Using a c++ function in an other c++ function (inside a R Package with Rcpp)

Question

I try to create a R package (using Rcpp). Everything works fine. But now I wrote a c++ function, which I want to call in an other c++ file. So in /src there is:

• function1 = linearInterpolation.cpp
• function2 = getSlope.cpp

Let's take function1, which just do a linear interpolation between two points at a specific position.

#include <Rcpp.h>

using namespace Rcpp;

//' @name linearInterpolation
//' @title linearInterpolation
//' @description Twodimensional linearinterpolation for a specific point
//' @param xCoordinates Two x coordinates
//' @param yCoordinates Coresponding two y coordinates
//' @param atPosition The Point to which the interpolation shall be done
//' @return Returns the linear interpolated y-value for the specific point
//' @examples
//' linearInterpolation(c(1,2),c(1,4),3)
//'
//' @export
// [[Rcpp::export]]
double linearInterpolation(NumericVector xCoordinates, NumericVector yCoordinates, double atPosition) {

  // start + delta y / delta x_1 * delta x_2
  return yCoordinates[1] + getSlope(xCoordinates, yCoordinates) * (atPosition - xCoordinates[1]);

}

and the slope is calculated in a different function (file).

#include <Rcpp.h>

using namespace Rcpp;

//' @name getSlope
//' @title getSlope
//' @description Calculates the slopes between two points in 2Dimensions
//' @param xCoordinates Two x coordinates
//' @param yCoordinates Coresponding two y coordinates
//' @return Returns the slope
//' @examples
//' getSlope(c(1,2),c(1,4),3)
//'
//' @export
// [[Rcpp::export]]
double getSlope(NumericVector xCoordinates, NumericVector yCoordinates) {
  return (yCoordinates[1] - yCoordinates[0]) / (xCoordinates[1] - xCoordinates[0]);
}

I do not have any deeper knowledge in Rcpp or c++. I read the Vignette and Writing a package that uses Rcpp I think I also read the right parts but I didn't get it.

Why is the getSlope function not "visible" in the other function - as they are both in the same package. How can I use the getSlope in the other file?

Sorry but I am really stuck.

Thanks and best Regards

Nico

Answer 1

Perhaps you should make another file, a header file .hpp, and put this in it:

#include <Rcpp.h>
using namespace Rcpp;
double getSlope(NumericVector xCoordinates, NumericVector yCoordinates);

or, better still,

#include <Rcpp.h>
double getSlope(Rcpp:: NumericVector xCoordinates, Rcpp:: NumericVector yCoordinates);

and put #include"myheader.hpp" at the top of both of your cpp files. This is in order to declare this function such that both cpp files can see it.

---

... as they are both in the same package

Just because two things are in the same R package, that does not mean they are in the same C++ translation unit (i.e. cpp file), and the translation unit is the important thing for two C++ functions to see each other. They must be in the same cpp file, or you must use a header file.