[Rcpp-devel] How to extract rows or columns by vector of indices

[Andy Leung]

Dear Rcpp users and developers,

I understand how to extract a single column or row from an
RcppArmadillo matrix, but I am still unsure how to extract columns or
rows based on a vector of indices. I wrote a rather awkward (and dumb)
way to extract, and here are some reproducible codes:

###############################
library(Rcpp)
library(RcppArmadillo)
library(inline)
src <-
'       arma::mat x = arma::randu(5, 5);
       arma::mat y = x;
       arma::uvec indice;
       indice << 0 << 3;
       int nRm = indice.n_elem;
       for(int j = 0; j < nRm; j++)
               y.shed_col( indice(j) - j );
       return List::create(Named("x")=x, Named("y")=y, Named("ind")=indice);'
colX <- cxxfunction(body=src,plugin="RcppArmadillo")
colX()
###############################

Although this code works, I am wondering if there are other more
clever ways of approaching this, especially a way to avoid any 'for'
loops.

Thank you for your help!
Andy Leung