mean-sd.clj

(defn ^:private mean-sd-step
  [[n m s] ^double x]
  (let [n (long n), m (double m), s (double s)
        n (inc n), d (- x m), m (+ m (/ d n)), s (+ s (* d (- x m)))]
    [n m s]))

(defn mean-sd
  "Calculate mean and (sample) standard-deviation of `vals`."
  [vals]
  (let [[n m s] (reduce mean-sd-step [0 0.0 0.0] vals)]
    [m (if (<= n 1) 0.0 (Math/sqrt (/ s (dec n))))]))

(mean-sd (range 1 100))
;; => [50.0 28.722813232690143]