从地图中删除零值?

你的 list comprehension返回映射列表,因此需要将此列表作为可选参数应用于merge函数:

user> (apply merge (for [[k v] record :when (not (nil? v))] {k v}))
{:b 2, :a 1}      

更简洁的解决方案是将地图作为一个序列进行过滤,并合并到地图中:

user> (into {} (filter second record))
{:a 1, :b 2}  

不删除 价值观:

user> (into {} (remove (comp nil? second) record))
{:a 1, :b false}  

使用 消散 要允许持久数据共享而不是创建一个全新的映射,请执行以下操作:

user> (apply dissoc                                                                                            
       record                                                                                                  
       (for [[k v] record :when (nil? v)] k))
{:a 1, :b 2}  

(defn remove-nils
  [m]
  (let [f (fn [[k v]] (when v [k v]))]
    (postwalk (fn [x] (if (map? x) (into {} (map f x)) x)) m)))

@Eelco回答的一个变体:

(defn remove-nils [m]
  (let [f (fn [x]
            (if (map? x)
              (let [kvs (filter (comp not nil? second) x)]
                (if (empty? kvs) nil (into {} kvs)))
              x))]
    (clojure.walk/postwalk f m)))

对于@broma0的点,它忽略了任何空的地图。

user> (def m {:a nil, :b 1, :c {:z 4, :y 5, :x nil}, :d {:w nil, :v nil}})
user> (remove-nils m)
{:b 1, :c {:z 4, :y 5}}
user> (remove-nils {})
nil

你可以把它压在地图上:

(into {} (remove (fn [[k v]] (nil? v)) {:a 1 :b 2 :c nil}))
=> {:a 1 :b 2}

改进Jrgen Htzel解决方案以解决nil/false问题

(into {} (filter #(not (nil? (val %))) {:a true :b false :c nil}))

(into {} (remove #(nil? (val %)) {:a true :b false :c nil}))

尽管Jrgen(filter second record)的方法得到了我对最漂亮的Clojure技巧的投票,但我想我会用另一种方法,这次使用 select-keys :

user> (select-keys record (for [[k v] record :when (not (nil? v))] k))
{:b 2, :a 1}

降低电压也可用于拔出钥匙

(reduce-kv (fn [m key value]
                (if (nil? value)
                  (dissoc m key)
                  m))
            {:test nil, :test1 "hello"}
            {:test nil, :test1 "hello"})

你可以使用reduce。

user> (reduce (fn [m [k v]] (if (nil? v) m (assoc m k v))) {} record)
{:b 2, :a 1}

dissoc .

user> (reduce (fn [m [k v]] (if (nil? v) (dissoc m k) m)) record record)
{:a 1, :b 2}

(defn compact
  [coll]
  (cond
    (vector? coll) (into [] (filter (complement nil?) coll))
    (map? coll) (into {} (filter (comp not nil? second) coll))))