class StepCounter {
    var totalSteps: Int = 0 {
        willSet(newTotalSteps) {
            println("About to set totalSteps to \(newTotalSteps)") }
        didSet {
            if totalSteps > oldValue {
                println("Added \(totalSteps - oldValue) steps")
            } 
        }
    }
}

struct Matrix {
    let rows: Int, columns: Int
    var grid: [Double]
    init(rows: Int, columns: Int) {
        self.rows = rows
        self.columns = columns
        grid = Array(count: rows * columns, repeatedValue: 0.0)
    }

    func indexIsValidForRow(row: Int, column: Int) -> Bool {
        return row >= 0 && row < rows && column >= 0 && column < columns
    }

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

        get {
            assert(indexIsValidForRow(row, column: column), "Index out of range")
            return grid[(row * columns) + column]
        }

        set {
            assert(indexIsValidForRow(row, column: column), "Index out of range")
            grid[(row * columns) + column] = newValue
        } 
   }
}

class StepCounter {
    var totalSteps: Int = 0 {
        willSet(newTotalSteps) {
            println("About to set totalSteps to \(newTotalSteps)")
        }
    }
}