How to Create a Random Int Number in Swift 2 - Swift Tips 7

Good random numbers in iPhone apps are really important (the same is true for the lottery!). A good random number is from a range [start, end] that gives every value an equal probability of occurring.

An equal probability means that if you were to flip a coin—you should always get close to 50% heads and 50% tails over 100 coin tosses.

Download the sample Xcode project that shows how to create random Int numbers in Swift with a working iPhone app.

Swift Random Int Range

In Swift you should use the arc4random_uniform(N) function because it creates better random numbers (i.e.: equal probability).

arc4random_uniform(N) creates a number within from [0, N - 1], which means if you want numbers in the range [0, 100] you need to call arc4random_uniform(N + 1).

1. Random Int [A, B] where A < B

   let A: UInt32 = 10 // UInt32 = 32-bit positive integers (unsigned)
    let B: UInt32 = 20
    let number = arc4random_uniform(B - A + 1) + A

2. Random Int [0, 100]

   // Using the equation (100 - 0 + 1) + 0 becomes (101)
    let number = arc4random_uniform(101) // [0, 100]

3. Random Int [1, 100]

   // Using the equation (100 - 1 + 1) + 1 becomes (100) + 1
    let number2 = arc4random_uniform(100) + 1 // [1, 100]

4. Random Int [50, 100]

   // Using the equation (100 - 50 + 1) + 50 becomes (51) + 50
    let number3 = arc4random_uniform(51) + 50 // [50, 100]

5. Unsafe: Swift method for a random Int within [A, B] where A < B

   Swift is strongly-typed which means that you need to explicitly change numbers that are Int into UInt32 and vice versa for the code to compile. Use the Int() and UInt32() initializers to convert between the types.

   func unsafeRandomIntFrom(start: Int, to end: Int) -> Int {
        return Int(arc4random_uniform(UInt32(end - start + 1))) + start
    }
   
    unsafeRandomIntFrom(50, to: 100)  // positive ascending ranges work
    unsafeRandomIntFrom(-50, to: 100) // negative ranges work!
    //unsafeRandomIntFrom(500, to: 100) // Crash! EXC_BAD_INSTRUCTION

   Crash: Technically this will crash if A > B with a EXC_BAD_INSTRUCTION because the parameter of arc4random_uniform() is UInt32 and can only hold positive 32-bit integer values.

6. Safe Swift method for a random Int within [A, B]

   func randomIntFrom(start: Int, to end: Int) -> Int {
        var a = start
        var b = end
        // swap to prevent negative integer crashes
        if a > b {
            swap(&a, &b)
        }
        return Int(arc4random_uniform(UInt32(b - a + 1))) + a
    }

Int Type Extension

You can make the randomIntFrom(_:end:) a static function on the type Int using a static function. This code block is then reusable in any of your projects to make random number generation simple.

extension Int {
    func randomIntFrom(start: Int, to end: Int) -> Int {
        var a = start
        var b = end
        // swap to prevent negative integer crashes
        if a > b {
            swap(&a, &b)
        }
        return Int(arc4random_uniform(UInt32(b - a + 1))) + a
    }
}

Where Does arc4random_uniform() Come From?

This random function is actually written in the C programming language (which you can use in Swift!). Read the arc4random man page (i.e.: manual page) for all the details.

Download the Code

Download the sample Xcode project that shows you how to create random Int numbers in Swift with a working iPhone app.

You can play change the range of numbers with the iPhone app to see how to use the arc4random_uniform() function correctly in your own apps.