What is the syntax for multidimensional array in Swift?

Question

CO fans: before you jump into conclusion it is a duplicate, there are 2 distinct flavours of arrays, and it seems I am asking about less popular one.

So far, I

Answer 1

Yes, there is nothing built-in (as far as I know). You can define a custom class/struct (as in Most efficient way to access multi-dimensional arrays in Swift? or How to Declare a Multidimensional Boolean array in Swift?) with a subscript operator, so that a[0,0] = 1 works.

Here is a mixture of those solutions, but as a generic struct instead of class. I have also changed the order of the rows and columns parameters because I find that more natural:

struct Array2D<T : IntegerLiteralConvertible > {
    let rows : Int
    let cols : Int
    var matrix: [T]

    init(rows : Int, cols : Int) {
        self.rows = rows
        self.cols = cols
        matrix = Array(count : rows * cols, repeatedValue : 0)
    }

    subscript(row : Int, col : Int) -> T {
        get { return matrix[cols * row + col] }
        set { matrix[cols*row+col] = newValue }
    }
}

I don't see how to create such a thing from a literal like [2, 3 ;; -1, 0]. But you could initialize it from a nested array:

extension Array2D {

    init(_ elements: [[T]]) {
        let rows = elements.count
        let cols = elements[0].count
        self.init(rows: rows, cols: cols)
        for i in 0 ..< rows {
            assert(elements[i].count == cols, "Array must have same number of elements for each row")
            self.matrix.replaceRange(cols * i ..< cols * (i+1), with: elements[i])
        }
    }
}

Example:

let array = Array2D([[1, 2, 3], [4, 5, 6]])
println(array.rows)  // 2
println(array.cols)  // 3
println(array[1, 2]) // 6
println(array[1, 0]) // 4

You can additionally implement the ArrayLiteralConvertible protocol to initialize a 2d array from a nested array literal:

extension Array2D : ArrayLiteralConvertible {

    init(arrayLiteral elements: [T]...) {
        self.init(elements)
    }
}

Example:

let array : Array2D = [[1, 2, 3], [4, 5, 6]]

For square arrays (rows == columns) you could alternatively initialize it from a plain array:

extension Array2D {

    init(_ elements: [T]) {
        let rows = Int(sqrt(Double(elements.count)))
        assert(rows * rows == elements.count, "Number of array elements must be a square")
        self.init(rows: rows, cols: rows)
        self.matrix = elements
    }
}

Example:

let squareArray = Array2D([2, 3, -1, 0])
println(squareArray.rows)  // 2
println(squareArray.cols)  // 3
println(squareArray[1, 0]) // -1

Answer 2

An example implementation supporting arbitrary number of dimensions:

struct ArrayMultiDimension<T> {
    private var _base:[T]
    let _dimensions: [Int]

    init(initialValue: T, dimensions:Int...) {
        _base = Array(count: reduce(dimensions, 1, *), repeatedValue: initialValue)
        _dimensions =  dimensions
    }
    private func _position2idx(position:[Int]) -> Int {
        assert(position.count == _dimensions.count)
        return reduce(Zip2(_dimensions, position), 0) {
            assert($1.0 > $1.1)
            return $0 * $1.0 + $1.1
        }
    }
    subscript(position:Int...) -> T {
        get { return _base[_position2idx(position)] }
        set { _base[_position2idx(position)] = newValue }
    }
}

// Usage:

var array3d = ArrayMultiDimension(initialValue: "", dimensions: 4,3,2)

for x in 0 ..< 4 {
    for y in 0 ..< 3 {
        for z in 0 ..< 2 {
            array3d[x,y,z] = "\(x)-\(y)-\(z)"
        }
    }
}

array3d[1,2,0] = "foo"

But, this could be very slow...