Swift字典默认值 [英] Swift Dictionary default value

问题描述

我已经习惯使用Python的默认模式是一个字典，如果给定键的值未被明确设置，则返回默认值。在Swift中尝试这样做是有点冗长的。

A pattern I've gotten used to with Python's defaultdicts is a dictionary that returns a default value if the value for a given key has not been explicitly set. Trying to do this in Swift is a little verbose.

var dict = Dictionary<String, Array<Int>>()
let key = "foo"
var value: Array<Int>! = dict[key]
if value == nil {
    value = Array<Int>()
    dict[key] = value
}

我意识到我可以做一个类，这样做，但是实际的字典必须通过属性访问以使用任何其他正常字典方法

I realize I can make a class that does this, but then the actual Dictionary has to be accessed through a property to use any of the other normal Dictionary methods

class DefaultDictionary<A: Hashable, B> {
    let defaultFunc: () -> B
    var dict = Dictionary<A, B>()

    init(defaultFunc: () -> B) {
        self.defaultFunc = defaultFunc
    }
    subscript(key: A) -> B {
        get {
            var value: B! = dict[key]
            if value == nil {
                value = defaultFunc()
                dict[key] = value
            }
            return value
        }
        set {
            dict[key] = newValue
        }
    }
}

有没有更好的模式？

推荐答案

使用Swift 2可以实现某些类似于python的版本，扩展名为字典：

Using Swift 2 you can achieve something similar to python's version with an extension of Dictionary:

// Values which can provide a default instance
protocol Initializable {
    init()
}

extension Dictionary where Value: Initializable {
    // using key as external name to make it unambiguous from the standard subscript
    subscript(key key: Key) -> Value {
        mutating get { return self[key, or: Value()] }
        set { self[key] = newValue }
    }
}

// this can also be used in Swift 1.x
extension Dictionary {
    subscript(key: Key, or def: Value) -> Value {
        mutating get {
            return self[key] ?? {
                // assign default value if self[key] is nil
                self[key] = def
                return def
            }()
        }
        set { self[key] = newValue }
    }
}

?? 用于类，因为它们不传播其值突变（仅指针突变;引用类型）。

The closure after the ?? is used for classes since they don't propagate their value mutation (only "pointer mutation"; reference types).

为了使用这些下标，字典必须是可变的（ var ）：

The dictionaries have to be mutable (var) in order to use those subscripts:

// Make Int Initializable. Int() == 0
extension Int: Initializable {}

var dict = [Int: Int]()
dict[1, or: 0]++
dict[key: 2]++

// if Value is not Initializable
var dict = [Int: Double]()
dict[1, or: 0.0]

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