类型(四) – 字典

mapValues

mapValues() 的方法更有用，因为它可以让你转换值，并使用原始键将它们放回字典。

let cities = ["Shanghai": 24_256_800, "Karachi": 23_500_000, "Beijing": 21_516_000, "Seoul": 9_995_000]
let roundedCities = cities.mapValues {
    "\($0 / 1_000_000) million people"
}
// ["Shanghai": "24 million people", "Beijing": "21 million people", "Seoul": "9 million people", "Karachi": "23 million people"]
print(roundedCities)

分组初始化器

可以将一个序列转换成一个由序列组成的字典，这些序列可以按你想要的任何方式分组。

let groupedCities = Dictionary(grouping: cities.keys) {
    $0.first!
}

// ["S": ["Shanghai", "Seoul"], "K": ["Karachi"], "B": ["Beijing"]]
print(groupedCities)

根据城市名称的长度进行分组

let groupedCities = Dictionary(grouping: cities.keys) {
    $0.count
}

// [8: ["Shanghai"], 5: ["Seoul"], 7: ["Karachi", "Beijing"]]
print(groupedCities)

访问字典的键值，并在建丢失时提供默认值。

let person = ["name": "Taylor", "city": "Nashville"]
let name = person["name", default: "Anonymous"]