making 3d array with arma::cube in Rcpp shows cube error - arrays

Question

I am making a Rcpp code for Gibbs sampling. Inside the code, I first want to make a 3 dimensional array with row number= number of iteration (500), column number=number of parameter(4) and slice number= number of chain(3). I wrote it in this way:
#include <RcppArmadillo.h>
#include <math.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace std;
using namespace arma;
//Gibbs sampling code starts here
Rcpp::List mcmc(const int iter,const int chains, const NumericVector data){
arma::cube posteriorC = arma::zeros(iter, 5, chains);
\\ rest of the codes
List out(Rcpp::List::create(Rcpp::Named("posteriorC") =posteriorC));
return out;
}
. While compelling it does not show any error. But when I want to run the code with:
res<- mcmc(iter=500,chains=2,data)
it shows the error:
Error: Cube::operator(): index out of bounds
. I want to know if there any mistake while making the 3D array. Please note that I want to get estimates of 5 parameters of my model.

Answer 1

You need to specify the template for arma::zeros to correctly fill an arma::cube, c.f. arma::zeros<template>
Generate a vector, matrix or cube with the elements set to zero
Usage:
vector_type v = zeros<vector_type>( n_elem )
matrix_type X = zeros<matrix_type>( n_rows, n_cols )
matrix_type Y = zeros<matrix_type>( size(X) )
cube_type Q = zeros<cube_type>( n_rows, n_cols, n_slices )
cube_type R = zeros<cube_type>( size(Q) )
Thus, in your case it would be:
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
Rcpp::List mcmc(const int iter, const int chains,
const Rcpp::NumericVector data){
arma::cube posteriorC = arma::zeros<arma::cube>(iter, 5, chains);
// --------------------------------- ^^^^^^^^
// Not Shown
Rcpp::List out = Rcpp::List::create(Rcpp::Named("posteriorC") =posteriorC);
return out;
}
Two final notes:
You explicitly state that the code as it stands now will create 4 columns to store 4 variables. However, you explicitly mention that you needed to estimate 5 parameters. You may need to increase this to prevent an out of bounds when saving into the arma::cube slices.
The way the Rcpp::List out is being created isn't quite correct. In general, the best way to create the list is to do: Rcpp::List out = Rcpp::List::create(Rcpp::Named("Blah"), Blah);