使用 RcppArmadillo 将 `arma::cube` 参数传递给函数时出错

Question

I am getting the following error when trying to compile using sourceCpp from Rcpp package:

`my path to R/.../Rcpp/internal/Exporter.h`
no matching function for call to 'arma::Cube<double>::Cube(SEXPREC*&)'

The object cube is the armadillo equivalent of an array in R .

EDIT : Note that the problem seems to be that the function can't accept a arma::cube object as an argument. If we change arma::cube B by arma::mat B it does work:

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

using namespace arma;

//  [[Rcpp::export]]
arma::cube ssmooth(arma::mat  A, 
                   arma::cube B) {

int ns = A.n_rows;    
int nk = A.n_cols;    
int np = B.n_rows;    

arma::mat C = zeros<mat>(nk, ns);
arma::cube D = zeros<cube>(nk, nk, ns);

return D;

}

I would appreciate any hint.

Answer 1

A basic example works:

R> cppFunction("arma::cube getCube(int n) { arma::cube a(n,n,n);\
                    a.zeros(); return a; }", depends="RcppArmadillo")
R> getCube(2)
, , 1

     [,1] [,2]
[1,]    0    0
[2,]    0    0

, , 2

     [,1] [,2]
[1,]    0    0
[2,]    0    0

R> 

so either you are doing something wrong or your installation is off.

Answer 2

I had the same issue. The problem seems to be related to the combination "Rcpp::export" and cube as an argument of the exported function. My guess is that the converter from sexp to cube may not be implemented yet (no pun intended ;-)). (Or we are both missing something...).

Workaround when you want to have an arma::cube argument in a Rcpp::export function: get it first as a NumericVector and simply create the cube afterward...

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

using namespace arma;

//  [[Rcpp::export]]
arma::cube ssmooth(arma::mat  A, 
                   NumericVector B_) {
  IntegerVector dimB=B_.attr("dim");
  arma::cube B(B_.begin(), dimB[0], dimB[1], dimB[2]);

  //(rest of your code unchanged...)
  int ns = A.n_rows;    
  int nk = A.n_cols;    
  int np = B.n_rows;    

  arma::mat C = zeros<mat>(nk, ns);
  arma::cube D = zeros<cube>(nk, nk, ns);

  return D;

}

Answer 3

I think your code fails because implicitly it tries to do casting like this:

#include<RcppArmadillo.h>
using namespace Rcpp;
using namespace arma;
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
cube return_thing(SEXP thing1){
  cube thing2 = as<cube>(thing1);
  return thing2;
}


/***R
thing <- 1:8
dim(thing) <- c(2, 2, 2)
return_thing(thing)
*/

which doesn't work, whereas it works for matrices:

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

//[[Rcpp::export]]
mat return_thing(SEXP thing1){
  mat thing2 = as<mat>(thing1);
  return thing2;
}


/***R
thing <- 1:4
dim(thing) <- c(2, 2)
return_thing(thing)
*/

Answer 4

I am able to read and return an arma cube with the following function :

#include <RcppArmadillo.h>

using namespace Rcpp;
using namespace arma;

// [[Rcpp::export]]
arma::cube return_cube(arma::cube X)
{
  return(X);
}

For example, I obtain the following result when I run the following in R :

my_cube <- array(data = rnorm(5 * 3 * 2), dim = c(5,3, 2))
return_cube(my_cube)
, , 1

          [,1]       [,2]       [,3]
[1,] 0.4815994  1.0863765  0.3278728
[2,] 1.4138699 -0.7809922  0.8341867
[3,] 0.6555752 -0.2708001  0.7701501
[4,] 1.1447104 -1.4064894 -0.2653888
[5,] 1.5972670  1.8368235 -2.2814959

, , 2

             [,1]       [,2]       [,3]
[1,] -0.485091067  1.1826162 -0.3524851
[2,]  0.227652584  0.3005968 -0.6079604
[3,] -0.147653664  1.3463318 -1.2238623
[4,]  0.067090580 -0.8982740 -0.8903684
[5,]  0.006421618 -1.7156955 -1.2813880