Using advanced constructors to convert array to armadillo vec - c++

Question

I am trying to convert an object of type ODE_vector to an armadillo vec such that the new object will inherit the linear algebraic functionality of the armadillo library. As a simple example of the problem I am having this is a demonstration using a C++ array:
#include <iostream>
#include <random>
#include <armadillo>
using namespace std;
using namespace arma;
int main() {
double b1 = 0.2;
double b2 = 0.1;
const double state[2] = {b1, b2};
rowvec B(&state[0], 2);
cout << B << endl;
mat A(2,2); A.fill(0.2); A.diag().ones();
cout << A << endl;
cout << B(0) * A(0,0) << endl; // x
return 0;
}
The data contained in b1 and b2 is saved in the elements of B so that running this script returns the following:
0.2000
0.1000
1.0000 0.2000
0.2000 1.0000
0.2
HOWEVER, if I replace line x above with
cout << B * A << endl;
I get errors suggesting B has not inherited the standard armadillo linear algebra functionality:
[ 50%] Building CXX object CMakeFiles/untitled.dir/main.cpp.o
[100%] Linking CXX executable untitled
CMakeFiles/untitled.dir/main.cpp.o: In function `void arma::blas::gemv<double>(char const*, int const*, int const*, double const*, double const*, int const*, double const*, int const*, double const*, double*, int const*)':
/usr/include/armadillo_bits/wrapper_blas.hpp:36: undefined reference to `wrapper_dgemv_'
CMakeFiles/untitled.dir/main.cpp.o: In function `void arma::blas::gemm<double>(char const*, char const*, int const*, int const*, int const*, double const*, double const*, int const*, double const*, int const*, double const*, double*, int const*)':
/usr/include/armadillo_bits/wrapper_blas.hpp:71: undefined reference to `wrapper_dgemm_'
collect2: error: ld returned 1 exit status
CMakeFiles/untitled.dir/build.make:94: recipe for target 'untitled' failed
make[3]: *** [untitled] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/untitled.dir/all' failed
make[2]: *** [CMakeFiles/untitled.dir/all] Error 2
CMakeFiles/Makefile2:79: recipe for target 'CMakeFiles/untitled.dir/rule' failed
make[1]: *** [CMakeFiles/untitled.dir/rule] Error 2
Makefile:118: recipe for target 'untitled' failed
make: *** [untitled] Error 2
Can anyone explain how to properly use the advanced constructors (arma documentation) to get this working?
Thanks

Answer 1

You need to have the right format in the multiplication. Type B is a 2x1 and A is 2x2. Change to
mat B(&state[0], 1,2);
or
rowvec B(&state[0], 2);