Swift - 使用命名变量声明一个集合

Question

I am trying to understand Sets in Swift and how to declare them correctly but I have found the following a little confusing.

I understand that this works:

let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7]
let setFromElements = Set(elements)

But I don't understand why the following doesn't:

let setFromElements : Set = elements

Or even:

let setFromElements : Set<Int> = elements

When the following is valid:

let setFromArray : Set = [ 1, 2, 4, 5]

Can someone please help me understand why this is the case?

Answer 1

let setFromArray: Set = [ 1, 2, 4, 5] works because Set conforms to ExpressibleByArrayLiteral and hence has an initializer that takes an ArrayLiteral . See Set.init(arrayLiteral:) . This conformance gives syntactic sugar for not having to explicitly call the init.

On the other hand, once you save the array literal into a variable using let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7] , elements becomes an Array , not an ArrayLiteral and hence another initializer of Set has to be called that takes an Array . This init does not provide syntactic sugar like ExpressibleByArrayLiteral does, so you explicitly have to call the init by doing Set(array) .

Answer 2

Set has an initializer that takes an array , and that makes a set, by taking the unique items in the array. But a set is not an array , two different types, so you can't just use = to assign one to the other.