enum ShapeDimensions {
    case point
    //原文：case square(side: Double)
    //去掉了side
    case square(Double)
    //原文：case rectangle(width: Double, height: Double)
    //去掉了width,height
    case rectangle(Double, Double)
    func area() -> Double {
        switch self {
        case ShapeDimensions.point:
            return 0
        //原文：case let ShapeDimensions.square(side: side):
        //原文中的side: side，2個(gè)side感覺有兩個(gè)變量名似的，看起來不好理解。
        //因?yàn)樽钌厦媸沁@樣寫的：case square(side: Double)
        //所以這里我覺得應(yīng)該這樣寫：case let ShapeDimensions.square(side: Double):
        //但是這樣寫編譯不通過
        //通過下面這樣的寫法就好理解為什么加let了，因?yàn)閟ide是個(gè)常量名，不能無中生有，必須用let定義出來。當(dāng)然let也可以改成var
        case let ShapeDimensions.square(side):
            return side * side
        //原文：case let ShapeDimensions.rectangle(width: w, height: h):
        //let加在case后面是這句的簡寫：case ShapeDimensions.rectangle(let w, let h):
        case let ShapeDimensions.rectangle(w, h):
            return w * h
        }
    }
}

var squareShape = ShapeDimensions.square(10.0) //原文：square(side: 10.0)
var rectShape = ShapeDimensions.rectangle(5.0, 10.0) //原文：rectangle(width: 5.0, height: 10.0)
var pointShape = ShapeDimensions.point
print("square's area = \(squareShape.area())")
print("rectangle's area = \(rectShape.area())")
print("point's area = \(pointShape.area())")