For more complex functions, or even whole sets of functions, it’s fairly clear that the string format is not going to be extremely useful. As an alternative, we can either place the code into a .cpp file, or into an Rcpp chunk in an R markdown file, with only marginally more boilerplate code. The following code is an example of how we might do this.

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector convolve_cpp(const NumericVector& a,
                           const NumericVector& b) {

  // Declare loop counters, vector sizes, and total length
  int i = 0;
  int j = 0;
  int na = a.size();
  int nb = b.size();
  int nab = na + nb;

  // Create vector filled with 0
  NumericVector ab(nab);

  // Crux of the algorithm
  for(i = 0; i < na; i++) {
    for(j = 0; j < nb; j++) {
      ab[i + j] += a[i] * b[j];
    }
  }

  // Return result
  return ab;
}

If we were to place this code in a separate convolve.cpp file to be used in our R script, we would need to call the following to make it available to our session.

Rcpp::sourceCpp("convolve.cpp")

This is not necessary if you simply place the code into an Rmd chunk with a Rcpp specifier.