[Rcpp-devel] Alternative way of calling R functions within C++

[George Vega Yon]

Dirk, good point. This is a very simple function and if I add complexity at
the end there's no significant difference between calling -fun-,
-cppFuncall- and -RcppFuncall-. Just to make this thread more complete, if
I modify -fun- from

fun <- function(x) {
  -cos(x[1])*cos(x[2])*exp(-((x[1] - pi)^2 + (x[2] - pi)^2))
}

to

fun <- function(x) {
  w<-sapply(1:1e3,function(x) -cos(x[1])*cos(x[2])*exp(-((x[1] - pi)^2 +
(x[2] - pi)^2)))
  1
}

Running the benchmark I get

Unit: relative
                expr min lq     mean median uq max neval
  cppFuncall(x, fun)   1  1 1.044215      1  1 6.1  1000
 RcppFuncall(x, fun)   1  1 1.018957      1  1 2.7  1000
              fun(x)   1  1 1.000000      1  1 1.0  1000

Which is way more reasonable from what I was getting at first. No
significant difference overall.

Kevin, thanks for the example, now I get why isn't a good idea! This has
been very useful to me.

Best,

George G. Vega Yon
+1 (626) 381 8171
http://www.its.caltech.edu/~gvegayon/

On Wed, Aug 3, 2016 at 12:34 PM, Kevin Ushey <kevinushey at gmail.com> wrote:

> The simplest demonstrating example I can think of:
>
> ---
>
> #include <Rcpp.h>
> using namespace Rcpp;
>
> struct A { ~A() { Rprintf("~A()"); } };
>
> // [[Rcpp::export]]
> void ouch() {
>   A a;
>   Rf_error("ouch!");
> }
>
> /*** R
> ouch()
> */
>
> ---
>
> Call 'Rcpp::sourceCpp()' on that and you'll see:
>
> > Rcpp::sourceCpp('~/Desktop/Untitled.cpp')
> > ouch()
>  Error in ouch() : ouch!
>
> Note that the destructor was not called. Replace `Rf_error` with
> `Rcpp::stop` and you will see the destructor is called.
>
> It's possible that you won't have a memory leak per-se (if the memory
> is all allocated on the stack, maybe the runtime still knows to just
> clear the entire stack after something like this) but not running
> destructors is definitely a big problem.
>
> Cheers,
> Kevin
>
> On Wed, Aug 3, 2016 at 12:22 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
> >
> > On 3 August 2016 at 11:38, George Vega Yon wrote:
> > | Thanks for the quick reply! What kind of errors are we talking about?
> I a new
> > | run I explicitly caused an error by passing a character vector, and
> had no
> > | memory leak (using Valgrind):
> > |
> > | cppFuncall(letters, fun)
> > | Error in cos(x[1]) : non-numeric argument to mathematical function
> > |
> > | If its not too much to ask, could you give an explicit example in
> which that
> > | happens (memory leak)? Just trying to learn here!
> >
> > You are misreading what Kevin said.  You short ten-line example runs
> fine.
> > We are not saying it has an error.
> >
> > What Kevin explained to you is that in the context of larger programs,
> > possibly with inputs you don't know yet, some errors may occur. And both
> > Rcpp:Function() and Rcpp::stop() can recover from that.
> >
> > Your example cannot.  So by all means use it as a small (local) script
> if the
> > few milliseconds matter to you.  But think twice about using it in a
> larger
> > context, or about promoting it as a general solution, and understand why
> we
> > can't put it into Rcpp as is.
> >
> > Hth, Dirk
> >
> > --
> > http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>
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