Swift setting a struct value within another struct init

Question

I have Cell struct values (position:, state:) which need to be set within the init of my Grid struct, but I can't seem to set these values of Cell.

struct Cell {
    var position: (Int,Int)
    var state: CellState

    init(_ position: (Int,Int), _ state: CellState) {
        self.position = (0,0)
        self.state = .empty
    }
}

func positions(rows: Int, cols: Int) -> [Position] {
    return (0 ..< rows)
        .map { zip( [Int](repeating: $0, count: cols) , 0 ..< cols ) }
        .flatMap { $0 }
        .map { Position(row: $0.0,col: $0.1) }
}

I've commented all of the ways that I've tried to set the position to (row, col)

struct Grid {
    static let offsets: [Position] = [
        (row: -1, col:  1), (row: 0, col:  1), (row: 1, col:  1),
        (row: -1, col:  0),                    (row: 1, col:  0),
        (row: -1, col: -1), (row: 0, col: -1), (row: 1, col: -1)
    ]

    var rows: Int = 10
    var cols: Int = 10
    var cells: [[Cell]] = [[Cell]]()

    init(_ rows: Int,
         _ cols: Int,
         cellInitializer: (Int, Int) -> CellState = { _,_ in .empty } ) {
        self.rows
        self.cols
        self.cells = [[Cell]](repeatElement([Cell](repeatElement(Cell((0,0), .empty), count: cols)),count: rows))


        positions(rows: rows, cols: cols).forEach { row, col in
            //  var position = cells(position: (row, col)) => cannot call value of non-function type '[[Cell]]'
            //  cells.position = (row, col)  => value type of '[[Cell]] has no member position'
            //  cells.position(row, col)  => value type of '[[Cell]] has no member position'
            //  position *= cells.position(row, col) => closure cannot implicitly capture a mutating self parameter


        }
    }
}

Clearly the Cell struct has a property of position, so why can't I access it?

Alvae · Accepted Answer · 2017-06-29 14:14:33Z

The problem is that none of your lines are actually accessing instances of your Cell struct.

Here's a functioning adaptation of your code. I allowed myself to remove extra stuff that seem to have been left out from your codebase:

struct Cell {
    var position: (Int,Int)

    init(_ position: (Int,Int)) {
        self.position = (0,0)
    }
}

func positions(rows: Int, cols: Int) -> [(Int, Int)] {
    return (0 ..< rows)
        .map { zip( [Int](repeating: $0, count: cols) , 0 ..< cols ) }
        .flatMap { $0 }
        .map { ($0.0, $0.1) }
}

struct Grid {
    var rows: Int = 10
    var cols: Int = 10
    var cells: [[Cell]] = [[Cell]]()

    init(_ rows: Int, _ cols: Int) {
        self.rows = rows
        self.cols = cols
        self.cells = Array.init(repeating: Array.init(repeating: Cell((0,0)), count: cols), count: cols)

        positions(rows: rows, cols: cols).forEach { row, col in
            cells[row][col].position = (row, col)
        }
    }
}

let g = Grid(1, 2)
print(g.cells[0][1].position)

Now, for a more detailed explanation of the errors you encountered:

var position = cells(position: (row, col))

Here you're not setting anything on any cell. Instead, you're trying to call your grid as if it was a function, with a parameter position: (Int, Int).

cells.position = (row, col)

Here you're trying to set a property position on your matrix ([[Cell]]). And obviously, Swift complains that such property does not exists in its builtin type Array.

cells.position(row, col)

Here you're trying to set a property position on your matrix ([[Cell]]) and call it as a function with two parameters Int. The problem is similar as above.

position *= cells.position(row, col)

Here I'm can't tell what's going on, since position does not seems to have been declared in your code. I guess it comes from elsewhere in your codebase, or maybe it's merely a typo.

Guillermo Alvarez · Accepted Answer · 2017-06-29 14:06:34Z

1

The issue here is that you are trying to access cells.position but cells is a two-dimensional array.

cells.position = (row, col)  => value type of '[[Cell]] has no member position'

You could loop through the cells and set the position of each one.

So in your forEach loop you could write instead

cells[row][column].position = (row, col)

and that should do it.

answered Jun 29, 2017 at 14:06

Guillermo Alvarez

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1 Comment

LayneSadler Over a year ago

Thank you - that compiles. So does [row] access the first layer of the array, and [column] accesses the 2nd layer?

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