At the risk of stating the obvious: instead of x =: 55(3})44(2})x
you can use x =: 55 44(3 2})x Note, ts =: 6!:2 , 7!:2@] x =: i.10000 ts 'x =: 55 44(3 2})x' 3e_6 2176 x 0 1 44 55 4 5 6 ... x =: i.10000 ts 'x =: 55(3})44(2})x' 2.1e_5 264576 x 0 1 44 55 4 5 6 ... You could use a similar rephrasing for your x=:55(3})(some verb returning a scalar)44(2})x example. Best, M On Fri, Jan 6, 2012 at 4:11 PM, Roger Hui <rogerhui.can...@gmail.com> wrote: > x=:44(2})x does not copy. All your other examples do. You can verify this > using the technique that David Mitchell used. > > > > On Fri, Jan 6, 2012 at 7:56 AM, Blake McBride <bl...@mcbride.name> wrote: > >> 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
