How to write multidimensional vector data to a file in C++ - c++

Question

I have a vector of objects obj (of class Holder) with N elements with members like x and y which are also vectors of double type with M elements. I would like to write a text file creating an MxN matrix from this. I have tried lots of different things to no avail up to now.
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;
}
}
}
But this just takes the last set of values. How can I create such an MxN matrix where rows are from the object member vector x and columns are from the object vector itself?
Thank you in advance.
--
The bigger version of the code is as follows:
//
//
#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
}
//
//*
//
Essentially I'm trying to write a txt file where particles_hold elements (from 1 to N) are columns and members of particles_hold elements like x (from 1 to some value M) are rows.

Answer 1

If you mean visually then the way is put endl or "\n" to the outer loop and remove endl from inner loop.But i do not know anythig about your Holder object and if you have [] operator defined there that is the answer.
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";
}
out<< "\n";
}
}

Answer 2

Your method is ok, however, made some minor change so that you have M lines, each lines represent obj[i], i = 0.. M-1. So, each column (jth index) is printed as tab separated in each line
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";
}
out << endl;
}
}