Write a table (of doubles) to binary file IO C++

Question

I need to write a matrix with four columns ("g_Grid.r", "g_Grid.t", "g_Grid.b", "g_Grid.ph")

Normally, I write to file using file stream:

ofstream fout; 
fout.open("GRID.dat");
 for (int m=0;m<N_PH;++m)
    {   
    for (int k=0;k<N_B;++k)
        {
          for (int j=0;j<N_T;++j)
            {   
            for (int i=0;i<N_R;++i)
                {
                  fout << setprecision(32) << g_Grid.r[i]<<" "<<g_Grid.t[j]<<" "<<g_Grid.b[k]<<" "<<g_Grid.ph[m]<< endl;

                }
            }
        }
    }
 fout.close();

It works fine, but now I'm dealing very large (long) matrix and it takes ages to write the formatted output (".txt"). Since I really do NOT need the file to be formatted, it prefer to write it as a binary file.

QUESTION: given four vectors/arrays (r,t,b,ph), how to write a table to binary file? (I use matlab to read this file later, and also need to read it as a binary table)

EDIT

Normally, I use the following simple code to write binary file:

ofstream myFile (fileName, ios::out | ios::binary);
double val;
for (int m=0;m<N_PH;++m)
    {   
    for (int k=0;k<N_B;++k)
        {
          for (int j=0;j<N_T;++j)
            {   
            for (int i=0;i<N_R;++i)
                {
                  val = g_N.Amp[m][k][j][i];
                  myFile.write(reinterpret_cast<const char*>(&val), sizeof(val));
                }
            }
        }
    }
myFile.close();

But this one will yield only "one-column" while it will be read by matlab later.

Answer 1

To write binary data into a file, you first need to open the file in binary mode, you need to use fstream::write. It takes a char * argument, and writes a number of bytes from that - the char * argument requires a cast if the data is not an array of char or pointer to char.

So as to avoid having several calls to write, it's better to stuff the data into a structure.

So, the following should do what you need:

ofstream fout; 
fout.open("GRID.dat", ios::binary);
 for (int m=0;m<N_PH;++m)
    {   
    for (int k=0;k<N_B;++k)
        {
          for (int j=0;j<N_T;++j)
            {   
            for (int i=0;i<N_R;++i)
                {
                  struct X
                  {
                      double a, b, c, d;
                  } x;

                  x.a = g_Grid.r[i];
                  x.b = g_Grid.t[j];
                  x.c = g_Grid.b[k];
                  x.d = g_Grid.ph[m];
                  fout.write(reinterpret_cast<char *>(&x), sizeof(x));
                }
            }
        }
    }
 fout.close();