fast reduce

[Yehonathan Sharvit]

The performance of array iteration in cljs is not good compared vs. native iteration on js array.
For example, I would like to sum all the members of on array.

I have tried (apply + arr) and (doseq ...). See below.

Is there a way to improve my code?

(def arr (range 20480))

(set! js/arr (clj->js arr))

(js/eval "function sum(arr) {
var total = 0,
i;
for (i = 0; i < arr.length; i++) {
total += arr[i];
}
return total;
}")

(defn my-sum[a]
(def res 0)
(doseq [x a]
(set! res (+ res x )))
res)

(time (js/sum js/arr)) ; less than 1 msec
(time (apply + arr)); around 10 msec
(time (my-sum arr)); around 10 msec

[David Nolen]

On Mon, Apr 7, 2014 at 5:26 PM, Yehonathan Sharvit <vie...@gmail.com> wrote:

(defn my-sum[a]
  (def res 0)
  (doseq [x a]
    (set! res (+ res x )))
  res)

def's are always top-level. Do not put def's in def's like this.

(defn my-sum [a]

  (loop [i 0 sum 0]

    (if (< i (alength a))

      (recur (inc i) (+ sum (aget a i))

      sum)))

Should give similar performance to the version you wrote in JavaScript.

David

[Yehonathan Sharvit]

Thanks David!

I see that ClojureScript can deal with js arrays both with:

1. aget and alength

2. get and count

The first option is much much faster!!!

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[Yehonathan Sharvit]

I have discovered that areduce is also very fast:

(areduce a i res 0 (+ res (aget a i)))

I feel that it preserves the functional programming spirit.

Any comment on that, David?

[David Nolen]

Yes areduce is preferred if applicable.

David

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