Returning duplicates in a sequence

Question

The best I could come up with was:

(defn dups [seq]
  (map (fn [[id freq]] id) 
       (filter (fn [[id freq]] (> freq 1))
               (frequencies seq))))

Is there a more concise way?

Answer 1

Use a list comprehension:

(defn dups [seq]
  (for [[id freq] (frequencies seq)  ;; get the frequencies, destructure
        :when (> freq 1)]            ;; this is the filter condition
   id))                              ;; just need the id, not the frequency