Armadillo Dot product C++

Question

I am currently on Ubuntu code::blocks and run into a few problems when trying to do linear algebra.

under compiler settings>search directories>compiler i have "/usr/include"

and under compiler settings>search directories>linker i have "/user/lib"

my liblapack-dev, libblas-dev, libboost-dev, libarmadillo-dev are installed via apt-get

i commented which part of the code give me error. without the difficult part of the code my code runs fine, so i think i have armadillo installed fine? so why cant i access all its functions?

#include <iostream>
#include <armadillo>
using namespace arma;
using namespace std;
int main()
{
    mat A;
    A<<1<<2<<endr<<3<<4;
    cout<<A;
    vec e=A.col(0);
    vec r=A.col(1);
    cout<<endl<<e<<endl<<r<<endl;//works perfectly up to here
    //if only there was not more of these codes
    cout<<e*r<<endl;//doesnt work from here anymore
    float y=dot(A,A);//from here on i get the error message:
    cout<<y<<endl;//'wrapper_ddot_' is not defined
    double z=as_scalar(e*r);//and wrapper_blas.hpp file opens
    double t=dot(e,r);
    cout<<z<<endl;//and points me to line 185
    return 0;//with an error
}

Answer 1

There is a bug in the code. You are using e*r but they are both 2x1 so you need to transpose e as et()*r so you get a 1x1 product. You also have the same problem three rows below. If you installed Armadillo using apt-get there is normally no need to add the blas/lapack libs. It should be sufficient to use the -larmadillo flag to the linker.