Column-wise dot product in Eigen C++

Question

Is there an easy way to evaluate the column wise dot product of 2 matrices (lets call them A and B, of type Eigen::MatrixXd) that have

Answer 1

Here is how I'd do it with an Eigen::Map (assuming real matrices, can extend to complex via taking the adjoint), where rows and cols denote the number of rows/columns:

#include 
#include 

int main()
{
    Eigen::MatrixXd A(2, 2);
    Eigen::MatrixXd B(2, 2);
    A << 1, 2, 3, 4;
    B << 5, 6, 7, 8;

    int rows = 2, cols = 2;

    Eigen::VectorXd vA = Eigen::Map(
                             const_cast(A.data()), rows * cols, 1);
    Eigen::VectorXd vB = Eigen::Map(
                             const_cast(B.data()), rows * cols, 1);

    double inner_prod = (vA.transpose() * vB).sum();

    std::cout << inner_prod << std::endl;
}