a. I believe only a very few APL interpreters (if any) would avoid making a copy of ,y, if you do x⍴y (x rho y).
b. To get a single number from comparing 3 3⍴⍳2 3 4 against 3 3⍴,⍳2 3 4, you need to use ≡ (match) instead of =. c. A key idea to remember, is that in J there is no look ahead in the parser. So the difference between x$,y and x($,)y, is that for x$,y the parser would do ,y, then do the x$ of that. When it does the ,y, it doesn't "know" that the next thing it's going to do is $ . In contrast, for x($,)y, it first derives a function for the hook ($,) and in such derivation "knows" that for the dyad you are doing a reshape of a ravel. d. > I don't think I can memorize everything I will need. Don't try to memorize the list. Just remember two things: - There is no look ahead (except for a couple of important cases involving =:). - There is special code for anything worthwhile that you can think of. If you find any deviation from the above, report the deviations to the J Forum. > There is a special case in there x (n|@^)y that > would have made the solution to the problem instant and trivial, without the > @ it takes more than an hour - and that is how I did it, sigh. I submit that for this example, if you keep the "two things" in mind, you coulda/shoulda/woulda have found the special code for x (n|@^) y . On Mon, Oct 17, 2011 at 9:50 AM, Nick Simicich <[email protected]> wrote: > ts '1 1 1 +./@E. (1 1 1 ,100000$0)' > 8.1599503875016434e_5 263936 > ts '+./3*./\ (1 1 1 ,100000$0)' > 0.00061567625668835938 395072 > ts '+./ 1 1 1 E. (1 1 1 ,100000$0)' > 0.0003145580874868281 394880 > > So the phrase +./@E. must have an early out for a single matched pattern. > But 3*./\ compares poorly to +./ 111 E. even without the early > > That is an enormous difference. > > I found the special code page, thanks to the above, I guess I need to study > it if I want to write excellent code. For one thing, had I read through > that page, there is one of the euler problems that would have been trivial > and short, it was something like computing the last few digits of a large > number to a large power. There is a special case in there x (n|@^)y that > would have made the solution to the problem instant and trivial, without the > @ it takes more than an hour - and that is how I did it, sigh. > > One thing that bothers me is the first thing in the list: > > In the old days of APL, one could reshape a structure directly (as an in > line result, of course, not as a side effect) with rho. I had noticed that > that didn't quite work here. 3 3 $ i. 2 3 4 is not the same as 3 3 $, i. 2 > 3 4. Once I learned that it didn't bother me. I had sort of presumed that > the difference between a vector and a matrix as an intermediate result might > have been nothing more than a flag saying, "this is a vector of L20' without > any actual rearranging. (I tried this in NARS2000, and it did not work as I > expected it to, but the malfunction is basic, I could not reduce the output > of (3 3 rho iota 2 3 4)=(3 3 rho , iota 2 3 4) to a single number no matter > how many ravels and compresses I stacked up, it kept displaying 1 1 1. > NARS2000 is very ambitious, and is admitted to not be prime time. But the > two sides of the expression agreed in shape and numeric value. > > So I was wrong about J, and there is some rearranging, to the point where > ($,) skips the rearranging while $, does not, and this saves some work. > > ($,) is faster than $, but why would you not always use the "better" code? > Why not use the better code any time you see $,? > > Now I understand that most of these things are only special when they are > glued with @ or &. Why is this one special as a hook without glue? Is it > because $@, does not have the same semantics as $,? Is this the only phrase > that needs to be expressed as a hook to crank up the special code? > > I'm taken aback by the number of cases that have special case code - some of > the code is functional, like the code that doesn't calculate unused parts of > the result, and some just seems to be code that was put in to speed things > up. I don't think I can memorize everything I will need. > -- > Of course I can ride in the carpool lane, officer. Jesus is my constant > companion. > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
