Counting occurrences of items in a collection

gistfile1.clj

;;Three ways to count the number of occurrences in a collection
;; ("orange" "bottle" "coke" "bottle") => [("bottle" "coke" "orange") (2 1 1)]
(let [c '("orange" "bottle" "coke" "bottle")]

  (let [counts (reduce #(conj %1 (hash-map %2 (inc (get %1 %2 0))))
                       {} c)]
    [(keys counts)
     (vals counts)])


  (let [l (for [[k v] (group-by identity c)]
            [k (count v)])]
    [(map first l)
     (map second l)])


  (let [counts (apply merge-with +
                      (map #(apply hash-map %)
                           (partition 2 (interleave c (repeat 1)))))]
    [(keys counts)
     (vals counts)]))