So you are saying that J treats the following code 1 with a copy, and code 2 without a copy?
1. y=:44(2})x 2. x=:44(2})x What about the following code: x=:55(3})44(2})x or: x=:55(3})(some verb returning a scalar)44(2})x Thanks for your response. It is very helpful! Blake McBride On Fri, Jan 6, 2012 at 9:51 AM, Roger Hui <rogerhui.can...@gmail.com> wrote: > David Mitchell already answered your question. > > x i}y is not indexed assign. (Where's the assign?) y=:x i}y *is *indexed > assign and is lean and fast. > > > > On Fri, Jan 6, 2012 at 7:42 AM, Blake McBride <bl...@mcbride.name> 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. >> > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
