Fail to call ones or eye function when using RcppArmadillo

Question

I want to use one or eye function in Armadillo to construct matrices or vectors. However, it does not allow me to do so. Here is a sample code:

#include <RcppArmadillo.h>

// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
// [[Rcpp::export]]
SEXP Generate(arma::mat Mat){
arma::mat Mat_2 = ones<mat>(5,6);
}

error message reminds me of use of undeclared idenfier of 'mat'. When I remove <mat>, another massage says use of undeclared idenfier of 'ones'.

I looked up the Armadillo tutorial which includes the ones function. I wonder why my code fails to call it. Did I miss something?

Answer 1

There are a few problems in your code:

• Why return SEXP? Use types to your advantage
• Why pass Mat in if you do not use it?
• No return statement
• Somewhat loose use of namespaces.

A cleaned up version follows:

#include <RcppArmadillo.h>

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

// [[Rcpp::export]]
arma::mat Generate(int n=5, int k=6){
    arma::mat m = arma::ones<arma::mat>(n,k);
    return m;
}

/*** R
Generate()
*/

It compiles and runs fine:

> Rcpp::sourceCpp("~/git/stackoverflow/67006975/answer.cpp")

> Generate()
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    1    1    1    1    1
[2,]    1    1    1    1    1    1
[3,]    1    1    1    1    1    1
[4,]    1    1    1    1    1    1
[5,]    1    1    1    1    1    1
>