clojure bootcamp: loop/recur vs. reduce vs. recursion

bootcamp.factorial.clj

(ns bootcamp.factorial)

(defn fast-factorial [number]
  (loop [n number factorial 1]
    (if (zero? n)
      factorial
      (recur (- n 1) (* factorial n)))))

(defn fast-no-loop-factorial
  ([number] (fast-no-loop-factorial number 1))
  ([number factorial]
   (if (zero? number)
     factorial
     (recur (- number 1) (* factorial number)))))

(defn recursive-factorial
  ([number] (recursive-factorial number 1))
  ([number factorial]
   (if (zero? number)
     factorial
     (recursive-factorial (- number 1) (* factorial number)))))

(defn slow-factorial [number]
  (reduce #(* %1 %2) 1 (range 1 (inc number))))

(println (time (fast-factorial 20)))
;"Elapsed time: 0.123 msecs"
;2432902008176640000

(println (time (fast-no-loop-factorial 20)))
;"Elapsed time: 0.125 msecs"
;2432902008176640000
;
(println (time (recursive-factorial 20)))
;"Elapsed time: 0.135 msecs"
;2432902008176640000

(println (time (slow-factorial 20)))
;"Elapsed time: 2.897 msecs"
;2432902008176640000

The performance difference is huge! Do you know why reduce is so much slower than the other solutions? It's the solution that I would certainly reach for for this problem.

great question :)
it was 10 years ago, things improved quite a bit since then:

dev=> (println (time (slow-factorial 20)))
;; "Elapsed time: 0.04197 msecs"
;; 2432902008176640000

dev=> (println (time (fast-factorial 20)))
;; "Elapsed time: 0.034636 msecs"
;; 2432902008176640000