How to pick a random element from an array
  • How to generate a random string

Want to learn more? Check out these resources:

Reinder de Vries

Hi, I'm Reinder.
I help developers play with code.

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Reinder de Vries

Reinder de Vries

Reinder de Vries is a professional iOS developer. He teaches app developers how to build their own apps at LearnAppMaking.com. Since 2009 he has developed a few dozen apps for iOS, worked for global brands and lead development at several startups. When he’s not coding, he enjoys strong espresso and traveling.

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