[英]Clojure get map key by value
I'm a new clojure programmer.我是一名新的 clojure 程序员。
Given...鉴于...
{:foo "bar"}
Is there a way to retrieve the name of the key with a value "bar"?有没有办法用值“bar”检索键的名称?
I've looked through the map docs and can see a way to retrieve key and value or just value but not just the key.我已经浏览了地图文档,可以看到一种检索键和值或仅检索值而不只是检索键的方法。 Help appreciated!
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There can be multiple key/value pairs with value "bar".可以有多个值为“bar”的键/值对。 The values are not hashed for lookup, contrarily to their keys.
与它们的键相反,这些值不会被散列用于查找。 Depending on what you want to achieve, you can look up the key with a linear algorithm like:
根据您想要实现的目标,您可以使用线性算法查找密钥,例如:
(def hm {:foo "bar"})
(keep #(when (= (val %) "bar")
(key %)) hm)
Or要么
(filter (comp #{"bar"} hm) (keys hm))
Or要么
(reduce-kv (fn [acc k v]
(if (= v "bar")
(conj acc k)
acc))
#{} hm)
which will return a seq of keys.这将返回一系列键。 If you know that your vals are distinct from each other, you can also create a reverse-lookup hash-map with
如果您知道您的 vals 彼此不同,您还可以创建一个反向查找哈希映射
(clojure.set/map-invert hm)
user> (->> {:a "bar" :b "foo" :c "bar" :d "baz"} ; initial map
(group-by val) ; sorted into a new map based on value of each key
(#(get % "bar")) ; extract the entries that had value "bar"
(map key)) ; get the keys that had value bar
(:a :c)
As in many other cases you can use for:与许多其他情况一样,您可以用于:
(def hm {:foo "bar"})
(for [[k v] hm :when (= v "bar")] k)
And with "some" you can return the first matching item instead of a list (as probably the original question implied):使用“some”,您可以返回第一个匹配项而不是列表(可能是原始问题所暗示的):
(some (fn [[k v]] (if (= v "bar") k)) hm)
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