Deal all,
Thank you for your time to read my question.
Eigen is really awesome, I'm using Eigen to write some FEM code.

I read the Eigen3.3.4 document, and in this website(http://eigen.tuxfamily.org/dox-devel/To ... Types.html), it says
we should use Ref<MatrixBase> to avoid the additioinal copy and obtain high performance.

So in my FEM code, for the sparse matrix assemble part, let's say the function is:

FormFE(const Ref<VectorXd> &U,const Ref<VectorXd> &V,Ref<SparseMatrix<double> > AMATRIX,Ref<VectorXd> RHS)

where U represent the displacement, V represent the velocity term. AMATRIX is my sparse matrix, RHS is the residual term.

Then I try to first initializing my AMATRIX before assemble(I have a tripletList which contains all the non-zero element and its value(I set the value to zero for initializing))
So I tried: AMATRIX.setFromTriplets(ZeroTripList.begin(),ZeroTripList.end());
But I have an error: class Eigen::Ref<Eigen::SparseMatrix<double, 0, int> >’ has no member named ‘setFromTriplets
So how could I solve this problem?

One of my solution is using
FormFE(const Ref<VectorXd> &U,const Ref<VectorXd> &V,SparseMatrix<double> &AMATRIX,Ref<VectorXd> RHS)

this is working very fine, but I'm not sure it is efficient or not.
I'm not very good at cpp

Actually, I have three questions:
1. How to efficiently use Eigen(especially for FEM calculation), I use Eigen's VectorXd and MatrixXd almost everywhere in each of my FEM-related function.
2. How to efficiently assemble a SparseMatrix?
3. Is it possible to do some OpenMP parallelization for the FEM assemble?

Thank you.
Best regards.

For the record, answered there: https://stackoverflow.com/questions/470 ... rse-matrix