Convert arma:vec to NumericVector in Rcpp

Question

I try to use the quantile function in RcppArmadillo, however, in the tutorial, the output of quantile function is arma::vec (The class of output in R is "matrix" "array").

quantile function in arma

I want to convert it to NumericVector. I have tried with the codes following:

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;


// quantile function
// [[Rcpp::export]]
NumericVector quantile_f(vec V, vec P) {
  vec Q = quantile(V, P);
  NumericVector Q_output = as<NumericVector>(wrap(Q));
  return (Q_output);
}

I followed the link to do this. But I still get class in R = "matrix" "array".

Output of my function in R

Can anyone help with this? Thank!

Answer 1

(Please don't post images. Links to the documentation are preferred.)

You don't need to convert it:

Code

// force return as a vector rather than single column matrix
#define RCPP_ARMADILLO_RETURN_ANYVEC_AS_VECTOR

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

// [[Rcpp::export]]
arma::vec quantile_f(arma::vec V, arma::vec P) {
    return arma::quantile(V, P);
}

/*** R
set.seed(123)
quantile_f(rnorm(1000), c(0, 0.25, 0.5, 0.75, 1.0))
*/

Output

> Rcpp::sourceCpp("~/git/stackoverflow/73979000/answer.cpp")

> set.seed(123)

> quantile_f(rnorm(1000), c(0, 0.25, 0.5, 0.75, 1.0))
[1] -2.80977468 -0.62874241  0.00920964  0.66478787  3.24103993
> 

Internally, of course, the equivalent conversion happens as the Armadillo vectors becomes the C++ variant of an R vector which is our Rcpp::NumericVector. But the existing code is nice enough to automate this for you.

Edit One can also explicitly reset the dimension attribute from an n by one matrix to a vector of length one:

// [[Rcpp::export]]
Rcpp::NumericVector quantile_f2(arma::vec V, arma::vec P) {
    arma::vec v = arma::quantile(V, P);
    Rcpp::NumericVector w = Rcpp::NumericVector(Rcpp::wrap(v));
    w.attr("dim") = static_cast<int>(v.n_elem);
    return w;
}