标

类，结构和枚举可以定义下标，下标是用于访问集合，列表或序列的成员元素的快捷方式。

例

struct DaysOfWeek {
  
  var days = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]
  
  subscript(index: Int) -> String {
    get {
      return days[index]
    }
    set {
      days[index] = newValue
    }
  }
}

下标用法

var week = DaysOfWeek()
//you access an element of an array at index by array[index].
debugPrint(week[1])
debugPrint(week[0])
week[0] = "Sunday"
debugPrint(week[0])

下标选项：

下标可以采用任意数量的输入参数，这些输入参数可以是任何类型。下标也可以返回任何类型。下标可以使用变量参数和可变参数，但不能使用输入输出参数或提供默认参数值。

例：

struct Food {
  
  enum MealTime {
    case Breakfast, Lunch, Dinner
  }
  
  var meals: [MealTime: String] = [:]
  
  subscript (type: MealTime) -> String? {
    get {
      return meals[type]
    }
    set {
      meals[type] = newValue
    }
  }
}

用法

var diet = Food()
diet[.Breakfast] = "Scrambled Eggs"
diet[.Lunch] = "Rice"

debugPrint("I had \(diet[.Breakfast]) for breakfast")