Rcpp sugar commands in armadillo

Question

I'm trying to use ifelse() command of Rcpp sugar with arma::vec. The code fails with error

'ifelse' was not declared in this scope

I could not find a solution. A simple example code (resulted with error) is below.

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

// [[Rcpp::export]]
arma::vec f(arma::vec x, arma::vec y) {
  arma::vec res1 = Rcpp::ifelse(x < y, x, y);
  arma::vec res = trans(res1)*y;
  return res;
}

/*** R
f(c(1,2,3),c(3,2,1))
*/

Sleigh

You need to use Rcpp::ifelse, but this won't work with arma::vec. – Flagpole

@F. Privé I edited the code according to your suggestion. Is there a way to use armadillo (for matrix computations) and rcpp sugar together? – Sleigh

I don't think so. Rcpp sugar is for Rcpp types. – Flagpole

C++ is a statically-typed object-oriented language. Something that works on one type (ie Rcpp::NumericVector) does not necessarily work on another (ie arma::vec). – Druse

Thanks. I just wonder if there is some commands to coerce the types like as.double or as.data.frame in R. For example as.Numeric.Vector here. But it seems there is not. – Sleigh

Answer 1

Using Armadillo's advanced constructors you can have Rcpp::NumericVector and arma::vec that refer to the same memory location. Then you can use both Rcpp functions and arma functions by using the correct front-end object for that piece of memory:

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

// [[Rcpp::export]]
arma::vec f(Rcpp::NumericVector xr, Rcpp::NumericVector yr) {
  arma::vec x(xr.begin(), xr.size(), false, true); 
  arma::vec y(yr.begin(), yr.size(), false, true);
  Rcpp::NumericVector res1r(xr.size());
  arma::vec res1(res1r.begin(), res1r.size(), false, true);  
  res1r = Rcpp::ifelse(xr < yr, xr, yr);
  arma::vec res = trans(res1)*y;
  return res;
}

/*** R
f(c(1,2,3),c(3,2,1))
*/

I am not 100% sure that this does not have any unwanted side-effects.

Tawannatawdry

Answer 2

This is the solution that I found I hope will work for you.

#include <RcppArmadillo.h>

// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]

arma::vec f(arma::vec x, arma::vec y) {
  int n = x.size();
  arma::vec res(n); 
    for(int i = 0; i < n; i++){
      if (x[i] < y[i]){res[i] = x[i];} else{res[i] = y[i];}
  }
  return trans(res)*y;
}

The output is

/*** R
f(c(1,2,3),c(3,2,1))
*/
     [,1]
[1,]    8

Calabro

Please do NOT include RcppArmadillo.h and Rcpp.h. The former pulls in the latter. You only need one, and order matters. And what you show is redundant as well. – Druse

Making the corrections that Dirk proposes, edit my answer. – Sacker

@Rafael Diaz thanks but your answer is not about my question. – Sleigh