How To: Random Numbers in Swift

Written by: Reinder de Vries, March 20 2017, in App Development

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

Random Number Functions In Swift

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.

Generating 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 it to Int.

Like this:

let n = Int(arc4random_uniform(42))

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

Generating Random Double’s with drand48()

What about double’s? 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

Easy!

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… Neat!

A Convenience 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.

You can generate a few random numbers, then, 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

So, how do you get a random number from an array? Easy!

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 array extension, 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

Want to shuffle an array? Check out this excellent answer on StackOverflow.

Generating A Random String

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

func random(_ n: Int) -> String  
{
    let a = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"

    var s = ""

    for _ in 0..<n  
    {
        let r = Int(arc4random_uniform(UInt32(a.characters.count)))

        s += String(a[a.index(a.startIndex, offsetBy: r)])  
    }

    return s  
}

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 s. This variable is then returned at the end of the function.

You use it like this:

print(random(8))  
// Output: 6FvUpkzp

A few things to keep in mind:

  • Strings in Swift 3 can be a bit weird… You can read more about them, here: https://oleb.net/blog/2016/08/swift–3-strings/
  • Don’t use this function to generate random passwords – they’re not random. Use this, instead: https://random.org/passwords
  • Be careful when generating random IDs or keys for your database, because it’s easy to create a collision. It’s often better to use the database framework functions, if there are any.

Conclusion

So, now you know:

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

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Written By: Reinder de Vries

Reinder de Vries is an indie app developer who teaches aspiring app developers and marketers how to build their own apps at LearnAppMaking.com. Since 2009 he has developed over 50 apps for iOS, Android and the web, and his code is used by millions of users all over the globe. When Reinder isn't building apps, he enjoys strong espresso and traveling.

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