If I remove elements from an array and store it in a variable like this:

let lastFive = myArray.suffix(5)

or this:

let lastFive = myArray.dropLast(5)

lastFive will be Array<SomeType>.SubSequence instead of the element type SomeType. How do I make lastFive become the [Element] rather than a SubSequence?

CodePudding user response：

I like to use this extension:

extension Sequence {
    func asArray() -> [Element] {
        return Array(self)
    }
}

That said, both the Collection and Sequence protocols define a sequence() method.

Sequence.prefix() returns [Element] while Collection.prefix() returns a SubSequence.

Given an Array defined as follows, we can do some interesting things.

let myArray = [1, 2, 3, 4, 5]

The following works because the compiler knows to use the Sequence version of suffix().

func lastFive() -> [Int] {
    return myArray.suffix(5)
}

Here I'm just using the extension from above.

func lastFive2() -> [Int] {
    return myArray.suffix(5).asArray()
}

The following also works because the compiler knows to use the Sequence version of suffix().

func lastFive3() -> [Int] {
    let suffix: [Int] = myArray.suffix(5)
    return suffix
}

This does not compile because the compiler assumes you want to use the Collection version of suffix().

func doesNotCompile() -> [Int] {
    let suffix = myArray.suffix(5)
    return suffix
}