(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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