efficiently sample from modified arma::vec object - r

Question

I am using Rcpp to speed up some R code. However, I'm really struggling with types - since these are foreign in R. Here's a simplified version of what I'm trying to do:
#include <RcppArmadillo.h>
#include <algorithm>
//[[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;
// [[Rcpp::export]]
NumericVector fun(SEXP Pk, int k, int i, const vec& a, const mat& D) {
// this is dummy version of my actual function - with actual arguments.;
// I'm guessing SEXP is going to need to be replaced with something else when it's called from C++ not R.;
return D.col(i);
}
// [[Rcpp::export]]
NumericVector f(const arma::vec& assignment, char k, int B, const mat& D) {
uvec k_ind = find(assignment == k);
NumericVector output(assignment.size()); // for dummy output.
uvec::iterator k_itr = k_ind.begin();
for(; k_itr != k_ind.end(); ++k_itr) {
// this is R code, as I don't know the best way to do this in C++;
k_rep = sample(c(assignment[assignment != k], -1), size = B, replace = TRUE);
output = fun(k_rep, k, *k_itr, assignment, D);
// do something with output;
}
// compile result, ultimately return a List (after I figure out how to do that. For right now, I'll cheat and return the last output);
return output;
}
The part I'm struggling with is the random sampling of assignment. I know that sample has been implemented in Rarmadillo. However, I can see two approaches to this, and I'm not sure which is more efficient/doable.
Approach 1:
Make a table of theassignment values. Replace assignment == k with -1 and set its "count" equal to 1.
sample from the table "names" with probability proportional to the count.
Approach 2:
Copy the relevant subset of the assignment vector into a new vector with an extra spot for -1.
Sample from the copied vector with equal probabilities.
I want to say that approach 1 would be more efficient, except that assignment is currently of type arma::vec, and I'm not sure how to make the table from that - or how much of a cost there is to converting it to a more-compatible format. I think I could implement Approach 2, but I'm hoping to avoid the expensive copy.
Thanks for any insights you can provide.

Answer 1

many variable declaration is not coherent with the assignment made by you, like assignment = k is impossible to compare as assignment has real value and k is a char. as the queston is bad written I feel free to change the variables type. this compile..
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
#include <RcppArmadilloExtensions/sample.h>
// [[Rcpp::export]]
arma::vec fun(const Rcpp::NumericVector& Pk, int k, unsigned int i, const arma::ivec& a, const arma::mat& D)
{
return D.col(i);
}
// [[Rcpp::export]]
Rcpp::NumericMatrix f(const arma::ivec& assignment, int k, unsigned int B, const arma::mat& D)
{
arma::uvec k_ind = find(assignment == k);
arma::ivec KK = assignment(find(assignment != k));
//these 2 row are for KK = c(assignment[assignment != k], -1)
//I dont know what is this -1 is for, why -1 ? maybe you dont need it.
KK.insert_rows(KK.n_rows, 1);
KK(KK.n_rows - 1) = -1;
arma::uvec k_ind_not = find(assignment != k);
Rcpp::NumericVector k_rep(B);
arma::mat output(D.n_rows,k_ind.n_rows); // for dummy output.
for(unsigned int i =0; i < k_ind.n_rows ; i++)
{
k_rep = Rcpp::RcppArmadillo::sample(KK, B, true);
output(arma::span::all, i) = fun(k_rep, k, i, assignment, D);
// do something with output;
}
// compile result, ultimately return a List (after I figure out how to do that. For right now, I'll cheat and return the last output);
return Rcpp::wrap(output);
}
this is not optimized (as the question is bogus), this is badly written, beccause as I think R would be sufficiently faster in searching index of a vector (so do this in R and implemement only fun in Rcpp)...is not useful to waste time here, there are other problems that need a solver implemented in Rcpp , not this searching stuff...
but this is not a useful question as you are asking more for an algorithm than for example signature of function