Unable to load armadillo Cube when using RcppArmadillo

Question

I was prepossessing data in C++ using the Armadillo library. The program end product is a ucube, which is a cube filled with unsigned integers. After its run, I want to load the ucube to R to perform some final statistical tests. To do so, I made a C++ function that load the ucube returning an array.

But it does not work!
I got the following warning: "warning: Cube::load(): incorrect header in B.bin" and the program returns a 0x0x0 array.

Trying to find why, I made a toy C++ program, which works fine. It is able to load the cubes without any problem.

#include <iostream>
#include <armadillo>
using namespace arma;

void read_cubes(char const* A, char const* B){

    cube C;
    ucube D;

    C.load(A, arma_binary);
    D.load(B, arma_binary);

}   

int main(int argc, char** argv){

    cube A = randu<cube>(5,5,5);
    ucube B = randi<ucube>(5,5,5, distr_param(1, 10));

    A.save(argv[1], arma_binary);
    B.save(argv[2], arma_binary);

    read_cubes(argv[1], argv[2]);

}

But I do not know why, doing the same steps in R does not work. To illustrate, please run the toy program as ./a.out A.bin B.bin. It will yield the Cube<double> A.bin and the Cube<uword> B.bin, which I will mention later.

The problem
If I source the following C++ code with Rcpp::sourceCpp and I try to read the Cube<double> A.bin with read_cube("A.bin") it works, but if I do the same for the Cube<uword> B.bin with read_ucube("B.bin") it does not (I get the warning).

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

// [[Rcpp::export]]
arma::cube read_cube(char const* x){
    arma::cube A;
    A.load(x, arma::arma_binary);
    return A;
}

// [[Rcpp::export]]
arma::ucube read_ucube(char const* x){
    arma::ucube B;
    B.load(x, arma::arma_binary);
    return B;
}

Of course I could cast the Cube<uword> to a Cube<double> before ending the C++ program, but I would like to know why this happen and if it is possible to load a Cube<uword> in RcppArmadillo. Because it should be possible, right?

R has no unsigned types, and no short words. We have `integer` for (signed) 32bit ints, and `double` at 64 bit. — Dirk Eddelbuettel, Sep 05 '19 at 10:32

score 2 · Accepted Answer · answered Sep 05 '19 at 14:30

Unfortunately R still only supports 32 bit integers, so RcppArmadillo forces Armadillo to use 32 bit integers. This is done by defining ARMA_32BIT_WORD before including the armadillo header. See RcppArmadillo's configuration here.

You can apply the same "trick" with your Armadillo programs like so:

#define ARMA_32BIT_WORD
#include <armadillo>

One of the effects is that ucube (Cube<uword>) will use 32 bit unsigned integers.

After doing the above trick, recompile your Armadillo programs and save the ucubes again. They can then be loaded in RcppArmadillo.

Unable to load armadillo Cube when using RcppArmadillo

1 Answers1