2D Array extension Swift 3.1.1

Question

I am trying to make an Array extension in Swift 3.1.1 that supports the addition of an object to a certain index in a 2D Array even if the array hasn't been populated yet. The extension should also provide the ability to get an object at certain indexPath. I have the code for this in Swift 2 but I don't seem to be able to migrate it to Swift 3. This is the Swift 2 code:

extension Array where Element: _ArrayProtocol, Element.Iterator.Element: Any {

    mutating func addObject(_ anObject : Element.Iterator.Element, toSubarrayAtIndex idx : Int) {
        while self.count <= idx {
            let newSubArray = Element()
            self.append(newSubArray) 
        }

        var subArray = self[idx]
        subArray.append(anObject)
    }

    func objectAtIndexPath(_ indexPath: IndexPath) -> Any {
        let subArray = self[indexPath.section]
        return subArray[indexPath.row] as Element.Iterator.Element
    }
}

The code is taken from this answer.

Answer 1

As Martin says in his answer here, _ArrayProtocol is no longer public in Swift 3.1, therefore meaning that you cannot use it as a constraint in your extension.

A simple alternative in your case is to instead constrain the Array's Element to being a RangeReplaceableCollection – which both defines an init() requirement meaning "empty collection", and an append(_:) method in order to add elements to the collection.

extension Array where Element : RangeReplaceableCollection {

    typealias InnerCollection = Element
    typealias InnerElement = InnerCollection.Iterator.Element

    mutating func fillingAppend(
        _ newElement: InnerElement,
        toSubCollectionAtIndex index: Index) {

        if index >= count {
            append(contentsOf: repeatElement(InnerCollection(), count: index + 1 - count))
        }

        self[index].append(newElement)
    }
}

Note also that we're doing the append as a single call (using append(contentsOf:), ensuring that we only have to resize the outer array at most once.

For your method to get an element from a given IndexPath, you can just constrain the inner element type to being a Collection with an Int Index:

// could also make this an extension on Collection where the outer Index is also an Int.
extension Array where Element : Collection, Element.Index == Int {

    subscript(indexPath indexPath: IndexPath) -> Element.Iterator.Element {
        return self[indexPath.section][indexPath.row]
    }
}

Note that I've made it a subscript rather than a method, as I feel it fits better with Array's API.

You can now simply use these extensions like so:

var arr = [[Int]]()

arr.fillingAppend(6, toSubCollectionAtIndex: 3)
print(arr) // [[], [], [], [6]]

let indexPath = IndexPath(row: 0, section: 3)
print(arr[indexPath: indexPath]) // 6

Although of course if you know the size of the outer array in advance, the fillingAppend(_:toSubCollectionAtIndex:) method is redundant, as you can just create your nested array by saying:

var arr = [[Int]](repeating: [], count: 5)

which will create an [[Int]] array containing 5 empty [Int] elements.