rep(x, each=3) equivalent in armadillo - c++

Question

I'm porting a R function to c++ for use in RcppArmadillo, and I cannot find an elegant (efficient) way to repeat a column vector N times, element-by-element. Here's a minimal example, where I had to first create a matrix with 3 columns repeated, then reshape to a row vector, then transpose.
library(RcppArmadillo)
sourceCpp(code = '
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
arma::colvec foo(const arma::colvec& u, const arma::colvec& v)
{
arma::colvec u_rep(12), result(12);
u_rep = trans(vectorise(repmat(u, 1, 3), 1)); // this seems inefficient
result = u_rep % v;
return(result);
}'
)
foo(1:4, 1:12)
The R equivalent would be,
fooR = function(u, v){
u_rep = rep(u, each=3)
u_rep * v
}

Answer 1

There is no known C++ operator or function that does this, so you may well have to do it by hand.
Worst case you just loop and copy (possibly in chunks). Armadillo does have indexing, so maybe that will help. R does a lot of checking when recycling so you probably have to account for that too.
By the way, you example mixes Attributes and inline. I'd just put the code
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;
// [[Rcpp::export]]
arma::colvec foo(const arma::colvec& u, const arma::colvec& v) {
arma::colvec u_rep(12), result(12);
u_rep = trans(vectorise(repmat(u, 1, 3), 1)); // this seems inefficient
result = u_rep % v;
return(result);
}
in a file bafoo.cpp and source it as follows:
R> sourceCpp("/tmp/bafoo.cpp")
R> foo(1:4, 1:12)
[,1]
[1,] 1
[2,] 2
[3,] 3
[4,] 8
[5,] 10
[6,] 12
[7,] 21
[8,] 24
[9,] 27
[10,] 40
[11,] 44
[12,] 48
R>