Storing Data From C++ Program to a file.

In summary, the speaker is using a verlet algorithm in c++ to model the motion of a satellite and wants to output position and velocity data from each timestep. They have encountered issues with creating and storing the data in a file and are unsure about adding new data without erasing previous data. They are seeking advice on how to achieve this and have received suggestions to open the file at the start of the run and write data every 10 time steps to avoid excess data. They have successfully stored time, speed, and altitude in one file and are now trying to open it as a three column array in MATLAB to plot graphs.
  • #1
engineer_ja
16
0
I am using a verlet algorithm in c++ to model the motion of a satellite, and want to output position and velocity data from each timestep so that I can then read it into MATLAB and plot graphs to see how well my model matches the 'real life' graphs. so far I have at the end of each iteration:

Code:
    ofstream fout; bool ok;
    
    ok = false;
    do{
        fout.clear();
        fout.open("height_data.dat");
        if (fout.good()) ok = true;
    }while (!ok);
    
    fout<<height<<endl; //height is the variable with current position to output and store
    
    fout.close();
As far as I have been taught, this should create the file height_data.dat in the same location as the c++program, but it doesn't.
I am also unsure about adding new data at each timestep without erasing previous data, and wondered if it is possible to store time, height and velocity all in one file to use with MATLAB?

Any hints on if and how I can achieve this would be greatly appreciated.
Thanks in advance
:)
 
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  • #2
engineer_ja said:
As far as I have been taught, this should create the file height_data.dat in the same location as the c++program

More like it should create a file in a current directory - whichever it is for your program. Doesn't have to be neither directory where the source is nor where the executable is. Details will depend on OS and configuration.
 
  • #3
engineer_ja said:
I am using a verlet algorithm in c++ to model the motion of a satellite, and want to output position and velocity data from each timestep so that I can then read it into MATLAB and plot graphs to see how well my model matches the 'real life' graphs. so far I have at the end of each iteration:

Code:
    ofstream fout; bool ok;
    
    ok = false;
    do{
        fout.clear();
        fout.open("height_data.dat");
        if (fout.good()) ok = true;
    }while (!ok);
    
    fout<<height<<endl; //height is the variable with current position to output and store
    
    fout.close();

You don't want to do this for a number of reasons.

One problem is the call to open. The version of the open call you are using will blow away the previous version of the file. You will get a file containing one entry for the very last step. This can be fixed by opening the file in append mode, but that isn't the right solution.

Opening and closing files are very expensive operations. Formatted output is also expensive, but not near as expensive as opening and closing. Doing this every time step turns your fast but inaccurate verlet method into a ridiculously slow (but still inaccurate) technique. What you should be doing instead is to open the file at the start of your run, writing to it occasionally (more on this later), and closing it when finished.

How often should you record? Not every step, particularly so for something like verlet. Verlet is a second order technique. To get anything close to reasonable accuracy you need to take very, very small time steps with verlet. If, for example, you are simulating an object that is circularly orbiting some other object, you need to have the time steps be such that the object moves about 1/1000 of a degree per step. Make your time steps smaller and you will start running into truncation error problems. Make your time steps bigger and you will start running into limitations of the verlet method itself. If you record every step you will have 360,000 or so points per orbit. You don't need anything close to this number of points. If you instead add an entry to the file for every ten or even 100 integration steps you will still see everything you need to see in a graph, and your program will run a whole lot faster.

Now for some individual snippets:

Code:
    ok = false;
    do{
        fout.clear();
        fout.open("height_data.dat");
        if (fout.good()) ok = true;
    }while (!ok);

Why bother with the loop? If you don't open the file successfully the first time around, what makes you think it will work the second, or third, or thousandth time around? Some errors are fatal. There is no recovery from them. Failing to open a key output file is one of those fatal kinds of errors. There's no point in continuing the run if you can't record the results, and there's not much you can do to recover from the error. You are doing nothing to recover from the error other than retrying the same command over and over (and over and over). It won't work. It's much easier to write an error message to standard output and exit the program if the open call doesn't work.
 
  • #4
OK, Thanks for your help.

I have managed to locate and change the working directory, so have now found my file.

This is just a one-off data collection function that I'm adding in, so I'm not to worried about it slowing the program down, but have altered it to work every 10 time steps to avoid an excess of data (I also replaced the loop with an error code).

By using the open in append mode, I have successfully stored time, speed and altitude in one file; all three on one line separated at a set width, with the next time on a new line. I'm now going to try and open this as a three column array in Matlab to plot the graphs.

Thanks.
 
  • #5


Hello,

Thank you for sharing your progress in using the verlet algorithm in C++ to model satellite motion. It's great that you are looking to store and analyze your data in MATLAB.

Based on your code, it seems like you are on the right track in terms of writing data to a file using the ofstream class. However, there are a few things you may want to consider to improve the functionality and efficiency of your code.

Firstly, instead of using a do-while loop to check if the file is open, you can use the good() function directly in the if statement. This will save you from having to use a separate boolean variable and a do-while loop.

Additionally, you may want to consider using the append mode when opening the file, so that new data is added to the end of the file instead of overwriting the existing data. This can be done by passing the ofstream::app flag as the second argument in the open() function.

In terms of storing multiple variables in the same file, you can use the << operator to write multiple values to the file in one line. For example, you can write:

fout << time << " " << height << " " << velocity << endl;

This will write the values of time, height, and velocity separated by a space in the file.

Finally, to ensure that your file is created in the desired location, you can specify the full path in the open() function instead of just the file name. For example:

fout.open("C:/Users/username/Documents/height_data.dat");

I hope these suggestions are helpful in achieving your goal of storing and analyzing data from your C++ program in MATLAB. Good luck with your project!
 

1. How do I store data from a C++ program to a file?

The process of storing data from a C++ program to a file involves opening the file, writing the data to the file, and then closing the file. This can be done using the ofstream class from the fstream library.

2. What is the best way to organize data in a file from a C++ program?

The best way to organize data in a file from a C++ program is to use a structured format such as CSV (comma-separated values) or JSON (JavaScript Object Notation). This will make it easier to read and manipulate the data later on.

3. Can I append data to an existing file from a C++ program?

Yes, you can append data to an existing file from a C++ program by opening the file in ofstream mode and using the ios::app flag. This will allow you to add new data to the end of the file without overwriting the existing data.

4. How do I handle errors when storing data to a file from a C++ program?

You can handle errors when storing data to a file from a C++ program by checking the status of the file after each operation. This can be done using the fail() or bad() functions of the ofstream class. You can also use try-catch blocks to handle exceptions that may occur.

5. Is it possible to encrypt data before storing it to a file from a C++ program?

Yes, it is possible to encrypt data before storing it to a file from a C++ program. You can use libraries such as OpenSSL or Crypto++ to implement encryption algorithms like AES or RSA. This will ensure that your data is secure and cannot be easily accessed by unauthorized users.

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