Most efficient option for build 3D structures using Eigen matrices

Question

I need a 3D matrix/array structure on my code, and right now I'm relying on Eigen for both my matrices and vectors.

Right now I am creating a 3D structure using new:

MatrixXd* cube= new MatrixXd[60];
for (int i; i<60; i++) cube[i]=MatrixXd(60,60);

and for acessing the values:

double val;
MatrixXd pos;
for (int i; i<60; i++){
    pos=cube[i];
    for (int j; j<60; j++){
        for (int k; k<60; k++){
            val=pos(j,k);
            //...
        }
    }
}

However, right now it is very slow in this part of the code, which makes me beleive that this might not be the most efficient way. Are there any alternatives?

Answer 1

While it was not available, when the question was asked, Eigen has been providing a Tensor module for a while now. It is still in an "unsupported" stage (meaning the API may change), but basic functionality should be mostly stable. The documentation is scattered here and here.

Answer 2

A solution I used is to form a fat matrix containing all the matrices you need stacked.

MatrixXd A(60*60,60);

and then access them with block operations

A0 = A.block<60,60>(0*60,0);
...
A5 = A.block<60,60>(5*60,0);