Column-wise ordering of matrix - c++

Question

I am new to RcppArmadillo. I am wondering how I can make a column-wise ordered matrix by the index of given vector. I know how to do it in R, but in RcppArmadillo it does not working. For example, in R,
aa = c(2,4,1,3)
# [1] 2 4 1 3
bb = cbind(c(1,5,4,2),c(3,1,0,8))
# [,1] [,2]
# [1,] 1 3
# [2,] 5 1
# [3,] 4 0
# [4,] 2 8
Trying to subset with R gives:
cc = bb[aa,]
# [,1] [,2]
# [1,] 5 1
# [2,] 2 8
# [3,] 1 3
# [4,] 4 0
I've tried the following using RcppArmadillo:
#include <RcppArmadillo.h>
using namespace Rcpp;
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
List example(arma::vec aa,arma::mat bb){
int p = bb.n_rows;
int n = aa.size();
arma::uvec index_aa=sort_index(aa);;
List cc(n);
for(int it=0; it<p; it++){
cc(it) = bb.each_col();
}
return List::create(cc);
}
and,
#include <RcppArmadillo.h>
using namespace Rcpp;
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
List example(arma::vec aa,arma::mat bb){
arma::uvec index_aa=sort_index(aa);
return List::create(bb.elem(index_aa));
}

Answer 1

Not sure why you are sorting the index here as that causes a new order to be introduced compared to bb[aa,].
Anyway, the idea here is to subset using the .rows() index, which requires a uvec or unsigned integer vector. As aa contains R indexes, we can translate them from R to C++ by subtracting 1 to take it from a 1-based index system to a 0-based index system.
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
arma::mat example_subset(arma::uvec aa, arma::mat bb){
// Convert to a C++ index from R (1 to 0-based indices)
aa = aa - 1;
return bb.rows(aa);
}
Test code:
aa = c(2, 4, 1, 3)
bb = cbind(c(1, 5, 4, 2), c(3, 1, 0, 8))
cpp_cc = example_subset(aa, bb)
r_cc = cbind(c(5,2,1,4),c(1,8,3,0))
all.equal(cpp_cc, r_cc)
# [1] TRUE