Eigenvector calculation in C++ - c++

Question

How can I make a function in cpp in order to calculate the first "Q" eigenvectors of a matrix M?
I tried using this code, but failed.
#include <RcppArmadillo.h>
using namespace arma;
mat M;
int Q;
vec getEigen(M,Q) {
return eig_sym(M, Q);
}
The error message says:
"no matching function for call to "arma::col(arma::mat&, int&)"
Any idea? I am new at cpp and don't know what the message means.
Thanks

Answer 1

As noted in the comments, there is no function in Armadillo that returns a subset of the eigenvalues. However, one can combine .head() or .tail() with eigen_sym() to extract a subset. In addition, it makes sense to use reverse(), since Armadillo returns eigenvalues in ascending order. For convenience I am using RcppArmadillo with Rcpp attributes here:
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
// [[Rcpp::export]]
arma::vec getEigen(const arma::mat& M, int Q) {
return arma::reverse(arma::eig_sym(M).tail(Q));
}
/*** R
set.seed(42)
N <- 10
m <- matrix(rnorm(N * N), N, N)
m <- m + t(m)
getEigen(m, N/2)
*/
Output upon calling Rcpp::sourceCpp on the file:
// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>
// [[Rcpp::export]]
arma::vec getEigen(const arma::mat& M, int Q) {
return arma::reverse(arma::eig_sym(M).tail(Q));
}
/*** R
set.seed(42)
N <- 10
m <- matrix(rnorm(N * N), N, N)
m <- m + t(m)
getEigen(m, N/2)
*/
This is only for the eigenvalues, not the eigenvectors. Extracting the Eigenvectors shouldn't be difficult, though.