Fail to call ones or eye function when using RcppArmadillo - rcpp

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
>