Cpp11 unnamed list

I added a function called squared_unnamed_ in src/code.cpp that will square each element in a vector of doubles, so the file content corresponds to the following:

#include <cpp11.hpp>
#include <omp.h>

using namespace cpp11;

[[cpp11::register]] list squared_unnamed_(doubles x) {
    // create vectors y = x^2 and z = thread number
    int n = x.size();
    writable::doubles y(n);
    writable::doubles z(n);
    #pragma omp parallel for
    for (int i = 0; i < n; ++i) {
        y[i] = x[i] * x[i];
        z[i] = omp_get_thread_num();
    }

    //create a list containing y and z
    writable::list out;
    out.push_back(y);
    out.push_back(z);
    return out;
}

The previous function returns an unnamed list with two elements: the squared vector and the thread number. The function is registered with [[cpp11::register]] so that it can be called from R.

If I try to run square_unnamed_(1:10), it will return an error because I am passing a vector of integers instead of doubles. C++ is strict with types, so I need to create a wrapper function that will convert the integers to doubles, and it will go inside R/cpp11omp-package.R:

#' Unnamed list with squared numbers and the threads used
#' @param x A vector of doubles
#' @export
squared_unnamed <- function(x) {
  squared_unnamed_(as.double(x))
}

The previous function is exported with @export so that it can be called by the end user. The function squared_unnamed_ is an internal function. This approach also has the advantage that I can document the function in a flexible way.

Cpp11 named list

I added a function called squared_named_ in src/code.cpp that does the same but returns a named list. The additional content corresponds to the following:

[[cpp11::register]] list squared_named_(doubles x) {
    // create vectors y = x^2 and z = thread number
    int n = x.size();
    writable::doubles y(n);
    writable::doubles z(n);
    #pragma omp parallel for
    for (int i = 0; i < n; ++i) {
        y[i] = x[i] * x[i];
        z[i] = omp_get_thread_num();
    }

    //create a list containing y and z
    writable::list out;
    out.push_back({"x^2"_nm = y});
    out.push_back({"thread"_nm = z});
    return out;
}

As in the previous part, I added a wrapper and documentation:

#' Named list with squared numbers and the threads used
#' @param x A vector of doubles
#' @export
squared_named <- function(x) {
  squared_named_(as.double(x))
}

Makevars

In order to make the #pragma instruction work, I need to add the following to src/Makevars:

PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS)
PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS)
CXX_STD = CXX11

If I don’t do this, the pragma instruction will be ignored and the functions will run in a single thread.