Swift: If let statement failing to handle empty array

Question

I have an application that is using the Foursquare API to download JSON data. I am using NSURLSession and the dataTaskWithRequest with completion block method to fetch the data. I am getting the data fine but sometimes a nested array named groups can be empty. And when I am parsing through the JSON like below, for some reason my conditional statement is not handling the empty array like I am expecting it should. Instead of evaluating the array as empty and proceeding to the "else" portion of the if let...else statement if instead throughs a runtime error stating: index 0 beyond bounds of empty array

if let response: NSDictionary = data["response"] as? [String: AnyObject],
            groups: NSArray = response["groups"] as? NSArray,
                        // Error here \|/ (sometimes groups array is empty)
            dic: NSDictionary = groups[0] as? NSDictionary,
            items: NSArray = dic["items"] as! NSArray {

}

else {

    // Never gets here. Why?
    // IF the groups array is empty, then the if let should return 
    // false and drop down to the else block, right?
}

I am relatively new to Swift, can someone tell me why this is happening and what I can do to fix this? Thanks

Answer 1

You have to check explicitly outside an if letstatement if the array is empty, because

An empty array is never an optional

if let response = data["response"] as? [String: AnyObject], groups = response["groups"] as? NSArray {
  if !groups.isEmpty {
    if let dic = groups[0] as? NSDictionary {
       items = dic["items"] as! NSArray
       // do something with items
       println(items)
    }
  }
} else ...

You can omit all type annotations while downcasting a type

However, you can perform the check with a where clause, this works in Swift 1.2 and 2

if let response = data["response"] as? [String: AnyObject],
                  groups = response["groups"] as? [AnyObject] where !groups.isEmpty,
                  let dic = groups[0] as? NSDictionary,
                  items = dic["items"] as? NSArray {
   // do something with items
   println(items)
} else {...