Constant Used Before Being Initialized - swift

Question

I'm converting a project to Swift 3 and I'm getting this error below:
fileprivate class func moveAi() -> DataModel {
let player = pathForPlayer(true)
let i: Int
let scope = GameModel.shared.scopeForPlayer(GameModel.shared.topPlayer.id, rival: GameModel.shared.downPlayer.id)
//for (i = player.count-1; i > 0; i -= 1) { // Original for loop prior to Swift 3
for i in stride(from: player.count-1, to: 0, by: -1) {
if scope.contains(player[i]) {
break
}
}
// Error is on this line
return DataModel.idConvertToPlayer(player[i], player: true)
}
// Path for Player
class func pathForPlayer(_ play: Bool) -> [Int] {
let player = play ? GameModel.shared.topPlayer.id : GameModel.shared.downPlayer.id
let end = play ? topEnd : downEnd
return pathForPlayer(Node(data: player, parent: -1), end: end)
}
If I move the return within the for loop the error goes away, but I still need to return a value here.
Any suggestions?
Updated Code:
fileprivate class func moveAi() -> DataModel {
let player = pathForPlayer(true)
let _: Int // i
var final: Int?
let scope = GameModel.shared.scopeForPlayer(GameModel.shared.topPlayer.id, rival: GameModel.shared.downPlayer.id)
**//for (i = player.count-1; i > 0; i -= 1)** {
for i in stride(from:player.count-1, to:0, by:-1) {
final = i
if scope.contains(player[i]) {
break
}
}
return DataModel.idConvertToPlayer(player[final!], player: true)
}
The above code does not provide the crash I was previously getting, but I am getting a new crash.
The best way to describe is my AI will prevent itself from finishing a game.
I'm building off the board game Quoridor. The AI will build walls around it's pawn so it can't move any further. This does not happen every game though. It's very random when it happens.
Could it be based on how I've written the for loop? Is the Swift way correct or could I get insight to how to right the commented for loop in Swift syntax?

Answer 1

Your loop gives i a value inside the loop, but not outside. After the loop ends, i has no value. If you want some value of i to be available after the loop finishes, you need to assign it to something that exists outside the loop.
One way would be to add this before the loop:
var final : Int?
Then in the loop, set final = i somewhere. After the loop finishes, the value will be in final.

Answer 2

Your function moveAi has 2 different things called i: An Int constant defined at the outer scope of the function and an inner constant that exists inside the scope of your for i in... loop. They are separate.
Inside the for loop, i hides the outer constant, and refers to the value extracted from your stride call.
Outside the for...in loop, that inner definition of i goes out of scope and no longer exists. Now you've got a constant that has never been initialized with a value.
You could rewrite your function like this:
fileprivate class func moveAi() -> DataModel {
let player = pathForPlayer(true)
var i: Int
let scope = GameModel.shared.scopeForPlayer(GameModel.shared.topPlayer.id, rival: GameModel.shared.downPlayer.id)
for index in stride(from: player.count-1, to: 0, by: -1) {
i = index
if scope.contains(player[i]) {
break
}
}
// Error is on this line
return DataModel.idConvertToPlayer(player[i], player: true)
}