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

Wed, Aug 3, 2016 1:09 PM

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

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