如何用C++将多维矢量数据写入文件

我有一个对象obj（类Holder）的向量，对象obj具有N个元素，成员为x和y，它们也是具有M个元素的双类型向量。我想写一个文本文件，从中创建一个MxN矩阵。到目前为止，我已经尝试了很多不同的方法，但都无济于事。

vector<Holder> obj(N);
void savedata(string filename, vector<Holder> obj, int M, int N) {
    ofstream out(filename);
    for(int i = 0; i < M; i++) {
        for(int j = 0; j < N; j++) {
          out << obj[i][j] << "t" << endl;  
     }
  }
}

但这只是取了最后一组值。如何创建这样一个MxN矩阵，其中行来自对象成员向量x，列来自对象向量本身？

提前谢谢。

--

代码的较大版本如下：

    //
    //
    #include <iostream>
    #include <cmath>
    #include <fstream>
    #include <string>
    #include <vector>
    #include <random>
    using namespace std;
    typedef vector< vector<double> > Matrix;
    // Particles making up the cell
    class Particle{
    public:
        double x; // x position
        double y; // y position
        double vx; // velocity in the x direction
        double vy; // velocity in the y direction
        double Fx; // force in the x direction
        double Fy; // force in the y direction
        // Default constructor
        Particle()
        : x(0.0),y(0.0),vx(0.0),vy(0.0),Fx(0.0),Fy(0.0){
        }
    };
    // Holder for storing data
    class HoldPar{
    public:
        vector<double> x;
        vector<double> y;
        vector<double> vx;
        vector<double> vy;
        // Default constructor
        HoldPar()
        : x(0.0),y(0.0),vx(0.0),vy(0.0){
        }
        // Add elements to vectors
        void add_Xelement(double a) {
            x.push_back(a);
        }
        void add_Yelement(double a) {
            y.push_back(a);
        }
        void add_VXelement(double a) {
            vx.push_back(a);
        }
        void add_VYelement(double a) {
            vy.push_back(a);
        }
    };
    int main() {
        // Initialization of x, v and F
        const float pi = 3.14;
        int N = 30; // Number of 'particles' that make up the cell
        float theta = 2*pi/N; // Angle between two particles in radians
        float x0 = 0; // Center of the cell [x]
        float y0 = 0; // Center of the cell [y]
        float R = 5e-6; // Radius of the cell
        vector<Particle> particles(N); // particles
        // Assigning the initial points onto the circle
        for(int i = 0; i < N; i++) {
            particles[i].x = x0 + R*cos(theta*i);
            particles[i].y = y0 + R*sin(theta*i);
        }
        float k = 4.3e-7; // Spring constant connecting the particles
        float m = 2e-8; // Mass of the particles
        // Calculating the initial spring force between the particles on the cell
        particles[0].Fx = -k*(particles[1].x - particles[N].x);
        particles[0].Fy = -k*(particles[1].y - particles[N].y);
        for(int i = 1; i < N-1; i++) {
            particles[i].Fx = -k*(particles[i+1].x - particles[i-1].x);
            particles[i].Fy = -k*(particles[i+1].y - particles[i-1].y);
        }
        particles[N].Fx = -k*(particles[0].x - particles[N-1].x);
        particles[N].Fy = -k*(particles[0].y - particles[N-1].y);
        // Initial velocities are given to each particle randomly from a Gaussian distribution
        random_device rdx; // Seed
        default_random_engine generatorx(rdx()); // Default random number generator
        random_device rdy; // Seed
        default_random_engine generatory(rdy()); // Default random number generator
        normal_distribution<float> distributionx(0,1); // Gaussian distribution with 0 mean and 1 variance
        normal_distribution<float> distributiony(0,1); // Gaussian distribution with 0 mean and 1 variance
        for(int i = 0; i < N; i++) {
            float xnumber = distributionx(generatorx);
            float ynumber = distributiony(generatory);
            particles[i].vx = xnumber;
            particles[i].vy = ynumber;
        }

        // Molecular dynamics simulation with velocity Verlet algorithm
        // 'Old' variables
        vector<Particle> particles_old(N);
        for(int i = 0; i < N; i++) {
            particles_old[i].x = particles[i].x;
            particles_old[i].y = particles[i].y;
            particles_old[i].vx = particles[i].vx;
            particles_old[i].vy = particles[i].vy;
            particles_old[i].Fx = particles[i].Fx;
            particles_old[i].Fy = particles[i].Fy;
        }
        // Sampling variables
        int sampleFreq = 2;
        int sampleCounter = 0;
        // MD variables
        float dt = 1e-4;
        float dt2 = dt*dt;
        float m2 = 2*m;
        int MdS = 1e+5; // Molecular dynamics step number
        // Holder variables
        vector<HoldPar> particles_hold(N);
        // MD
        for(int j = 0; j < MdS; j++) {
            // Update x
            for(int i = 0; i < N; i++) {
                particles[i].x = particles_old[i].x + dt*particles_old[i].vx + dt2*particles_old[i].Fx/m2;
                particles[i].y = particles_old[i].y + dt*particles_old[i].vy + dt2*particles_old[i].Fy/m2;
            }
            // Update F
            particles[0].Fx = -k*(particles[1].x - particles[N].x);
            particles[0].Fy = -k*(particles[1].y - particles[N].y);
            for(int i = 1; i < N-1; i++) {
                particles[i].Fx = -k*(particles[i+1].x - particles[i-1].x);
                particles[i].Fy = -k*(particles[i+1].y - particles[i-1].y);
            }
            particles[N].Fx = -k*(particles[0].x - particles[N-1].x);
            particles[N].Fy = -k*(particles[0].y - particles[N-1].y);
            // Update v
            for(int i = 0; i < N; i++) {
                particles[i].vx = particles_old[i].vx + dt*(particles_old[i].Fx + particles[i].Fx)/m2;
                particles[i].vy = particles_old[i].vy + dt*(particles_old[i].Fy + particles[i].Fy)/m2;
            }
            // Copy new variables to old variables
            for(int i = 0; i < N; i++) {
                particles_old[i].x = particles[i].x;
                particles_old[i].y = particles[i].y;
                particles_old[i].vx = particles[i].vx;
                particles_old[i].vy = particles[i].vy;
                particles_old[i].Fx = particles[i].Fx;
                particles_old[i].Fy = particles[i].Fy;
            }
            // Store variables
            if(j % sampleFreq == 0) {
                for(int i = 0; i < N; i++) {
                    particles_hold[i].add_Xelement( particles[i].x );
                    particles_hold[i].add_Yelement( particles[i].y );
                    particles_hold[i].add_VXelement( particles[i].vx );
                    particles_hold[i].add_VYelement( particles[i].vy );
                }
                sampleCounter += 1;
            }
        }
        //* End of molecular dynamics simulation
    }
    //
    //*
    //

本质上，我试图编写一个txt文件，其中particles_hold元素（从1到N）是列，particles_hold元素的成员（如x（从1至某个值m））是行。