Concatenate sparse matrix Eigen

Question

I have two sparse matrices in Eigen, and I would like to join them vertically into one. As an example the target of the code would be:

SparseMatrix         


        

Answer 1

I ended up doing the following:

MATRIX_JOIN.resize(matrix1.rows() + matrix2.rows(), matrix1.cols() + matrix2.cols());
MATRIX_JOIN.setZero();

// Fill MATRIX_JOIN with triples from the other matrices
std::vector > tripletList;
for (int k = 0; k < matrix1.outerSize(); ++k)
{
    for (SparseMatrix::InnerIterator it(matrix1, k); it; ++it)
    {
        tripletList.push_back(Triplet(it.row(), it.col(), it.value()));
    }
}
for (int k = 0; k < matrix2.outerSize(); ++k)
{
    for (SparseMatrix::InnerIterator it(matrix2, k); it; ++it)
    {
        tripletList.push_back(Triplet(it.row(), it.col(), it.value()));
    }
}
FINALMATRIX.setFromTriplets(tripletList.begin(), tripletList.end());

There can be a speedup by calling tripleList.reserve(X) with X being the expected amount of triplets to insert.