clojure - 在clojure中使用reduce进行分组

Question

我想聚合大型数据集以获得类似的东西

SELECT SUM(`profit`) as `profit`, `month` FROM `t` GROUP BY `month`

所以，我像这样修改了clojure的group-by函数

(defn group-reduce [f red coll]
  (persistent!
   (reduce
    (fn [ret x]
      (let [k (f x)]
        (assoc! ret k (red (get ret k) x))))
    (transient {}) coll)))

这是用法：

(group-reduce :month (fn [s x]
                       (if s
                         (assoc s :profit (+ (:profit s) (:profit x)))
                         x))
              [{:month 10 :profit 12}
               {:month 10 :profit 15}
               {:month 12 :profit 1}])

#_=> {10 {:profit 27, :month 10}, 12 {:profit 1, :month 12}}

它可以工作，但也许还有另一种方法可以做到这一点，使用 clojure 标准库？

Answer 1

最接近核心的是merge-with：

(def t [{:month 10 :profit 12}
        {:month 10 :profit 15}
        {:month 12 :profit 1}])

(apply merge-with + (for [x t] {(:month x) (:profit x)}))
;=> {12 1, 10 27}

Answer 2

一些例子：

user=> (def groups (group-by :month [{:month 10 :profit 12}
  #_=>                               {:month 10 :profit 15}
  #_=>                               {:month 12 :profit 1}])
{10 [{:profit 12, :month 10} {:profit 15, :month 10}], 12 [{:profit 1, :month 12}]}

user=> (for [[k v] groups] {:month k :sum-profit (apply + (map :profit v))})
({:month 10, :sum-profit 27} {:month 12, :sum-profit 1})

user=> (into {} (for [[k v] groups] [k (apply + (map :profit v))]))
{10 27, 12 1}