Reading/Writing to a binary file in C++

Question

I have a large matrix of values that I am trying to store in external memory.

I am writing the matrix to a file and then editing the file using read/write commands one row at a time. Originally, I was reading the matrix from the file using the getLine() method and that was too inefficient in terms of time.

Now, I am trying to write to a binary file using the fstream class read/write commands. My program terminates when it reaches the line:

IOfile.read(reinterpret_cast<char*>(&d1), (n - i) * sizeof(double)); //gets all elements in the row after element i

Why is it not reading the values from the file into vector d1?

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

vector<double> vector_solve(vector <double>& B) {
      vector <double> MI(B.size());
      int n = B.size();

      /* Writing a matrix of values to the file */
      fstream IOfile;
      IOfile.open("test_matrix.bin", ios::in | ios::out | ios::binary);

      if (!IOfile.is_open())
          cout << "Error opening file: test_matrix.bin" << endl;
      else {
          for (int i = 0; i < n; i++)          //rows
               for (int j = 0; j <= n; j++)    //columns
                    if (j == n) //write final column as original vector
                        IOfile.write(reinterpret_cast<char*>(&B[i]), sizeof(double));
                    else {
                        double d1 = 500; //TO-DO: double d1 = (*Mfunc)(i, j, dtMU);
                        IOfile.write(reinterpret_cast<char*>(&d1), sizeof(double));
                   }

        //loop to perform the gauss elimination
        for (int i = 0; i < (n - 1); i++) {
            for (int k = (i + 1); k < n; k++) {
                 vector <double> d1(n + 1); //row1
                 vector <double> d2(n + 1); //row2

                 int SG1 = i * sizeof(double); //file get pointer
                 cout << "SG1 is: " << SG1 << endl;
                 IOfile.seekg(SG1); //Sets the get position to element i
                 cout << "ABOUT TO READ" << endl;
                 IOfile.read(reinterpret_cast<char*>(&d1), (n - i) * sizeof(double)); //gets all elements in the row after element i
                 cout << "d1[0] is equal to: " << d1[0] << endl;
                 cout << "d1[1] is equal to: " << d1[1] << endl;
                 cout << "d1[2] is equal to: " << d1[2] << endl;
                 cout << "d1[3] is equal to: " << d1[3] << endl;

                 int SG2 = k * sizeof(double);
                 IOfile.seekg(SG2); //Sets the get position to element k
                 IOfile.read(reinterpret_cast<char*>(&d2), (n - k) * sizeof(double)); //gets all elements in the row after element k

                 double t = d2[i] / d1[i];

                 for (int j = 0; j <= n; j++) {
                      double temp1 = d1[j];
                      d2[j] = d2[j] - t * temp1;
                 }
                 IOfile.seekp(SG2);
                 IOfile.write(reinterpret_cast<char*>(&d2), sizeof(d2));
            }

            for (int i = (n - 1); i >= 0; i--) {
                 vector <double> d1(n + 1);

                 int SG1 = i * sizeof(double);
                 IOfile.seekg(SG1);
                 IOfile.read(reinterpret_cast<char*>(&d1), (n - i) * sizeof(double));

                 MI[i] = d1[n];
                 for (int j = (i + 1); j < n; j++)
                      if (j != i)
                          MI[i] = MI[i] - d1[j] * MI[j];

                 MI[i] = MI[i] / d1[i];
            }

            IOfile.close();
        }
     }

     return MI;
}

Why is it able to write to the file in binary but not read from it? Any help is appreciated, thanks.

Answer 1

Try

IOfile.read(reinterpret_cast<char*>(d1.data()), (n - i) * sizeof(double));

The address of the vector is different than the underlying array you're trying to write to. This also applies to d2 etc.