An error with a three dimensional field (of vectors) in rcpp armadillo

Question

I am trying to create a field in rcpp arma due to more than three iterations, but got an error. As an example, see the following simple code:

//[[Rcpp::export]]
field<vec> testgg(int k, int h, int g){
  field<vec> res(k, h, g);
  return(res);
}

I do nothing in this code, so this code should give me something. However, I got an error when I do this function like this.

> testgg(3,4,5)
Error in testgg(3, 4, 5) : 
  dims [product 12] do not match the length of object [60]
> testgg(3,4,1)
     [,1]      [,2]      [,3]      [,4]     
[1,] numeric,0 numeric,0 numeric,0 numeric,0
[2,] numeric,0 numeric,0 numeric,0 numeric,0
[3,] numeric,0 numeric,0 numeric,0 numeric,0

My guess is that the rcpp does not get this three dimensional field, but can get the field up to 2 dimensions. Why does this happen and how can I get a three dimensional field?

Answer 1

Please see the open issue #263 and maybe also other ones. There was a bug, there is a fix, but you currently need to set a #define to enable the fix (as we need to sort out if this has side-effects on other packages assuming the current behaviour).

So in short do for example

> Sys.setenv(PKG_CPPFLAGS="-DRCPP_ARMADILLO_FIX_Field")  ## sets #define RCPP_ARMADILLO_FIX_Field
> Rcpp::cppFunction("arma::field<arma::vec> testgg(int k, int h, int g) { arma::field<arma::vec> res(k, h, g); return(res);}", depends="RcppArmadillo")
> testgg(2,3,4)
, , 1

     [,1]      [,2]      [,3]     
[1,] numeric,0 numeric,0 numeric,0
[2,] numeric,0 numeric,0 numeric,0

, , 2

     [,1]      [,2]      [,3]     
[1,] numeric,0 numeric,0 numeric,0
[2,] numeric,0 numeric,0 numeric,0

, , 3

     [,1]      [,2]      [,3]     
[1,] numeric,0 numeric,0 numeric,0
[2,] numeric,0 numeric,0 numeric,0

, , 4

     [,1]      [,2]      [,3]     
[1,] numeric,0 numeric,0 numeric,0
[2,] numeric,0 numeric,0 numeric,0

> 

So if you compile with the correct flag, the field dimension is not reduced down.

Edit And maybe you wanted cube instead of field ?

> Rcpp::cppFunction("arma::cube testhh(int k, int h, int g) { arma::cube res(k, h, g); return(res);}", depends="RcppArmadillo")
> testhh(4,3,2)
, , 1

     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0
[4,]    0    0    0

, , 2

     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0
[4,]    0    0    0

>