(make-array (.getComponentType (class arr)) n)  seems to work.

On Sunday, July 21, 2013 12:22:41 PM UTC-7, Brian Craft wrote:
>
> Is there a way to create an array with the type of another array? (type 
> arr) returns the array type, but make-array wants the element type not the 
> array type, so 
>
> (make-array (type arr) n)
>
> doesn't work as one might hope.
>
>
> On Sunday, July 21, 2013 8:36:22 AM UTC-7, Alex Fowler wrote:
>>
>> Java's System.arraycopy is the fastest you can get, since it delegates 
>> execution to a function implemented in C inside JVM. Simply, this is the 
>> fastest that your computer hardware can get. All in all Java arrays meet 
>> the same difficulties and implications as C arrays and that is why 
>> concationation of raw arrays is so "complex", in contrast to higher-level 
>> collections which use objects and pointers (e.g. LinkedList). In other 
>> words, difficulties you experience are natural outcome of how computer's 
>> memory management is made and there is no way around them. You get the most 
>> of the speed from arrays because they are solid (not fragmented) chunks of 
>> bytes allocated in memory in the moment of their creation. For that very 
>> reason you cannot extend an existing array (the size cannot be changed 
>> after creation) and you can't concatenate it with another array since first 
>> it would have to be concatenated.
>>
>> The natural outcome also is that only arrays of same types can be 
>> concatenated with System.arraycopy since only array pointers store type 
>> data, and the contents are simply untyped bytes. And this is why it is 
>> byte-level and no type-checks are ever done besiedes the initial 
>> type-check. Again, higher-level pointer-based data structures like 
>> LinkedList or Queue can introduce boxed typed values, but that'd be waaay 
>> slower. Considering that only arrays of same type are concatenateable, 
>> creating a polymorphic function is easy - simply check the argument type 
>> like:
>>
>> ; first save types to use them later
>> (def arr-type-int (class (ints 3)))
>> ; ... same for other primitives...
>>
>> ; then in your func:
>> (cond
>>   (= (class arr) arr-type-int) (do-int-concat)
>>   ...)
>>
>> For more reference:
>> http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html
>> http://docstore.mik.ua/orelly/java-ent/jnut/ch02_09.htm
>>
>> As an alternative, try looking into Java NIO buffers - they too are fast 
>> and too have some limits. But maybe you could make good of them, depends on 
>> your use case.
>>
>> Although somewhat in another vein, but still relating fast data 
>> management is 
>> https://groups.google.com/forum/?hl=en#!topic/clojure/BayfuaqMzvs which 
>> brings in C-like structs in.
>>
>> On Sunday, July 21, 2013 2:39:38 AM UTC+4, Brian Craft wrote:
>>>
>>> Here are some experiments that aren't polymorphic. The System/arraycopy 
>>> version is fastest, by far. Is there any good way to make the other 
>>> versions faster, or make them handle any array type?
>>>
>>> (defn bconcat [& arrays]
>>>  (let [sizes (map count arrays)
>>>        sizes_r (vec (reductions + sizes))
>>>        offsets (cons 0 (drop-last sizes_r))
>>>        total (last sizes_r)
>>>        out (float-array total)]
>>>    (dorun (map #(System/arraycopy %2 0 out %1 %3) offsets arrays sizes))
>>>    out))
>>>
>>> (defn cconcat [& arrays]
>>>  (let [vs (map vec arrays)
>>>        cc (apply concat vs)]
>>>    (float-array cc)))
>>>
>>> (defn dconcat [& arrays]
>>>  (let [vs (map vec arrays)
>>>        cc (reduce into [] vs)]
>>>    (float-array cc)))
>>>
>>> (defn econcat [& arrays]
>>>  (let [cc (reduce into [] arrays)]
>>>    (float-array cc)))
>>>
>>>
>>> On Saturday, July 20, 2013 2:24:14 PM UTC-7, Brian Craft wrote:
>>>>
>>>> Is there an easy, fast way to concat primitive arrays? I was hoping 
>>>> java arrays had some common interface for this, but I haven't found much 
>>>> of 
>>>> use. I mostly see code like this:
>>>>
>>>> byte[] c = new byte[a.length + b.length];
>>>> System.arraycopy(a, 0, c, 0, a.length);
>>>> System.arraycopy(b, 0, c, a.length, b.length);
>>>>
>>>> which only works for bytes (in this case).
>>>>
>>>

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