clojure - 汇总(分组和计数)一系列 map

Question

我试图在 Clojure 中找到一种惯用的方法，通过特定键对一系列映射进行分组并提供计数。有点像 SQL 中的“SELECT X, Y, COUNT(*) FROM Z GROUP BY X, Y”。数据如下所示:

({:status "Academy Sponsor Led",
  :pupil-population "",
  :locality "Northamptonshire",
  :pupil-gender "Mixed",
  :county "Northamptonshire",
  :pupil-age "11-18",
  :school "Wrenn School",
  :website ""}
 {:status "Academy Sponsor Led",
  :pupil-population "915",
  :locality "Plymouth",
  :pupil-gender "Mixed",
  :county "Devon",
  :pupil-age "11-19",
  :school "The All Saints Church of England Academy",
  :website "http://www.asap.org.uk/"}
 {:status "Academy Converter",
  :pupil-population "735",
  :locality "Somerset",
  :pupil-gender "Mixed",
  :county "Somerset",
  :pupil-age "11-16",
  :school "Stanchester Academy",
  :website "www.Stanchester-Academy.co.uk"}
 {:status "Community School",
  :pupil-population "",
  :locality "Herefordshire",
  :pupil-gender "Mixed",
  :county "Herefordshire",
  :pupil-age "11-18",
  :school "Lady Hawkins High School",
  :website "http://www.lhs.hereford.sch.uk"}...

我的解决方案是这样的:

(defn summarise-locality-status
  "Return counts of status within locality"
  [data]
  (let [locality (group-by :locality data)
        locality-status (map #(vector (first %) (group-by :status (second %))) locality)
        counts-fn (fn [locality-status-item]
                    (let [statuses (second locality-status-item)]
                      (map #(vector % (count (get statuses %))) (keys statuses))))]
    (map #(vector (first %) (counts-fn %)) locality-status)))

但是感觉有点笨拙。这样做的更好方法是什么？

Answer 1

根据您的需要，

(frequencies (for [r data] (select-keys r [:locality :status])))

更接近 SQL，因为它没有嵌套。