Swift 字典获取值的键 [英] Swift dictionary get key for value

问题描述

我正在使用 [UIImage:UIImage] 类型的 swift 字典，并且我正在尝试为给定值查找特定键.在 Objective-C 中，我可以使用 allKeysForValue，但对于 Swift 字典似乎没有这样的方法.我应该使用什么?

I'm using a swift dictionary of type [UIImage:UIImage], and I'm trying to find a specific key for a given value. In Objective-C I could use allKeysForValue, but there appears to be no such method for a Swift dictionary. What should I be using?

推荐答案

Swift 3:一种针对双射词典特殊情况的更高效方法

如果反向字典查找用例涵盖键和值之间具有一对一关系的双射字典，则集合穷举filter 操作的另一种方法是使用更快的短路找到某些键(如果存在)的方法.

Swift 3: a more performant approach for the special case of bijective dictionaries

If the reverse dictionary lookup use case covers a bijective dictionary with a one to one relationship between keys and values, an alternative approach to the collection-exhaustive filter operation would be using a quicker short-circuiting approach to find some key, if it exists.

extension Dictionary where Value: Equatable {
    func someKey(forValue val: Value) -> Key? {
        return first(where: { $1 == val })?.key
    }
}

示例用法:

let dict: [Int: String] = [1: "one", 2: "two", 4: "four"]

if let key = dict.someKey(forValue: "two") { 
    print(key)
} // 2

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