修改词典

向 Dictionary 添加键和值

var books = [Int: String]()
//books = [:]
books[5] = "Book 5"
//books = [5: "Book 5"]
books.updateValue("Book 6", forKey: 5)
//[5: "Book 6"]

updateValue 返回原始值（如果存在或为零）。

let previousValue = books.updateValue("Book 7", forKey: 5)
//books = [5: "Book 7"]
//previousValue = "Book 6"

使用类似的语法删除值及其键

books[5] = nil
//books [:]
books[6] = "Deleting from Dictionaries"
//books = [6: "Deleting from Dictionaries"]
let removedBook = books.removeValueForKey(6)
//books = [:]
//removedValue = "Deleting from Dictionaries"