从可选容器中提取非可选值的最佳方法

Question

This is a best practices question, and I have a feeling I'm missing an obvious solution...

I was stumped why this bit of code made it past the if when wantsGroundAlways was false and I1 was undefined:

let i = c.data["I1"]?.integerValue
if !wantsGroundAlways && i == 0 { return }

The problem was that data["I1"] is, by definition, optional, so Swift inferred i as int? , and nil != 0. Subtle, but obvious in retrospect.

But what is the best way to deal with this common case? One solution is:

let i = (c.data["I1"] ?? 0).integerValue

But personally I think that looks terrible and hides the intent. Something along the lines of:

guard let i = c.data["I1"]?.integerValue else { i = 0 }

would make it obvious what you're trying to do, but it doesn't work because the i cannot be accessed in the else clause and { let i = 0 } is not the same i (try it, you'll see what I mean).

Now there is this:

guard let arcradius = c.data["F1"]?.doubleValue else { return }

which seems really close to what I want to do, but I'm not sure this is really what I think it means - will the return really fire if F1 is not in the dict? And what is the difference between that version and this:

guard case let arcradius = c.data["F1"]?.doubleValue else { return }

Which tells me it's always true?

I think I am missing something key here... so what do you all do in these situations?

Answer 1

I think this is the most straightforward and expressive way of solving your first question:

let i = c.data["I1"]?.integerValue ?? 0

It doesn't require brackets, and shows intent.

?? is the nil coalescence operator . It's a binary operator. If the left operand is not nil , it's returned, otherwise the right operand is returned.

c.data["I1"] can be nil (because there might be no value for the "I1" key). In such a case, c.data["I1"]?.integerValue will be nil . The ?? will then return the 0 . If all goes well, and the left side isn't nil , it'll be returned, ignoring the 0 .

Answer 2

If you want only to check if a key exist or not then a "type cast" to Int or Double is irrelevant.

Why not simply

guard c.data["I1"] == nil && !wantsGroundAlways else { return }

It passes the test if I1 is not in the dictionary and wantsGroundAlways is false .

if let or guard let is not needed either because according the condition the value for key is never used.