Silence error while solving the eigenvalues of a 2-by-2 matrix.

[BangShiuh]

The below code failed to obtain the eigenvalues for a 2-by-2 sparse matrix. At least an error message should pop up. (version 1.01)

#include <Eigen/Core>
#include <Eigen/SparseCore>
#include <Spectra/GenEigsSolver.h>
#include <Spectra/MatOp/SparseGenMatProd.h>
#include <iostream>
#include <Eigen/Sparse>
#include <Eigen/SparseLU>

using namespace Spectra;

double solver(Eigen::SparseMatrix<double> M, SortRule rule, int nKrylov)
{
    // Construct matrix operation object using the wrapper class SparseGenMatProd
    SparseGenMatProd<double> op(M);

    // Construct eigen solver object, requesting the largest three eigenvalues
    // how to set the size of Krylov subspace? from 5 to 10
    GenEigsSolver<SparseGenMatProd<double>> eigs(op, 1, nKrylov);

    // Initialize and compute
    eigs.init();
    int nconv = eigs.compute(rule);

    // Retrieve results
    Eigen::VectorXcd evalues;
    Eigen::MatrixXcd evecs;
    if(eigs.info() == CompInfo::Successful) {
        evalues = eigs.eigenvalues();
        evecs = eigs.eigenvectors();
    }

    // max eigenvalue
    std::complex<double> eigmax = evalues.coeff(0,0);

    return eigmax.real();
}

int main()
{
    // Define the matrix A
    Eigen::SparseMatrix<double> M(2, 2);
    M.insert(0, 0) = 1.0;
    M.insert(1, 1) = 2.0;
    M.insert(0, 1) = 0;
    M.insert(1, 0) = 0;

    double eigmax = solver(M,SortRule::LargestReal,2);
    std::cout << "Max real Eigenvalues found:\n" << eigmax << std::endl;

    return 0;
}

[yixuan]

An exception nev must satisfy 1 <= nev <= n - 2, n is the size of matrix should be thrown. Did you capture the exception somewhere else?

[BangShiuh]

I am using the visual studio 2022 compiler, which does not show any message.

[yixuan]

It depends on how the compiler handles the exception, but indeed the program will terminate due to the error. You can explicitly use try-catch statements to detect such exceptions.

[BangShiuh]

include/Spectra/JDSymEigsBase.h

void check_argument() const
    {
        if (m_number_eigenvalues < 1 || m_number_eigenvalues > m_matrix_operator.cols() - 1)
            throw std::invalid_argument("nev must satisfy 1 <= nev <= n - 1, n is the size of matrix");
    }

It seems that it is n-1 in the code.

[yixuan]

I think you were using GenEigsSolver, so this is the code: https://github.com/yixuan/spectra/blob/master/include/Spectra/GenEigsBase.h#L336. As a result, for any 2x2 matrix, GenEigsSolver will raise an error.