This thread suggest a way to copy the data of an Eigen::VectorXd to a std::vector

However, I am wondering what can be the cleanest way to typecast/copy a two dimensional array into a std::vector<std::vector<double>>

CodePudding user response：

You could do the same as in the thread you shared, but do it for each column (default Eigen::ColMajor) or row (Eigen::RowMajor):

Eigen::MatrixXd mat(2,3);
mat << 1.1, 2.1, 3.1,
       4.1, 5.1, 6.1;

std::vector<std::vector<double>> matrix(mat.cols());
auto ptr = mat.data();
for (auto& vec : matrix)
{
    vec = std::vector<int>(ptr, ptr   mat.rows());
    ptr  = mat.rows();
}

CodePudding user response：

One way might be using loop to traverse all the values of array, then store them in vector.

int ar[2][2];
vector<vector<int>> vec;

int main()
{
    ar[0][0] = 1;
    ar[0][1] = 2;
    ar[1][0] = 3;
    ar[1][1] = 4;

    for(int i = 0; i < 2; i  )
    {
        vector<int>v;
        for(int j = 0; j < 2; j  )
        {
            v.push_back(ar[i][j]);
        }
        vec.push_back(v);
    }

    for(int i = 0; i < vec.size(); i  )
    {
        for(int j = 0; j < vec[i].size(); j  ) cout<<vec[i][j]<<" ";
        cout<<endl;
    }
}

Another way: traverse each row and store them in vector.

int ar[2][2];
vector<vector<int>> vec;

int main()
{
    int row = 2, col = 2;
    ar[0][0] = 1;
    ar[0][1] = 2;
    ar[1][0] = 3;
    ar[1][1] = 4;

    for(int i = 0; i < row; i  )
    {
        vector<int> currV = {vector<int> (begin(ar[i]), end(ar[i]))};
        vec.push_back(currV);
    }

    for(int i = 0; i < vec.size(); i  )
    {
        for(int j = 0; j < vec[i].size(); j  ) cout<<vec[i][j]<<" ";
        cout<<endl;
    }
}