[Rd] C function, double problem

Dirk Eddelbuettel edd at debian.org
Sat Dec 8 16:58:35 CET 2012

That is great example of why folks should NOT use the .C() interface.

Rcpp can help here, __even if you do not use any Rcpp language features__.
Below is a showcase for the recent sourceCpp() function which takes a C++
file (shown below), converts it and optionally runs the embedded R
code. Think of as something like Sweave for mixing C++ and R rather than
Latex and R.

Source file is shown via cat on the cmdline:

edd at max:/tmp$ cat dblvec.cpp 

#include <Rcpp.h>

#include <vector>

// [[Rcpp::export]]
std::vector<double> dbl(std::vector<double> x) {
  std::vector<double> y(x.size());
  for (unsigned int i=0; i<y.size(); i++)
    y[i] = 2.0*x[i];
  return y;

/*** R
     myx <- seq(1.1, 5.5, by=1.1)

edd at max:/tmp$
edd at max:/tmp$

We then simply source it to show off how it transform a C++ vector of doubles:

edd at max:/tmp$ R --quiet -e 'library(Rcpp); sourceCpp("dblvec.cpp")'
R> library(Rcpp); sourceCpp("dblvec.cpp")

R> myx <- seq(1.1, 5.5, by = 1.1)

R> print(dbl(myx))
[1]  2.2  4.4  6.6  8.8 11.0
edd at max:/tmp$ 

No make, compiler invocation, linking, ... which all happen behind the
scenes.  We just enjoy the C++ function to double the content of a double

Hope this helps,  Dirk

Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com

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