Clojure：如何计算列表中的出现次数？

Question

我对 clojure 还是很陌生，所以如果这有点微不足道，我深表歉意。基本上，问题出在 if 语句的“then”部分：（if（符号？（第一个 slist））。

;counts the number of occurences of 
(defn count-occurrences [s slist]
 (if (empty? slist)
   0
   (if (symbol? (first slist))
     (if (= (first slist) s)
       (+ 1 (count-occurrences s (rest slist)))
       (+ 0 (count-occurrences s (rest slist))))
     (count-occurrences s (first slist)))))                  ;Problem on this line

(println (count-occurrences 'x '((f x) y (((x z) x)))))

Answer 1

要计算嵌套列表中的元素，您可以试试这个函数：

(defn count-occurrences [s slist]
  (->> slist
       flatten
       (filter #{s})
       count))

测试：

user> (count-occurrences 'x '((f x) y (((x z) x))))
;; => 3
user> (count-occurrences 'y '((f x) y (((x z) x))))
;; => 1
user> (count-occurrences 'z '((f x) y (((x z) x))))
;; => 1