Er... actually you only had one other question to be answered. Oops. Thanks,
-- Raul On Mon, Mar 3, 2014 at 2:13 PM, Raul Miller <rauldmil...@gmail.com> wrote: > Answering your questions in order: > 2&%~ > is equivalent to > (2&%)~ > > If you want a more detailed explanation of why this is, don't hesitate > to ask, but you have a number of other questions to be answered... > > As for if statements, J actually has them > (http://www.jsoftware.com/help/dictionary/cif.htm) but they only work > in explicit definitions, they cannot be a part of sentence. And, like > Michael Dykman points out, agenda is probably what you are looking for > (http://www.jsoftware.com/help/dictionary/d621.htm). > > But there's another way: you can multiple one of these expressions by > zero, and another by 1 and then add them together. > even=: %&2 > odd=: 1 + *&3 > isOdd=: 2&| > ((even,:odd) +/@(* isOdd) 1 0 +/ ]) i. 5 > 0 4 1 10 2 > > There's other ways of expressing that, of course. For example: > isEven=: isOdd@+&1 > ((even * isEven) + (odd * isOdd)) i. 5 > 0 4 1 10 2 > > Note that the parenthesis around odd * isOdd in that last expression > was unnecessary. > > Still, agenda also works, and some people prefer it. > > Note also that at the machine code level, the multiply and add > approach would typically give better performance than the test and > branch approach. This has to do with the architecture of modern > machines (branch prediction is expensive). > > Thanks, > > -- > Raul > > On Mon, Mar 3, 2014 at 11:30 AM, Jon Hough <jgho...@outlook.com> wrote: >> Beginner question again.I quick task I set myself was to write ONE ITERATION >> of the Collatz function for a given positive >> integer.http://en.wikipedia.org/wiki/Collatz_conjecture >> >> So my verb is supposed to do 3*n+1 if n is odd and n/2 if n is even.In a >> more imperative/OOE based language (C/C++/Java) I could write this in less >> than a minute. Unfortunately, I fell at the very first hurdle in J. >> I originally wrote my tacit verb for even ints:collatz_even =.2&(%~) >> collatz_even 4 >> 2 >> This works, but I had a terrible time trying to put the brackets in the >> right place. I am not sure why %~ needs to be bracketed. Won't J parse %~ as >> dyadic and "know" that the left operand is 2? >> Next I tried to do the case for odd n: >> collatz_odd =. 1&+@(3&*) collatz_odd 3 >> 10 >> That seems to work ok.Now I am not sure how to do an if statement in J. In >> plain English I want "If n is even do collatz_even else do collatz_odd".The >> verb I wrote to test for even-ness is >> ones =. {:@#: NB. Finds the ones column values. 1 => odd, 0=> even >> 1 = ones 2 >> 0 >> So I have a test but I am not sure how to utilize this test. How should I go >> about doing:"If n is even do collatz_even else do collatz_odd"? >> Thanks and regards,Jon >> >> ---------------------------------------------------------------------- >> For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
