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struct Matrix {

    let rows: Int, columns: Int

    var grid: [Double]

    init(rows: Int, columns: Int) {

        self.rows = rows

        self.columns = columns

        grid = Array(repeating: 0.0, count: rows * columns)

    }

    func indexIsValid(row: Int, column: Int) -> Bool {

        return row >= 0 && row < rows && column >= 0 && column < columns

    }

    subscript(row: Int, column: Int) -> Double {

        get {

            assert(indexIsValid(row: row, column: column), "Index out of range")

            return grid[(row * columns) + column]

        }

        set {

            assert(indexIsValid(row: row, column: column), "Index out of range")

            grid[(row * columns) + column] = newValue

        }

    }

}

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var matrix = Matrix(rows: 2, columns: 2)

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matrix[0, 1] = 1.5

matrix[1, 0] = 3.2

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func indexIsValidForRow(row: Int, column: Int) -> Bool {

    return row >= 0 && row < rows && column >= 0 && column < columns

}

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let someValue = matrix[2, 2]

// this triggers an assert, because [2, 2] is outside of the matrix bounds