使用 clojure 的两个哈希映射的交集

Question

我是 Clojure 的新手。我的问题是如何获得两个哈希图的交集，例如

(def map-1 {"a" 2, "b" 1, "c" 4, "d" 3})
(def map-2 {"a" 3, "b" 6, "e" 5})

当我们定义了两个映射时，预期的结果是{"a" 3, "b" 6}，它在一个具有相交键和键的最大值的映射中。

不知何故，我想出了一个解决方案并实施了它，但它部分正确。

基本思想是找到其中项目数量最少的地图，并将其用作参考。对于参考地图中的每个项目，检查其他地图是否包含它。如果包含，则将其最大值放入输出映射的键（使用(max num1 num2)）

以下是我的示例代码：

(defn max-intersect [map1 map2]
  (let [smaller-map (if (< (count map1) (count map2))
                      map1
                      map2)
        longer-map (if (= smaller-map map1)
                     map2
                     map1)]
        (loop [output {}
               reference-map smaller-map]
          (if (empty? reference-map)
            output
            (recur (let [[item-key item-val] (first smaller-map)]
                     (when (contains? longer-map item-key)
                       (assoc output item-key (max item-val (get longer-map item-key)))))
                   (rest reference-map))))))

这是我的 repl 结果：

test-intersect.core=> (def map1 {"a" 2, "b" 1, "c" 4, "d" 3})
#'test-intersect.core/map1
test-intersect.core=> (def map2 {"a" 3, "b" 6, "e" 5})
#'test-intersect.core/map2
test-intersect.core=> (max-intersect map1 map2)
{"a" 3}

这可能看起来很复杂，我也在等待任何伟大而有效的解决方案。

非常感谢！

Answer 1

这可以通过merge-with 使用max 作为合并函数来完成：

(def map-1 {"a" 2, "b" 1, "c" 4, "d" 3})
(def map-2 {"a" 3, "b" 6, "e" 5})
(merge-with max map-1 map-2)
=> {"a" 3, "b" 6, "c" 4, "d" 3, "e" 5}

要仅包含出现在两个映射中的键，您可以使用 select-keys 与两个映射键的集合交集：

(select-keys (merge-with max map-1 map-2)
             (clojure.set/intersection
               (set (keys map-1))
               (set (keys map-2))))

Answer 2

(require '[clojure.set :refer [intersection]])

(defn merge-with-max-intersecting
  [m1 m2]
  (let [intersecting (intersection (set (keys m1))
                                   (set (keys m2)))]
    (merge-with max
                (select-keys m1 intersecting)
                (select-keys m2 intersecting))))

(merge-with-max-intersecting {"a" 2, "b" 1, "c" 4, "d" 3}
                             {"a" 3, "b" 6, "e" 5})
;;=> {"a" 3, "b" 6}

Answer 3

你也可以像这样一次性完成：

(defn merge-maps [m1 m2]
  (reduce-kv (fn [acc k v]
               (if (contains? m1 k)
                 (assoc acc k (max v (m1 k)))
                 acc))
             {}
             m2))

user> (merge-maps map-1 map-2)
;;=> {"a" 3, "b" 6}