swift2 Subscripts

下標腳本語法

subscript(index: Int) -> Int {
    get {
      // 返回與入參匹配的Int類型的值
    }

    set(newValue) {
      // 執行賦值操作
    }
}

實例如下：

struct TimesTable {
    let multiplier: Int
    subscript(index: Int) -> Int {
        return multiplier * index
    }
}
let threeTimesTable = TimesTable(multiplier: 3)
print("3的6倍是\(threeTimesTable[6])")
// 輸出 "3的6倍是18"

下標腳本選項

可以定義多個參數的形式

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

let someValue = matrix[2, 2]
// 斷言將會觸發，因爲 [2, 2] 已經超過了matrix的最大長度