From the official J documentation: http://www.jsoftware.com/help/dictionary/special.htm
Appendix B. Special Code Many primitives contain special code for certain arguments to effect time and/or space savings not available to general arguments. Moreover, some phrases are “recognized” and are supported by special code. For example, the dyad of the hook ($,) is exactly the reshape of APL (denoted by r); its implementation avoids actually ravelling the right argument, and in so doing saves both time and space: ... y=: x i}y - in-place ... On 1/6/2012 10:42, Blake McBride wrote: > This is a very important point. I'd like to get an official response to this. > > Modifying an existing array could easily be 1,000 times faster than > creating a new, modified array. Naturally, this would depend on the > size and type of the array. Seeing how 'fast' it is in human or even > computer time means nothing! What is important is the relative time > between performing a real indexed update to a copy operation. Even a > small difference multiplied over a lot of operations can become > unbelievable significant. And, we are not talking about a small > difference! > > Imagine an application specific need to maintain a very large array > and make a lot of changes to individual elements. A program that does > only this on a large dataset could easily by 1,000 times more time > consuming. Imagine a situation where one program takes an hour to > complete a calculation and another takes 1,000 hours for the same > calculation. This is a significant issue. > > Blake McBride > > > On Fri, Jan 6, 2012 at 9:11 AM, David Mitchell<davidmitch...@att.net> wrote: >> I believe that optimization is happening behind the scenes. See the >> following >> tests actually timing indexed assignment, first without replacing the >> original >> array and then replacing the original array: >> >> ts=: 6!:2 , 7!:2@] >> a=:i.1e6 >> ts'_777 (2 9 43556)}a' >> 0.0238083 8.39078e6 >> ts'a=:_777 (2 9 43556)}a' >> 1.00571e_5 2432 >> >> On 1/6/2012 9:54, Blake McBride wrote: >>> Greetings, >>> >>> Going through J I found that: >>> >>> x=:1+i.5 >>> x >>> 1 2 3 4 5 >>> 44 (2}) x >>> 1 2 44 4 5 >>> x >>> 1 2 3 4 5 >>> >>> It seems that J doesn't actually make an indexed assignment. It seems >>> more like it creates a copy of the array with the specified >>> modifications. This is actually quite shocking. Index assignment to >>> a very large array in most languages is a very fast and simple >>> operation. It is also very memory efficient. Creating a modified >>> copy of a very large array has a much, much greater time and space >>> cost. Further, multiplying this (unnecessary) cost over a great >>> number of operations can drastically (and unnecessarily) affect >>> performance negatively. >>> >>> Am I missing something here? >>> >>> Thanks. >>> >>> Blake McBride >>> ---------------------------------------------------------------------- >>> For information about J forums see http://www.jsoftware.com/forums.htm >>> >> ---------------------------------------------------------------------- >> For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
