Question

我必须使用值从地图中提取密钥。除了自己实现反向查找之外，有没有办法做到这一点？

Answer 1

我认为map-invert是执行此操作的正确方法。

;; Despite being in clojure.set, this has nothing to do with sets. 

user=> (map-invert {:a 1, :b 2})
{2 :b, 1 :a}

;; If there are duplicate keys, one is chosen:

user=> (map-invert {:a 1, :b 1})
{1 :b}

;; I suspect it'd be unwise to depend on which key survives the clash.

Answer 2

您可以使用2行功能轻松地反转地图：

(defn reverse-map [m]
  (into {} (map (fn [[a b]] [b a]) m)))

(def a {:a 1 :b 2 :c 3})

(reverse-map a)
=> {1 :a, 3 :c, 2 :b}

((reverse-map a) 1)
=> :a

Answer 3

尝试

(some #(if (= (val %) your-val) (key %)) your-map)

Answer 4

如果您使用的是ClojureScript，或者还需要一个替代方案：）

root

Answer 5

另一个：

(defn reverse-map [m]                                                                                                                          
  (apply hash-map  (mapcat reverse m)))  

(defn reverse-lookup [m k]                                                                                                                     
  (ffirst (filter (comp #{k} second) m)))

Answer 6

如果你想保留按键，最好只是反转地图，但是在集合/列表等中收集旧密钥......

(defn map-inverse [m]
  (reduce (fn [m' [k v]] (update m' v clojure.set/union #{k})) {} m))

(defn map-inverse [m]
  (reduce (fn [m' [k v]] (update m' v conj k)) {} m))

在地图中反向查找

6 个答案: