Eigen library --> initialize matrix with data from file or existing std::vector content (c++)

Question

My question is how to initialize an eigen Matrix, but NOT this way:

matrix << 1,0,1,0,
          1,0,1,0,
          1,0,1,0,


        

Answer 1

Here is my solution:

#include <istream>
#include <string>
#include <sstream>
#include <vector>
#include <fstream>
#include <iostream>
#include <eigen>

using namespace std;
using namespace Eigen;

// load matrix from an ascii text file.
vector<vector<double>> LoadMatrix(istream* filePath, const string &delim = " \t")
{
    string line;
    string strnum;

    auto data = vector<vector<double>>();

    // clear first
    data.clear();

    // parse line by line
    while (getline(*filePath, line))
    {
        data.push_back(vector<double>());

        for (string::const_iterator i = line.begin(); i != line.end(); ++i)
        {
            // If i is not a delim, then append it to strnum
            if (delim.find(*i) == string::npos)
            {
                strnum += *i;
                if (i + 1 != line.end()) // If it's the last char, do not continue
                    continue;
            }

            // if strnum is still empty, it means the previous char is also a
            // delim (several delims appear together). Ignore this char.
            if (strnum.empty())
                continue;

            // If we reach here, we got a number. Convert it to double.
            double       number;

            istringstream(strnum) >> number;
            data.back().push_back(number);

            strnum.clear();
        }
    }
    return data;
}

Eigen::MatrixXd ConvertToEigenMatrix(std::vector<std::vector<double>> data)
{
    Eigen::MatrixXd eMatrix(data.size(), data[0].size());
    for (int i = 0; i < data.size(); ++i)
        eMatrix.row(i) = Eigen::VectorXd::Map(&data[i][0], data[0].size());
    return eMatrix;
}
MatrixXd LoadEigenMatrix(istream* filePath, const string &delim = " \t")
{
    auto data = LoadMatrix(filePath, delim);
    return ConvertToEigenMatrix(data);
}