2D arrays: Index out of range - arrays

Question

I'm pretty new to Swift, and I'm trying to store my data as a 2D AnyObject array. I have declared var internalData = [[AnyObject]]() and a struct like so:
struct WiFiData {
var latency: Double
var duration: Double
}
Now, in another function, I would like to switch on the instance variable currentExit, which is an enum:
private var currentExit = Exit.A
enum Exit {
case A
case B
case C
case NotSelected
}
func someFunc() {
switch self.currentExit {
case .A:
self.internalData[0].append(WiFiData(latency: 1.5, duration: 4.0) as AnyObject) // <- CRASHES ON THIS LINE
......// other cases
}
However, it always crashes on the line self.internalData[0].append(WiFiData(latency: 1.5, duration: 4.0) as AnyObject) with the following error:
fatal error: Index out of range
Can anyone tell me why and how I could fix it? Any help is appreciated, thanks!

Answer 1

You instantiated a 2d array with [[AnyObject]](), but it's still empty, meaning there's nothing at self.internalData[0] for you to append to. There are a lot of things you could do to handle this, but if you know you're going to have 3 arrays inside self.internalData you may as well instantiate each internal array like so:
self.internalData = [[AnyObject](), [AnyObject](), [AnyObject]()]
Now you do have an empty array at self.internalData[0] that you can append items to.
It seems, though, like if you already know you're going to have 3 arrays of WiFiData it'd be even better just to create 3 variables:
var a = [WiFiData]()
var b = [WiFiData]()
etc.
and then you can access the individual arrays by name.

Answer 2

There are no "2D arrays" in Swift. There are arrays of arrays, which are different. For example, there is no promise that each row will have the same number of columns in an array of arrays.
In your case, the problem is that internalData has no elements in it. There has to be an internalData[0] element already created before you can append to it. You're assignment of var internalData = [[AnyObject]]() means there's no element 0. It's an empty array (of arrays).
Also keep in mind that the type [[AnyObject]] is very likely to cause a lot of problems for you. AnyObject is useful for working with Cocoa APIs, but generally creates a lot of headaches and should very rarely be part of a Swift property. You should almost certainly create some more specific type to hold your data. It's unclear from your example what you expect internalData to hold. Your code suggests, though, that you mean something more like:
var internalData: [Exit: [ExitRecord]] = [:]
enum Exit {
case A
case B
case C
case NotSelected
}
enum ExitRecord {
case wifi(latency: Float, duration: Float)
}
func someFunc() {
switch self.currentExit {
case .A:
var currentRecords = internalData[.A] ?? []
currentRecords.append(.wifi(latency: 1.5, duration: 4.0))
internalData[.A] = currentRecords
......// other cases
}