Why does the reverse() function in the Swift standard library return ReverseRandomAccessCollection?

Question

Now that I\'ve learned Swift (to a reasonable level) I\'m trying to get to grips with the standard library, but in truth it\'s mainly ελληνικά to me!

So a specific q

Answer 1

It is an performance optimization for both time and memory. The ReverseRandomAccessCollection presents the elements of the original array in reverse order, without the need to create a new array and copying all elements (as long as the original array is not mutated).

You can access the reversed elements with subscripts:

let el0 = arr[arr.startIndex]
let el2 = arr[arr.startIndex.advancedBy(2)]

or

for i in arr.indices {
    print(arr[i])
}

You can also create an array explicitly with

let reversed = Array(["Mykonos", "Rhodes", "Naxos"].reversed())

---

A dictionary is also a sequence of Key/Value pairs. In

let dict = ["greek" : "swift sometimes", "notgreek" : "ruby for this example"].reverse()

a completely different reversed() method is called:

extension SequenceType {
    /// Return an `Array` containing the elements of `self` in reverse
    /// order.
    ///
    /// Complexity: O(N), where N is the length of `self`.
    @warn_unused_result
    public func reversed() -> [Self.Generator.Element]
}

The result is an array with the Key/Value pairs of the dictionary in reverse order. But this is of limited use because the order of the Key/Value pairs in a dictionary can be arbitrary.