将对象数组映射到Swift中的Dictionary

Question

我有一个Person对象的数组:

class Person {
   let name:String
   let position:Int
}

而数组是:

let myArray = [p1,p1,p3]

我想映射myArray成[position:name]经典解决方案的字典是:

var myDictionary =  [Int:String]()

for person in myArray {
    myDictionary[person.position] = person.name
}

Swift是否有任何优雅的方式来实现功能性方法map,flatMap......或其他现代Swift风格

Answer 1

在Swift 4你可以使用它的into版本更干净,更有效地做@Tj3n的方法reduce它摆脱了临时字典和返回值,因此它更快更容易阅读.

struct Person { 
    let name: String
    let position: Int
}
let myArray = [Person(name:"h", position: 0), Person(name:"b", position:4), Person(name:"c", position:2)]
let myDict = myArray.reduce(into: [Int: String]()) {
    $0[$1.position] = $1.name
}
print(myDict)

注意:在Swift 3中不起作用.

Answer 2

好吧map不是一个很好的例子,因为它与循环相同,你可以使用reduce它,它将你的每个对象组合并变成单个值:

let myDictionary = myArray.reduce([Int: String]()) { (dict, person) -> [Int: String] in
    var dict = dict
    dict[person.position] = person.name
    return dict
}

//[2: "b", 3: "c", 1: "a"]

Answer 3

从Swift 4开始,您可以轻松完成此操作.有两个 新的初始化器,它们从一系列元组(键和值对)构建字典.如果密钥保证是唯一的,您可以执行以下操作:

let persons = [Person(name: "Franz", position: 1),
               Person(name: "Heinz", position: 2),
               Person(name: "Hans", position: 3)]

Dictionary(uniqueKeysWithValues: persons.map { ($0.position, $0.name) })

=> [1: "Franz", 2: "Heinz", 3: "Hans"]

如果任何密钥重复,则会因运行时错误而失败.在这种情况下,您可以使用此版本:

let persons = [Person(name: "Franz", position: 1),
               Person(name: "Heinz", position: 2),
               Person(name: "Hans", position: 1)]

Dictionary(persons.map { ($0.position, $0.name) }) { _, last in last }

=> [1: "Hans", 2: "Heinz"]

这表现为你的for循环.如果您不想"覆盖"值并坚持第一个映射,您可以使用:

Dictionary(persons.map { ($0.position, $0.name) }) { first, _ in first }

=> [1: "Franz", 2: "Heinz"]

Swift 4.2添加了第三个初始化程序,它将序列元素分组到字典中.字典键由闭包派生.具有相同键的元素按照与序列中相同的顺序放入数组中.这使您可以获得与上面类似的结果.例如:

Dictionary(grouping: persons, by: { $0.position }).mapValues { $0.last! }

=> [1: Person(name: "Hans", position: 1), 2: Person(name: "Heinz", position: 2)]

Dictionary(grouping: persons, by: { $0.position }).mapValues { $0.first! }

=> [1: Person(name: "Franz", position: 1), 2: Person(name: "Heinz", position: 2)]

Answer 4

您可以为Dictionary类型编写自定义初始值设定项,例如来自元组:

extension Dictionary {
    public init(keyValuePairs: [(Key, Value)]) {
        self.init()
        for pair in keyValuePairs {
            self[pair.0] = pair.1
        }
    }
}

然后map用于你的数组Person:

var myDictionary = Dictionary(keyValuePairs: myArray.map{($0.position, $0.name)})

Answer 5

基于 KeyPath 的解决方案怎么样？

extension Array {

    func dictionary<Key, Value>(withKey key: KeyPath<Element, Key>, value: KeyPath<Element, Value>) -> [Key: Value] {
        return reduce(into: [:]) { dictionary, element in
            let key = element[keyPath: key]
            let value = element[keyPath: value]
            dictionary[key] = value
        }
    }
}

这是你将如何使用它：

struct HTTPHeader {

    let field: String, value: String
}

let headers = [
    HTTPHeader(field: "Accept", value: "application/json"),
    HTTPHeader(field: "User-Agent", value: "Safari"),
]

let allHTTPHeaderFields = headers.dictionary(withKey: \.field, value: \.value)

// allHTTPHeaderFields == ["Accept": "application/json", "User-Agent": "Safari"]