frequencies-by function

gistfile1.txt

(defn frequencies-by
  "Returns a map from distinct results of (f item) in coll 
  to a vector duple of all (k item) (if k is provided, or items otherwise) 
  with the same (f item) value, and the number of times (f item) appears.
"
  ([f coll] 
   (frequencies-by f nil coll))
  ([f k coll] 
   (persistent! ;; copied from clojure.core
    (reduce (fn [counts x]
              (assoc! counts (f x) 
                      (let [[xs kount] (get counts (f x) [[] 0])] 
                        [(conj xs (if k (k x) x)) 
                         (inc kount)])))
            (transient {}) coll))))

Examples:

(frequencies-by :age :id  [{:age 10 :id 1} {:age 12 :id 2} {:age 10 :id 3}])
 ;;=> {10 [[1 3] 2] 12 [[2] 1]}

(frequencies-by even?  [1 2 3 4 5])
;;=> {true [[2 4] 2]
        false [[1 3 5] 3]}