In Swift, Array [String] slicing return type doesn

Question

I'm slicing an array of strings and setting that to a [String] variable, but the type checker is complaining. Is it a possible compiler bug?

var tags = ["this", "is", "cool"]
tags[1..<3]
var someTags: [String] = tags[1..<3]

Answer 1

Subscripting an array with a range doesn't return an array, but a slice. You can create an array out of that slice though.

var tags = ["this", "is", "cool"]
tags[1..<3]
var someTags: Slice<String> = tags[1..<3]
var someTagsArray: [String] = Array(someTags)

Answer 2

var tags = ["this", "is", "cool"]
var someTags: [String] = Array(tags[1..<3])
println("someTags: \(someTags)") // "someTags: [is, cool]"

Answer 3

You can also do this to get a new array of the slice:

var tags = ["this", "is", "cool"]
var someTags = [String]()
someTags += tags[1..<3]
println(someTags[0])  //prints ["is", "cool"]

Answer 4

Another convenient way to convert an ArraySlice to Array is this:

var tags = ["this", "is", "cool"] var someTags: [String] = tags[1..<3] + []

It's not perfect because another developer (or yourself) who looks at it later may not understand its purpose. The good news is that if that developer (maybe you) removes the + [] they will immediately be met with a compiler error, which will hopefully clarify its purpose.

Answer 5

Another way to do that in one place is combine variable declaration let someTags: [String] and map(_:), that will transform ArraySlice<String> to [String]:

let tags = ["this", "is", "cool"]
let someTags: [String] = tags[1..<3].map { $0 } // ["is", "cool"]