Krdytkyu

I'm writing a simple function: given a number, it will return a sequence or a collection of the digits in the number (in the correct order). i.e (get-digits-fn 1234567) => (1 2 3 4 5 6 7) / [1 2 3 4 5 6 7]

Below are five attempts at the same function:

(defn get-digits-1 [num]

  (->> [num '()]

       (iterate (fn [[num digits]]

                  (when (> num 0)

                    [(quot num 10) (conj digits (rem num 10))])))

       (take-while some?)

       (last)

       (second)))



(defn get-digits-2 [num]

  (when (> num 0)

    (lazy-seq (concat (get-digits-2 (quot num 10)) '((rem num 10))))))



;; Suggested by Carcigenate

(defn get-digits-3 [num]

  (->> (str)

       (map str)

       (map int)

       (into '())))



(defn get-digits-4 [num]

  (loop [n num

         res '()]

    (if (= n 0)

      res

      (recur (quot n 10) (conj res (rem n 10))))))



(defn get-digits-5 [num]

  (->>

   (iterate (fn [[n digits]]

              [(quot n 10) (conj digits (rem n 10))])

            [num '()])

   (drop-while #(not= 0 (first %)))

   (first)

   (second)))

A helper function for testing performance:

(defn quick-bench-get-digits [fn range]

  (quick-bench (->> range

                    (map fn)

                    (map (partial apply vector))

                    (into ))))

The perf results (output truncated to only show execution time mean):

eul> (quick-bench-get-digits get-digits-1 (range 1 100000))

Execution time mean : 129.516521 ms



eul> (quick-bench-get-digits get-digits-2 (range 1 100000))

Execution time mean : 128.637055 ms



eul> (quick-bench-get-digits get-digits-3 (range 1 100000))

Execution time mean : 24.267716 ms



eul> (quick-bench-get-digits get-digits-4 (range 1 100000))

Execution time mean : 25.083393 ms



eul> (quick-bench-get-digits get-digits-5 (range 1 100000))

Execution time mean : 145.430443 ms

It looks like get-digits-3 is the fastest while get-digits-4 is closely behind. (As the numbers increase, get-digits-3 outperforms get-digits-4. i.e try (range 1000000 2000000))

1. Any way to increase performance more without leaving Clojure land?


2. If mutability and Java inter-op is allowed, is there a way to increase performance?

p.s. functions 1 and 5 are almost identical. This was incremental exploration.

I'm afraid your code has errors, so your benchmarking is invalid.

• The get-digits-3 function is wrong.


• A corrected version runs about a hundred times slower than
  get-digits-4.

Repairing get-digits-3

Your get-digits-3 doesn't work:

=> (get-digits-3 123456789)

()

The function chain should be applied to the argument num:

(defn get-digits-3 [num]

  (->> num

       (str)

       (map str)

       (map int)

       (into '())))

But this fails:

=> (get-digits-3 123456789)

ClassCastException java.base/java.lang.String cannot be cast to java.base/java.lang.Character  clojure.lang.RT.intCast (RT.java:1213)

The culprit is the (map int) line. You can't apply int to strings. If you drop the offending line, you get the character values:

=> (get-digits-3 123456789)

(57 56 55 54 53 52 51 50 49)

Notice, too, that they are in reverse order. So we

• subtract the value of character 0 and


• 
  conj onto the other end by using a vector.

This gives us ...

(defn get-digits-3 [num]

  (->> num

       (str)

       (map int)

       (map #(- % (int )))

       (into )))



=> (get-digits-3 123456789)

[1 2 3 4 5 6 7 8 9]

Benchmarking

I went about this differently:

• I used a single example instead of a range.


• I used Criterium's bench rather than its quick-bench.

The results were as follow:

For your original (faulty) version of get-digits-3:

=> (bench (get-digits-3 123456789))

...

             Execution time mean : 228.998292 ns

For the corrected version:

=> (bench (get-digits-3 123456789))

...

             Execution time mean : 3.440024 µs

And for get-digits-4:

=>(bench (get-digits-4 123456789))

...

             Execution time mean : 247.502418 ns

This confirms that your faulty get-digits-3 is about as fast as get-digits-4. And get-digits-3 ignores its argument, so is bound to get relatively faster as the numbers get bigger. You can see that the corrected version is nowhere near competitive.