In Rcpp How to create a NumericMatrix by a NumbericaVector? - rcpp

Question

In Rcpp How to create a NumericMatrix by a NumbericaVector?
Something like
// vector_1 has 16 element
NumericMatrix mat = NumericMatrix(vector_1, nrow = 4);
Thanks.

Answer 1

Edit: I knew we had something better. See below for update.
Looks like we do not have a matching convenience constructor for this. But you can just drop in a helper function -- the following is minimally viable (one should check that n + k == length(vector)) and taken from one of the unit tests:
// [[Rcpp::export]]
Rcpp::NumericMatrix vec2mat(Rcpp::NumericVector vec, int n, int k) {
Rcpp::NumericMatrix mat = Rcpp::no_init(n, k);
for (auto i = 0; i < n * k; i++) mat[i] = vec[i];
return mat;
}
Another constructor takes the explicit dimensions and then copies the payload for you (via memcpy()), removing the need for the loop:
// [[Rcpp::export]]
Rcpp::NumericMatrix vec2mat2(Rcpp::NumericVector s, int n, int k) {
Rcpp::NumericMatrix mat(n, k, s.begin());
return mat;
}
Full example below:
> Rcpp::sourceCpp("~/git/stackoverflow/66720922/answer.cpp")
> v <- (1:9) * 1.0 # numeric
> vec2mat(v, 3, 3)
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
> vec2mat2(v, 3, 3)
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
>
Full source code below.
#include <Rcpp.h>
// [[Rcpp::export]]
Rcpp::NumericMatrix vec2mat(Rcpp::NumericVector vec, int n, int k) {
Rcpp::NumericMatrix mat = Rcpp::no_init(n, k);
for (auto i = 0; i < n * k; i++) mat[i] = vec[i];
return mat;
}
// [[Rcpp::export]]
Rcpp::NumericMatrix vec2mat2(Rcpp::NumericVector s, int n, int k) {
Rcpp::NumericMatrix mat(n, k, s.begin());
return mat;
}
/*** R
v <- (1:9) * 1.0 # numeric
vec2mat(v, 3, 3)
vec2mat2(v, 3, 3)
*/
Depending on what you want to do with the matrix object (linear algrebra?) you may want to consider RcppArmadillo (or RcppEigen) as those packages also have plenty of vector/matrix converters.