Convert an Eigen matrix to Triplet form C++

Question

I think Eigen uses compressed methods to store sparse matrices. Is there any way that I can extract Triplet-format vectors of an Eigen sparse matrix in from of std::vectors?

Thanks.

More info (an example of triplet format) Triplet format of matrix :

A=
3 0 4 0
0 0 1 0
0 2 0 5
4 0 0 0

i = 1 1 2 3 3 4  // row
j = 1 3 3 2 4 1 // column
S = 3 4 1 2 5 4  // values

Answer 1

The answer to the question, which is:

// Is there some method such as:

std::vector<Eigen::Triplet<double>> T = SparseMat.to_triplets();

// in Eigen?

Is no, there does not appear to be such a function.

Instead,

std::vector<Eigen::Triplet<double>> to_triplets(Eigen::SparseMatrix<double> & M){
    std::vector<Eigen::Triplet<double>> v;
    for(int i = 0; i < M.outerSize(); i++)
        for(typename Eigen::SparseMatrix<double>::InnerIterator it(M,i); it; ++it)
            v.emplace_back(it.row(),it.col(),it.value());
    return v;
}

auto t = to_triplets(SparseMat);

And if you want to do it faster, open it in an IDE, look around for pointers to the data arrays, and write a convoluted function that will have no effect on runtime, since the matrix is sparse, and copying is linear in terms of nonzero elements.