Reputation: 799
I see with new Eigen 3.2, you can get row, column or even block from a sparse matrix, is there a way to set any of these to 0?
Eigen::SparseMatrix<float, Eigen::RowMajor> A( 5, 5 );
A.block(1, 1, 2, 2) = 0; // won't work
A.row(1) = 0; // won't work
A.col(1) = 0; // won't work
Thanks!
Upvotes: 2
Views: 5283
Reputation: 3250
For beginners, the simplest way set to zero a row/column/block is just to multiply it by 0.0.
Eigen::SparseMatrix<float, Eigen::RowMajor> A;
A = MatrixXf::Random(8,8).sparseView(); //Initialize to random values
A.block(1, 1, 2, 2) *= 0; //Set a block to 0
A.row(1) *= 0; //Set a row to 0
A.col(1) *= 0; //Set a column to 0
std::cout << A << "\n";
Using prune() to set a block to zero can be faster if the density of your sparse matrix is high, but for most cases *= 0
is quite fast and you don't have to rewrite your code depending of RowMajor/ColMajor ordering.
If you are really interested in free memory after setting the row/col to 0, just add a A.prune(0,0)
after you have finished editing the matrix.
Upvotes: 2
Reputation: 11
Eigen's Tutorial for sparse matrices (Tutorial Sparse: block operations) explain that for ColumnMajor (resp. RowMajor) sparse matrices, the column (resp. row) sub-matrix operators have write-access.
So, for example, to set to zero the 2nd, 3rd and 4th rows of a rowmajor sparse matrix you could do:
Eigen::SparseMatrix<float, Eigen::RowMajor> A;
A = MatrixXf::Random(5,5).sparseView();
A.middleRows(1,3) = Eigen::SparseMatrix<float,Eigen::RowMajor>(3,A.cols());
And leave prune()
for setting to zero columns of row major matrices and vicevesa.
Upvotes: 1
Reputation: 29265
For 5x5 matrices, it is overkill to use a sparse matrix. Better use a MatrixXd
, or even a Matrix<float,5,5>
. In this case you can set a row to zero with A.row(1).setZero()
. Sparse matrices are worth it for matrices of size about 1000x1000 and greater.
Anyway, the best to suppress multiple columns and rows of a sparse matrix at once is to use the prune method. Here is an example removing the second row and third column:
#include <Eigen/Sparse>
#include <iostream>
using namespace Eigen;
int main()
{
Eigen::SparseMatrix<float, Eigen::RowMajor> A;
A = MatrixXf::Random(5,5).sparseView();
A.prune([](int i, int j, float) { return i!=1 && j!=2; });
std::cout << A << "\n";
}
Upvotes: 6