Clojure function to map a given function over a given dataset every nth interval.

every-nth.clj

(defn every-nth [f data n & {:keys [n-times] :or {n-times -1}}]
  "Applies the function to every nth element, leaving other elements unaltered"
  (map-indexed #(if (and
                      (zero? (mod (Math/abs (+ %1 1)) n))
                      (> n (quot %1 (* (Integer/signum n) n-times))))
                  (f %2) %2)
                data))