As mentioned by Colin and Andy, I would guess it would be some form of
boxing and reflection going on. I tried the following:
(defn array-max [^doubles arr]
(let [len (alength arr)]
(loop [m Double/NEGATIVE_INFINITY indx 0]
(if (< indx len)
(recur (max m (aget arr indx)) (unchecked-inc indx))
m))))
user=> (let [vs (amap (double-array 1280000) idx ret (Math/random))]
(time (array-max vs)))
"Elapsed time: 3.719835 msecs"
To note, if you check out the source of areduce:
user=> (source areduce)
(defmacro areduce
"Reduces an expression across an array a, using an index named idx,
and return value named ret, initialized to init, setting ret to the
evaluation of expr at each step, returning ret."
{:added "1.0"}
[a idx ret init expr]
`(let [a# ~a]
(loop [~idx 0 ~ret ~init]
(if (< ~idx (alength a#))
(recur (unchecked-inc ~idx) ~expr)
~ret))))
It's just a macro, and so typehinting is going to play a factor. For
example, with areduce and a type hint on the array:
(defn array-max2 [^doubles arr]
(areduce arr idx ret Double/NEGATIVE_INFINITY (max ret (aget arr idx))))
user=> (let [vs (amap (double-array 1280000) idx ret (Math/random))] (time
(array-max vs)))
"Elapsed time: 3.314599 msecs"
But with no type hint on arr:
(defn array-max2 [arr]
(areduce arr idx ret Double/NEGATIVE_INFINITY (max ret (aget arr idx))))
user=> (let [vs (amap (double-array 1280000) idx ret (Math/random))] (time
(array-max2 vs)))
"Elapsed time: 35612.919192 msecs"
Without a typehint on the arr argument, I also do get boxed math and
reflection warnings:
Reflection warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:3
- call to static method alength on clojure.lang.RT can't be resolved
(argument types: unknown).
Boxed math warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:3
- call: public static boolean
clojure.lang.Numbers.lt(long,java.lang.Object).
Reflection warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:58
- call to static method aget on clojure.lang.RT can't be resolved (argument
types: unknown, int).
Boxed math warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:49
- call: public static java.lang.Object
clojure.lang.Numbers.max(double,java.lang.Object).
form-init1595291808747030463.clj:2 recur arg for primitive local: ret is
not matching primitive, had: Object, needed: double
Auto-boxing loop arg: ret
Reflection warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:3
- call to static method alength on clojure.lang.RT can't be resolved
(argument types: unknown).
Boxed math warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:3
- call: public static boolean
clojure.lang.Numbers.lt(long,java.lang.Object).
Reflection warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:58
- call to static method aget on clojure.lang.RT can't be resolved (argument
types: unknown, int).
Boxed math warning,
/private/var/folders/0k/xj_drd990xxf4q99n2bdknrc0000gn/T/form-init1595291808747030463.clj:2:49
- call: public static java.lang.Object
clojure.lang.Numbers.max(java.lang.Object,java.lang.Object).
On Wednesday, June 10, 2015 at 4:07:09 PM UTC-4, Ritchie Cai wrote:
>
> I'm working on a java array of double with 1280000 elements. I need the
> max and min values of the array. So I initially tried areduce and loop,
> both gives runs around 20 seconds. But when try (apply max (vec array)) I
> get result under 90 ms.
> Can anyone explain why there is such a big difference?
> Also if want to iterate large java array like this to do some other
> operations, e.g. convolution, what's the best way to go? Is there another
> fast way to iterate through array or do I need to convert array into vector?
>
> Thanks
> Ritchie
>
>
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