Why does the following code crash on an iPhone 5 but not an iPhone 5S?

Question

func rand(max: Int?) -> Int {
    var index = Int(arc4random())
    return max? != nil ? (index % max!) : index
}

I get an exception on the last

Answer 1

As it seems like you found out, arc4random returns an unsigned 32 bit integer. So 0 to 4,294,967,295. Also, Int is a different size depending on the system that it is running on.

From "Swift reference:"

On a 32-bit platform, Int is the same size as Int32.
On a 64-bit platform, Int is the same size as Int64.

On an iPhone 5, Int will only hold −2,147,483,648 to +2,147,483,647. On an iPhone 5S, Int can hold −9,223,372,036,854,775,808 to +9,223,372,036,854,775,807. An unsigned 32 bit integer can overflow a Int32 but never a Int64.

More information on what random function to use and why.