Swift: Mutating array in struct

Question

I am trying to create a Matrix class in Swift but I am getting an error on the self.data[row * columns..<(row + 1) * columns] = data line in my setRow() function. The error is 'Cannot assign value of type '[Double]' to type 'ArraySlice''

struct Matrix: CustomStringConvertible {
    let rows:Int
    let columns:Int
    var data:[Double]

    // Description
    public var description: String {
        var description = ""
        for r in 0..<rows {
            description += data[r * columns..<(r + 1) * columns].description + "\n"
        }
        return description
    }

    // Initialisation
    init(rows: Int, columns: Int) {
        self.rows = rows
        self.columns = columns
        self.data = Array(repeating: 0.0, count: rows * columns)
    }

    init(rows: Int, columns: Int, data:[Double]) {
        assert(data.count == (rows * columns),"Number of elements must equal rows * columns")
        self.rows = rows
        self.columns = columns
        self.data = data
    }

    // Validity
    func validRow(row: Int) -> Bool {
        return row > 0 && row < rows
    }

    func validColumn(column: Int) -> Bool {
        return column > 0 && column < columns
    }

    func validIndex(row: Int, column: Int) -> Bool {
        return validRow(row: row) && validColumn(column: column)
    }

    // Setters and getters
    func get(row: Int, column: Int) -> Double {
        assert(validIndex(row: row,column: column), "Index out of range")
        return data[(row * columns) + column]
    }

    mutating func set(row: Int, column: Int, value: Double) {
        assert(validIndex(row: row,column: column), "Index out of range")
        data[(row * columns) + column] = value
    }

    func getRow(row: Int) -> [Double] {
        assert(validRow(row: row), "Index out of range")
        return Array(data[row * columns..<(row + 1) * columns])
    }

    mutating func setRow(row: Int, data:[Double]) {
        assert(validRow(row: row), "Index out of range")
        assert(data.count == columns, "Data must be same length ans the number of columns")
        self.data[row * columns..<(row + 1) * columns] = data
    }

    // Swapping
    mutating func swapRow(row1: Int, row2: Int) {
        assert(validRow(row: row1) && validRow(row: row2), "Index out of range")
        let holder = getRow(row: row2)
        setRow(row: row2, data: getRow(row: row1))
        setRow(row: row1, data: holder)
    }
}

Answer 1

As the error says, Array's ranged subscript deals with ArraySlice, not Array.

One solution, as @vacawama says, is to just create a slice of the entire input array. This can be done by subscripting with the array's indices:

mutating func setRow(row: Int, data newData: [Double]) {
    assert(validRow(row: row), "Index out of range")
    assert(data.count == columns, "Data must be same length ans the number of columns")
    data[row * columns ..< (row + 1) * columns] = newData[newData.indices]
}

Or in Swift 4, you can take advantage of the ... 'operator', which does the exact same thing:

data[row * columns ..< (row + 1) * columns] = newData[...]

But IMO, a better tool for the job here would be replaceSubrange(_:with:):

mutating func replaceSubrange<C>(_ subrange: Range<Int>, with newElements: C) 
     where Element == C.Element, C : Collection

as it allows you to deal with an arbitrary Collection of new elements, meaning that you could also make setRow generic:

mutating func setRow<C : Collection>(row: Int, data newData: C)
    where C.Iterator.Element == Double { // <- in Swift 4, remove ".Iterator"

    assert(validRow(row: row), "Index out of range")
    assert(data.count == columns, "Data must be same length ans the number of columns")
    data.replaceSubrange(row * columns ..< (row + 1) * columns, with: newData)
}