SparseMatrix in Eigen - c++

Question

If I set the value of a SparseMatrix entry in Eigen as follows:
sparse_matrix->coeffref(10, 10) = 0;
Would this actually shrink the storage required by the matrix or would it try and store a 0 and use up 4 bytes there (assuming integer type)?
if the answer is the latter, how can I set columns to 0, so that it does not use any extra space?
Also, what about something like this:
typedef Eigen::Triplet<double> TripletType;
std::vector<TripletType> t;
for (int i = 0; i < some_value; ++i) {
for (int j = 0; j < some_value; ++j) {
t->push_back(TripletType(i, j, 0);
}
}
sparse_matrix->setFromTriplets(t);
Would this result in explicit zeros in the sparse matrix?

Answer 1

After insertion with coeffRef you can prune the sparse matrix like:
Eigen::SparseMatrix<double, Eigen::ColMajor> A(5,5);
// fill A
A.insert(0,0)=9.;
A.insert(1,0)=3.0/2.0;
A.insert(0,1)=3.0/2.0;
A.insert(2,0)=6.0;
A.insert(0,2)=6.0;
A.insert(3,0)=3.0/4.0;
A.insert(0,3)=3.0/4.0;
A.insert(4,0)=3.0;
A.insert(0,4)=3.0;
A.insert(1,1)=1.0/2.0;
A.insert(2,2)=12.0;
A.insert(3,3)=5.0/8.0;
A.insert(4,4)=16.0;
std::cout << A << std::endl;
std::cout << A.data().size() << std::endl;
A.coeffRef(3,0) = 0;
A.prune(0,0); // Suppresses all nonzeros which are much smaller than reference under the tolerence epsilon
std::cout << A << std::endl;
std::cout << A.data().size() << std::endl;`
Output:
Nonzero entries:
(9,0) (1.5,1) (6,2) (0.75,3) (3,4) (_,_) (_,_) (_,_) (1.5,0) (0.5,1) (6,0) (12,2
) (0.75,0) (0.625,3) (3,0) (16,4)
Outer pointers:
0 8 10 12 14 $
Inner non zeros:
5 2 2 2 2 $
9 1.5 6 0.75 3
1.5 0.5 0 0 0
6 0 12 0 0
0.75 0 0 0.625 0
3 0 0 0 16
16
Nonzero entries:
(9,0) (1.5,1) (6,2) (3,4) (1.5,0) (0.5,1) (6,0) (12,2) (0.75,0) (0.625,3) (3,0)
(16,4)
Outer pointers:
0 4 6 8 10 $
9 1.5 6 0.75 3
1.5 0.5 0 0 0
6 0 12 0 0
0 0 0 0.625 0
3 0 0 0 16
12
You can see that the size has changed from 16 to 12, as also the three (_,_) are removed.
I didn't check with sizeof() if memory storage that is needed is really less.