组用Clojure中

Question

减少我想要聚合大量的数据可能要达到这样的

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

所以，我修改Clojure的组，按功能，像这样

(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))) 

而这里的用法：

(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}} 

它的工作，但也许有另一种方式来做到这一点，使用clojure标准库？

Answer 1

最近的核心是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}