[Rcpp-devel] Beginner Question: Two misc. questions on usage of Rcpp

[Sunny Srivastava]

Dear Rcpp-List:

I have been working on speeding my R code. A while ago, I realized that no
further vectorization would help and interacting R with C/C++/.. was
absolutely necessary. I don't program in C/C++ extensively, so the
transition was a difficult process. I tried to use the R API but the error
messages were intimidating. A few weeks ago I saw Professor Chambers
presentation (pdf from David Smith's website) about R in Stanford. I saw him
mentioning Rcpp which motivated me to try Rcpp (I confess, I am lazy to try
new stuff). And I haven't been dissapointed. Thank you very much Dirk and
Romain (and other developers, including Professor Chambers and Professor
Bates).

Rcpp reduced the length of my code considerably (like: I didn't need to
bother about PROTECT .. ) and the informative syntax (like:
Rcpp::IntegerMatrix ..) made my code more readable. These are great
advantages, but the best part was testing and compiling with the Inline
package. I am a statistician; writing a C program and then using dyn.load
was the way I learned to do it in grad school. Rcpp + inline makes this
process way easier.

Thank you for the great documentation on your webpage Dirk. It helped me a
lot.

I am still a beginner in using Rcpp and friends (2 weeks old) and have a few
miscellaneous questions. The answer to these questions may be very basic or
obvious, so please bear with me. (Dirk showed me a simple way to test things
using Rcpp + inline here --
http://stats.stackexchange.com/questions/6690/what-is-the-difference-between-rf-dpois-in-rmath-h-and-the-dpois-that-i-use-direc
)

1. I have been using Rcpp + inline to test if the faster implementation
gives the right results (which is obtained from R code). I was wondering if
there are other strategies which people use to test their Rcpp code? (sorry,
if this doesn't make sense.)

2. I have a MCMC simulation (say: foo) which uses several functions (say:
fun1, fun2). These functions (fun1, fun2) are themselves computation
intensive and I used Rcpp to speed them. Until now I do something like this:

In R file:

foo <- function (...){

    src1 <- paste(readLines("fun1-rcpp.cpp"), collapse="\n")
    RcppFun1 <- cxxfunction( .., plugin="Rcpp",body=src1)

   ## similar cpp code for fun2

    for (i in 1:10000){
          ## use RcppFun1, RcppFun2 here
    }
}

I was wondering if this is OK (it already speeds up my code by zillion
times) or is there a more elegant way of doing this. I am thinking about
using Rcpp to increase the speed of foo (for more speed) but I am not too
sure on how to do this. Towards this end, I checked a few packages mentioned
on Dirk's website namely, pcaMethods. I am thinking RcppExport and .Call is
the way to do this?

Thank you for your answers or pointers.

S.
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