sparse matrix from triplet

Question

With eigen (c++) i want to have a similar result as Matlab's one :

 % Matlab code: Create a 1500-by-1500 sparse matrix from the triplets i, j, 
            and v 
 i = [900 1000];
   j = [900 1000];
   v = [10 100];
   S = sparse(i,j,v,1500,1500)

   %result
    S =

 (900,900)     10
 (1000,1000)   100

I want to do the same thing in C++, i saw in different posts that we can handle sparse matrices with Eigen Library, but i didn't really understand how to do this. Can you help me ?

Thanks

Answer 1

Create a vector of triplets

typedef Eigen::Triplet<int> Trip;
std::vector<Trip> trp;

Create the triplets

trp.push_back(Trip(900,900,10)); // (index, index, value)
trp.push_back(Trip(1000,1000,100));

Assign them to the sparse Eigen matrix

int rows, cols;
rows = cols = 1500;
Eigen::SparseMatrix<int> A(rows,cols);

A.setFromTriplets(trp.begin(), trp.end());

Print the entire (dense) matrix

std::cout << A << std::endl;

or print just the indices and values

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