Question

I have a research project I’m working on. I am a beginner in C++ and programming in general. I have already made a program that generates interacting particles that move on continuous space as time progresses. The only things my program outputs are the XY coordinates for each particle in each time-step.

I want to visualize my findings, to know if my particles are moving as they should. My professor said that I must use gnuplot. Since I could not find a way to output my data in one file so that gnuplot would recognize it, I thought of the following strategy:

a) For each time-step generate one file with XY coordinates of the form “output_#.dat“.

b) Generate a .png file for each one of them in gnuplot.

c) Make a movie of the moving particles with all the .png files.

I am going to worry about b and c later, but up to now, I am able to output all my data in one file using this code:

    void main()
{
    int i = 0;
    int t = 0; // time
    int j = 0; 
    int ctr = 0;
    double sumX = 0;
    double sumY = 0;
    srand(time(NULL)); // give system time as seed to random generator


    Cell* particles[maxSize]; // create array of pointers of type Cell.

    for(i=0; i<maxSize; i++) 
    {
        particles[i] = new Cell(); // initialize in memory
        particles[i]->InitPos(); // give initial positions
    }

    FILE *newFile = fopen("output_00.dat","w"); // print initial positions
    for(i=0; i<maxSize; i++)
    {
        fprintf(newFile, "%f %3 ", particles[i]->getCurrX());
        fprintf(newFile, "%f %3 \n", particles[i]->getCurrY());
    }
    fclose(newFile);

    FILE *output = fopen("output_01.dat","w");
    for(t = 0; t < tMax; t++)
    {
        fprintf(output, "%d ", t);
        for(i=0; i<maxSize; i++) // for every cell
        {
            sumX = 0;
            sumY = 0;
            for(j=0; j<maxSize; j++) // for all surrounding cells
            {
                sumX += particles[i]->ForceX(particles[i], particles[j]);
                sumY += particles[i]->ForceY(particles[i], particles[j]);
            }
            particles[i]->setVelX(particles[i]->getPrevVelX() + (sumX)*dt); // update speed
            particles[i]->setVelY(particles[i]->getPrevVelY() + (sumY)*dt);
            particles[i]->setCurrX(particles[i]->getPrevX() + (particles[i]->getVelX())*dt); // update position
            particles[i]->setCurrY(particles[i]->getPrevY() + (particles[i]->getVelY())*dt);

            fprintf(output, " ");
            fprintf(output, "%f %3 ", particles[i]->getCurrX());
            fprintf(output, "%f %3 \n", particles[i]->getCurrY());
        }
    }
    fclose(output);
}

This indeed generates 2 files, output_00.dat and output01.dat with the first one containing the initial randomly generated positions and the second one containing all my results.

I can feel that in the nested for loop, where I’m updating the speed and position for the XY coordinates, I can have a FILE* that will store the coordinates for each time step and then close it, before incrementing time. In that way, I will not need multiple pointers to be open at the same time. At least that is my intuition.

I do not know how to generate incrementing filenames. I have stumbled upon ofstream, but I don’t understand how it works…

I think what I would like my program to do at this point is:

1) Generate a new file name, using a base name and the current loop counter value.

2) Open that file.

3) Write the coordinates for that time-step.

4) Close the file.

5) Repeat.

Any help will be greatly appreciated. Thank you for your time!

Answer 1

Using ofstream instead of fopen would be a better use of the C++ standard library, whereas now you are using C standard library calls, but there is nothing wrong per se with what you are doing doing now.

It seems like your real core question is how to generate a filename from an integer so you can use it in a loop:

Here is one way:

// Include these somewhere
#include <string>
#include <sstream>

// Define this function
std::string make_output_filename(size_t index) {
   std::ostringstream ss;
   ss << "output_" << index << ".dat";
   return ss.str();
}

// Use that function with fopen in this way:
for (size_t output_file_number=0; /* rest of your for loop stuff */)  {
  FILE *file = fopen(make_output_filename(output_file_number).c_str(), "w");
  /* use the file */
  fclose(file);
}

This uses a std::ostringstream” to build a filename using stream operations, and returns the built std::string. When you pass it to fopen, you have to give it a const char * rather than a std::string, so we use the .c_str() member which exists just for this purpose.