Swift: If let statement failing to handle empty array - ios

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 {...

Answer 2

Use && operator between statments.
For example this won't crash:
var a = 4;
var c = 5;
var array = Array<Int>();
if a > 2 && c > 10 && array[0] != 0 {
}

Answer 3

Make sure the array is not empty before trying to access it:
if let response: NSDictionary = data["response"] as? [String: AnyObject],
let groups: NSArray = response["groups"] as? NSArray where groups.count > 0 {
if let dic: NSDictionary = groups[0] as? NSDictionary,
let items: NSArray = dic["items"] as? NSArray {
// Do something..
return // When the 2nd if fails, we never hit the else clause
}
}
// Do logic for else here
...