下标的语法 xxxxx[6] -->threeTimesTable[6] || xxxxx["11"]

subscript(index: Int) -> Int {
    get {
        // return an appropriate subscript value here
    }
    set(newValue) {
        // perform a suitable setting action here
    }
}
// 只读计算属性一样，你可以给只读下标省略 get
subscript(index: Int) -> Int {
    // return an appropriate subscript value here
}

//eg:
struct TimesTable {
    let multiplier: Int
    subscript(index: Int) -> Int {
        return multiplier * index
    }
}
let threeTimesTable = TimesTable(multiplier: 3)
print("six times three is \(threeTimesTable[6])")
// prints "six times three is 18"

下标用法

// “下标”确切的意思取决于它使用的上下文。通常下标是用来访问集合、列表或序列中元素的快捷方式。你可以在你自己特定的类或结构体中自由实现下标来提供合适的功能。

// eg: Dictionary
var numberOfLegs = ["spider": 8， "ant": 6， "cat": 4]
numberOfLegs["bird"] = 2

下标选项(下标可以接收任意数量的输入形式参数，并且这些输入形式参数可以是任意类型。下标也可以返回任意类型)

//eg:
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
        }
    }
}

var matrix = Matrix(rows: 2, columns: 2)

matrix[0, 1] = 1.5
matrix[1, 0] = 3.2

func indexIsValidForRow(row: Int, column: Int) -> Bool {
    return row >= 0 && row < rows && column >= 0 && column < columns
}
// 断言在下标越界时触发：
let someValue = matrix[2, 2]