conservativeResize() with zero values for the new values

Question

How to set the new values to zero after resizing a matrix? It is really weird that after resizing the matrix, the new values are set to trash values instead of at least set to zero.

N = 0;
Eigen::MatrixXd CO;
CO.setZero(3+3*N, 3+3*N);
std::cout << CO << std::endl << std::endl;
Nt = 1;
CO.conservativeResize(3+3*Nt,3+3*Nt);
std::cout << CO << std::endl << std::endl;

The result

Deglutinate

It is a common practice to avoid initialization unless it was explicitly required. Otherwise you'd fill in the new values twice: first with 0, and then with the values you need. – Flatter

Answer 1

I've solved the problem by using conservativeResizeLike()

int Nt = 0;
Eigen::MatrixXd  CO;
CO.setOnes(3+3*Nt, 3+3*Nt);
std::cout << CO << std::endl << std::endl;
Nt = 1;
CO.conservativeResizeLike(Eigen::MatrixXd::Zero(3+3*Nt,3+3*Nt));
std::cout << CO << std::endl << std::endl;

The result

Also, I found out you can set them as ones Eigen::MatrixXd::Ones(3+3*Nt,3+3*Nt) or identity Eigen::MatrixXd::Identity(3+3*Nt,3+3*Nt)

For Identity

Deglutinate

Answer 2

Those values are not so much "trash" values as they are "uninitialized memory" values. It is your responsibility to set them to whatever values make sense to you. It should not be difficult to iterate over the new values and zero them if you wish.

Anzac

I'm sorry but this is not a solution. I can iterate over the new values but it seems odd that there is no default values at least with zeros. – Deglutinate

@Deglutinate If you want the value to be set "automatically" when you resize a matrix, you do have the option of creating a derivative class of Eigen::MatrixXd that would do that (unless it is declared with the final keyword). – Anzac

@Deglutinate Well, the documentaton here: eigen.tuxfamily.org/dox/classEigen_1_1PlainObjectBase.html says this: Matrices are resized relative to the top-left element. In case values need to be appended to the matrix *they will be uninitialized*. – Infant

Answer 3

I am not sure which version of Eigen you are working with. But as of today, Eigen has a setZero() function which sets all coefficients in this expression to zero.

Here is the documentation: Derived & Eigen::DenseBase< Derived >::setZero ( )

Allyn

I've asked it 5 years ago. They may fix it but thanks for the update. – Deglutinate

yeah, it is nice to have a simple initializer filling zeros. – Allyn