add-rcpp-integration

Add Rcpp Integration

Integrate C++ code into an R package using Rcpp for performance-critical operations.

When to Use

• R function is too slow and profiling confirms a bottleneck
• Need to interface with existing C/C++ libraries
• Implementing algorithms that benefit from compiled code (loops, recursion)
• Adding RcppArmadillo for linear algebra operations

Inputs

• Required: Existing R package
• Required: R function to replace or augment with C++
• Optional: External C++ library to interface with
• Optional: Whether to use RcppArmadillo (default: plain Rcpp)

Procedure

Step 1: Set Up Rcpp Infrastructure

usethis::use_rcpp()

This:

• Creates src/ directory
• Adds Rcpp to LinkingTo and Imports in DESCRIPTION
• Creates R/packagename-package.R with @useDynLib and @importFrom Rcpp sourceCpp
• Updates .gitignore for compiled files

For RcppArmadillo:

usethis::use_rcpp_armadillo()

Got: src/ directory created, DESCRIPTION updated with Rcpp in LinkingTo and Imports, and R/packagename-package.R contains @useDynLib directive.

If fail: If usethis::use_rcpp() fails, manually create src/, add LinkingTo: Rcpp and Imports: Rcpp to DESCRIPTION, and add #' @useDynLib packagename, .registration = TRUE and #' @importFrom Rcpp sourceCpp to the package-level documentation file.

Step 2: Write C++ Function

Create src/my_function.cpp:

#include <Rcpp.h>
using namespace Rcpp;

//' Compute cumulative sum efficiently
//'
//' @param x A numeric vector
//' @return A numeric vector of cumulative sums
//' @export
// [[Rcpp::export]]
NumericVector cumsum_cpp(NumericVector x) {
  int n = x.size();
  NumericVector out(n);
  out[0] = x[0];
  for (int i = 1; i < n; i++) {
    out[i] = out[i - 1] + x[i];
  }
  return out;
}

For RcppArmadillo:

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

//' Matrix multiplication using Armadillo
//'
//' @param A A numeric matrix
//' @param B A numeric matrix
//' @return The matrix product A * B
//' @export
// [[Rcpp::export]]
arma::mat mat_mult(const arma::mat& A, const arma::mat& B) {
  return A * B;
}