subscripts(下标)是访问对象数据中的快捷方式，所谓下标脚本语法就是能够通过，实例[索引值]来访问实例中的数据，类似于我们访问数组和字典，其实Swift中的数组和字典是一个结构体

Array:public subscript(index: Int) -> Element
arr[0] == arr.subscript(0)
Dictionary:public subscript(key: Key) -> Value?
dict["key"] == dict.subscript("key")

使用案列

struct Student {
    var name:String = "xiaoming"
    var math:Double = 99.0
    var chinese:Double = 99.0
    var english:Double = 99.0
    
    func score(course:String) ->Double?
    {
        switch course{
        case "math":
            return math
        case "chinese":
            return chinese
        case "english":
            return english
        default:
            return nil
        }
    }
    
    
    //要想实现下标访问, 必须实现subscript方法
    subscript(course:String) ->Double?{
        //如果想要通过下标访问, 必须实现get方法
        get{
            switch course{
            case "math":
                return math
            case "chinese":
                return chinese
            case "english":
                return english
            default:
                return nil
            }
        }
        
        //如果想要通过下表赋值, 必须实现set方法
        set{
            switch course{
            case "math":
                math = newValue!// 因为返回的是可选类型,所以使用了强制解包
            case "chinese":
                chinese = newValue!
            case "english":
                english = newValue!
            default:
                print("not found")
            }
            
        }
    }
}

var stu = Student(name: "zs", math: 99.0, chinese: 88.0, english: 10.0)
print(stu.score(course: "math") ?? "nil")
stu["chinese"] = 100.0
print(stu["chinese"]!)

Swift中允许多索引的下标的

struct Mul {
    subscript(a:Int, b:Int,c:Int) -> Int
    {
        return a * b * c
    }
}
var m = Mul()
print(m[3, 5,10]) //150