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c++matrixindexingsparse-matrixeigen

How to access a specific (row,col) index in an C++ Eigen sparse matrix?

I'm working in C++ with a sparse matrix in Eigen. I would like to read the data stored in a specific row and column index just like I would with a regular eigen matrix.

std::vector<Eigen::Triplet<double>> tripletList;

// TODO: populate triplet list with non-zero entries of matrix

Eigen::SparseMatrix<double> matrix(nRows, nCols);
matrix.setFromTriplets(tripletList.begin(), tripletList.end());

// TODO:  set iRow and iCol to be valid indices.

// How to read the value at a specific row and column index?
// double value = matrix(iRow, iCol);  // Compiler error

How do I go about performing this type of indexing operation?

Solution

  • Try coeff:

    double value = matrix.coeff(iRow, iCol);

    If you want a non-const version use coeffRef instead. Note that when using coeffRef if the element doesn't exist, it will be inserted.