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eigensparse-matrix
Nina
Nina

Reputation: 71

Eigen Sparse Matrix get Indices of Nonzero Elements

I am using Eigen Sparse Matrices for the first time, and now I would like to know how to get the indices of the nonzero elements. I constructed my Sparse Matrix as follows:

Eigen::SparseMatrix<Eigen::ColMajor> Am(3,3);

and I can see some indices in VS by looking into the m_indices variable. But I can't access them. Can anyone please help me? For a Matrix like

( 1 0 1 
  0 1 1
  0 0 0 )

I would like the indices to be like (0,0), (0,2), (1,1), (1,2).

Is there any way to do it?

P.S. My matrices are way bigger than 3x3.

Upvotes: 7

Views: 6819

Answers (3)

Alec Jacobson
Alec Jacobson

Reputation: 6264

Using libigl's igl::find, you can extract the indices of non-zeros into Eigen vectors:

Eigen::VectorXi I,J;
Eigen::VectorXd V;
igl::find(Am,I,J,V);

In your example these will contain:

I: 0 1 0 1
J: 0 1 2 2
V: 1 1 1 1

Upvotes: 0

zeng sylvia
zeng sylvia

Reputation: 1

Eigen::SparseMatrix<int, Eigen::ColMajor> A(2,3);
for (int k=0; k < A.outerSize(); ++k)
{
    for (Eigen::SparseMatrix<int,Eigen::ColMajor>::InnerIterator it(A,k); it; ++it)
    {
        std::cout << "(" << it.row() << ","; // row index
        std::cout << it.col() << ")\t"; // col index (here it is equal to k)
    }
}

Upvotes: 0

Avi Ginsburg
Avi Ginsburg

Reputation: 10596

The tutorial has code similar to this:

for (int k=0; k < A.outerSize(); ++k)
{
    for (SparseMatrix<int>::InnerIterator it(A,k); it; ++it)
    {
        std::cout << "(" << it.row() << ","; // row index
        std::cout << it.col() << ")\t"; // col index (here it is equal to k)
    }
}

Upvotes: 9

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