Сабскрипты для пользовательской индексации

struct Box {
    private var items: [String] = []
    init(_ values: [String]) { self.items = values }

    subscript(index: Int) -> String {
        get { items[index] }           // may trap if out of bounds (like Array)
        set { items[index] = newValue }
    }
}
var b = Box(["a","b","c"])
print(b[1])     // "b"
b[1] = "B"
print(b[1])     // "B"

extension Array {
    subscript(safe i: Int) -> Element? {
        return indices.contains(i) ? self[i] : nil
    }
}
let arr = [10,20]
print(arr[safe: 1] ?? -1)  // 20
print(arr[safe: 5] ?? -1)  // -1

struct Matrix {
    let rows: Int, cols: Int
    private var grid: [Int]
    init(rows: Int, cols: Int, fill: Int = 0) {
        self.rows = rows; self.cols = cols
        self.grid = Array(repeating: fill, count: rows*cols)
    }
    private func index(_ r: Int, _ c: Int) -> Int { r*cols + c }

    subscript(r: Int, c: Int) -> Int {
        get { grid[index(r,c)] }
        set { grid[index(r,c)] = newValue }
    }
}
var m = Matrix(rows: 2, cols: 3)
m[0, 2] = 7
print(m[0, 2])

struct Roman {
    static let map: [Int:String] = [1:"I", 5:"V", 10:"X"]
    static subscript(_ n: Int) -> String? {
        return map[n]
    }
}
print(Roman[5] ?? "?")   // "V"