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
