如何遍历可选集合

Question

What's the Swifty way to unwrap an optional collection directly before a loop? Consider the following:

let elements: [Element]?
for element in elements { ... }

where elements may contain values. The compiler produces the error: For-in loop requires '[Element]?' to conform to 'Sequence'; did you mean to unwrap optional? For-in loop requires '[Element]?' to conform to 'Sequence'; did you mean to unwrap optional? , similarly with forEach .

Using the where -Keyword to tell the compiler that I only want to iterate if there are values in elements like this

for element in elements where elements != nil { ... }

does not work. I am basically looking for a way to unwrap the collection where I don't have to write verbose guard let or if let before iterating through the loop.

I am not looking to extend Sequence with a property that returns unwrapped self , I just don't think it's very elegant (including force-unwrapping).

Answer 1

You have a couple of options. You can provide an empty array as the default value for elements in a for ... in loop:

let elements: [Element]?

for element in elements ?? [] {

}

Or you can use forEach and optional chaining on elements

elements?.forEach { element in

}

Bear in mind that optional collections are an anti-pattern in Swift. You can represent the lack of value using an empty collection, so wrapping a collection in an optional doesn't provide any extra value, while complicating the interface.

Answer 2

Your elements are not optional, but array is. In your case just unwrap the array:

let elements: [Element]?
for element in elements ?? [] {
    // do stuff
}

But if your elements are optional, you can use compactMap:

let elements: [Element?] // elements are optional instead of array
for element in elements.compactMap { $0 } {
    // do stuff
}