SparseMatrix and Vector addition broadcasting in Eigen - c++

Question

I want to add a dense VectorXf to a SparseMatrix, and I know that I could do matrix + vector easily for dense matrix, like this
Eigen::MatrixXf mat(2,4);
Eigen::VectorXf v(2);
mat << 1, 2, 6, 9,
3, 1, 7, 2;
v << 0,
1;
//add v to each column of m
mat.colwise() += v;
but how to do this for sparse matrix?

Answer 1

The simplest way to do this is to iterate over all columns and add a sparse vector to your sparse matrix. And you should keep in mind, that if you need colwise operation, your matrix should be stored as Column-Major.
// allocate dense object
Eigen::MatrixXd M(2,4);
Eigen::VectorXd v(2);
// allocate sparse vector
Eigen::SparseVector<double> spV(2);
// allocate Column-major sparse matrix
Eigen::SparseMatrix<double, Eigen::ColMajor> spM(2, 4);
// initialize dense objects values
M << 1, 0, 0, 2,
4, 5, 0, 0;
v << 1,
3;
// convert dense objects to sparse representation
spM = M2.sparseView();
spV = v2.sparseView();
// iterate over sparse Matrix columns
for(int i = 0; i< spM.cols(); ++i) {
spM.col(i) += spV;
}
It can't be done in such way for dense vector and one of the reason is that this operation does not make any sense, since the resulting matrix will be dense. In this case you will need to create some dense matrix and perform colwise operation with your dense vector like this:
Eigen::MatrixXd(spM).colwise() + v