Rcpp select/subset NumericMatrix column by a NumericVector - c++

Question

I can select all the rows of a matrix and a range of columns of a matrix as follows:
library(Rcpp)
cppFunction('
NumericMatrix subset(NumericMatrix x){
return x(_, Range(0, 1));
}
')
However, I would like to select columns based on a NumericVector y which, for instance, could be something like c(0, 1, 0, 0, 1). I tried this:
library(Rcpp)
cppFunction('
NumericMatrix subset(NumericMatrix x, NumericVector y){
return x(_, y);
}
')
but it doesn't compile. How do I do it?

Answer 1

Alas, Rcpp doesn't have great support for non-contiguous views or selecting in a single statement only columns 1 and 4. As you saw, selecting contiguous views or selecting all columns is accessible with Rcpp::Range(). You'll likely want to upgrade to RcppArmadillo for better control over matrix subsets.
RcppArmadillo subset examples
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
arma::mat matrix_subset_idx(const arma::mat& x,
const arma::uvec& y) {
// y must be an integer between 0 and columns - 1
// Allows for repeated draws from same columns.
return x.cols( y );
}
// [[Rcpp::export]]
arma::mat matrix_subset_logical(const arma::mat& x,
const arma::vec& y) {
// Assumes that y is 0/1 coded.
// find() retrieves the integer index when y is equivalent 1.
return x.cols( arma::find(y == 1) );
}
Test
# Sample data
x = matrix(1:15, ncol = 5)
x
# [,1] [,2] [,3] [,4] [,5]
# [1,] 1 4 7 10 13
# [2,] 2 5 8 11 14
# [3,] 3 6 9 12 15
# Subset only when 1 (TRUE) is found:
matrix_subset_logical(x, c(0, 1, 0, 0, 1))
# [,1] [,2]
# [1,] 4 13
# [2,] 5 14
# [3,] 6 15
# Subset with an index representing the location
# Note: C++ indices start at 0 not 1!
matrix_subset_idx(x, c(1, 3))
# [,1] [,2]
# [1,] 4 13
# [2,] 5 14
# [3,] 6 15
Pure Rcpp logic
If you do not want to take on the dependency of armadillo, then the equivalent for the matrix subset in Rcpp is:
#include <Rcpp.h>
// [[Rcpp::export]]
Rcpp::NumericMatrix matrix_subset_idx_rcpp(
Rcpp::NumericMatrix x, Rcpp::IntegerVector y) {
// Determine the number of observations
int n_cols_out = y.size();
// Create an output matrix
Rcpp::NumericMatrix out = Rcpp::no_init(x.nrow(), n_cols_out);
// Loop through each column and copy the data.
for(unsigned int z = 0; z < n_cols_out; ++z) {
out(Rcpp::_, z) = x(Rcpp::_, y[z]);
}
return out;
}