Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence.
下标的形式和函数相同，并且set和get合一
subscript(row: Int, column: Int) -> Double
比较：
In addition to simple properties that are stored, properties can have a getter and a setter.
class EquilateralTriangle: NamedShape {var sideLength: Double = 0.0init(sideLength: Double, name: String) {self.sideLength = sideLengthsuper.init(name: name)numberOfSides = 3}var perimeter: Double {get {return 3.0 * sideLength}set {sideLength = newValue / 3.0}}override func simpleDescription() -> String {return "An equilateral triangle with sides of length \(sideLength)."}}var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")print(triangle.perimeter)triangle.perimeter = 9.9print(triangle.sideLength)