How to call a R function from C++ with passing the parameters

Question

I am trying to call my R function from C++ program.

rtest = function(input ,output) {
  a <- input
  b <- output 
  outpath <- a+b
  print(a+b)
  return(outpath)
}

This is my R function. I need to find a way for calling this function from C with passing arguments. calling a R function from python code with passing arguments. Here i have made similar method for calling R from python. So i need to specify the path to R script and the name of the function, and also able to pass the arguments through python. I am looking for similar way in C. But didn't get results. This may be a simple thing. Any help is appreciated.

Answer 1

This question has multiple interpretations, which is why I did not attempt an answer before. Here Solutions for several possible interpretations:

C++ function defined with Rcpp that is called from R and uses a user defined R function. This follows http://gallery.rcpp.org/articles/r-function-from-c++/:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector callFunction(NumericVector x, NumericVector y, Function f) {
  NumericVector res = f(x, y);
  return res;
}

/*** R
set.seed(42)
x <- rnorm(1e5)
y <- rnorm(1e5)

rtest <- function(x, y) {
  x + y
}

head(callFunction(x, y, rtest))
head(x + y)
*/

The R function rtest is defined in R and passed to the C++ function callFunction together with it's two arguments. Partial result from Rcpp::sourceCpp():

> head(callFunction(x, y, rtest))
[1]  0.95642325 -0.57197358 -1.45084989 -0.18220091  0.07592864  0.56367202

> head(rtest(x, y))
[1]  0.95642325 -0.57197358 -1.45084989 -0.18220091  0.07592864  0.56367202

Calling the function in R and via C++ gives the same result.

C++ program using RInside which calls an user defined R function on data present in C++. Here we have two possibilities: either transfer the data to R and call the function there or move the function to C++ and call the R function in C++ like above:

#include <RInside.h>

int main(int argc, char *argv[]) {
    // define two vectors in C++
    std::vector<double> x({1.23, 2.34, 3.45});
    std::vector<double> y({2.34, 3.45, 1.23});
    // start R
    RInside R(argc, argv);
    // define a function in R
    R.parseEvalQ("rtest <- function(x, y) {x + y}");
    // transfer the vectors to R
    R["x"] = x;
    R["y"] = y;
    // call the function in R and return the result
    std::vector<double> z = R.parseEval("rtest(x, y)");
    std::cout << z[0] << std::endl;

    // move R function to C++
    Rcpp::Function rtest((SEXP) R.parseEval("rtest"));
    // call the R function from C++
    z = Rcpp::as<std::vector<double> >(rtest(x, y));
    std::cout << z[0] << std::endl;
    exit(0);
}

In order to compile this, I am using the GNUmakefile that comes with the examples in RInside. Result:

$ make -k run
ccache g++ -I/usr/share/R/include -I/usr/local/lib/R/site-library/Rcpp/include -I/usr/local/lib/R/site-library/RInside/include -g -O2 -fdebug-prefix-map=/home/jranke/git/r-backports/stretch/r-base-3.5.0=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g -Wno-ignored-attributes -Wall    call_function.cpp  -Wl,--export-dynamic -fopenmp -Wl,-z,relro -L/usr/lib/R/lib -lR -lpcre -llzma -lbz2 -lz -lrt -ldl -lm -licuuc -licui18n  -lblas -llapack  -L/usr/local/lib/R/site-library/RInside/lib -lRInside -Wl,-rpath,/usr/local/lib/R/site-library/RInside/lib -o call_function

Running call_function:
3.57
3.57