ifstream, getline, and istringstream slow performance on big file in c++

Question

I want to ask about my code below: (The code below basically read an input file named inputVelocity.dat with format stated in the code. The code reads with istringstream to pass each value to each particular arrays)

std::ifstream inputVelocity("input/inputVelocity.dat");
std::string lineInputVelocity;
while (std::getline(inputVelocity, lineInputVelocity))  {
  std::istringstream issVelocity(lineInputVelocity);
  double a, b, c, d, e;

  if (!(issVelocity >> a >> b >> c >> d >> e))  {
    std::cout << "ISS ERROR" << std::endl;
  }

  for (int k=0; k<=nz+1; k++)  {
    for (int j=0; j<=ny+1; j++)  {
      for (int i=0; i<=nx+1; i++)  {
        ux[i][j][k]     =   a;
        uy[i][j][k]     =   b;
        uz[i][j][k]     =   c;
        pressure[i][j][k]   =   d;
        temperature[i][j][k]    =   e;
      }
    }
  }
}

inputVelocity.close();

The code is fine when reading around 20000 lines, but when I change the file into around 1.6 million lines, the code run very slow even on a server.

I did std::cout on each getline loop and it read like 5 lines/second, with around 1.6 million lines.

I have found some related questions here but still can't understand what's the problem source and how to fix it. Anyone can help? Thank you.

Answer 1

I changed the code into this:

std::ifstream inputVelocity("input/inputVelocity.dat");
std::string lineInputVelocity;
int getI, getJ, getK;
getI = 0;
getJ = 0;
getK = 0;
while (std::getline(inputVelocity, lineInputVelocity))  {
  std::istringstream issVelocity(lineInputVelocity);
  double a, b, c, d, e;

  if (!(issVelocity >> a >> b >> c >> d >> e))  {
    std::cout << "ISS ERROR" << std::endl;
  }
  std::cout << getI << " " << getJ << " " << getK << std::endl;
  ux[getI][getJ][getK]              =   a;
  uy[getI][getJ][getK]              =   b;
  uz[getI][getJ][getK]              =   c;
  pressure[getI][getJ][getK]        =   d;
  temperature[getI][getJ][getK]     =   e;

  getK = getK + 1;

  if (getK == nz+2)  {
    getJ = getJ + 1;
    getK = 0;
  }

  if (getJ == ny+2)  {
    getI = getI + 1;
    getJ = 0;
  }

}

inputVelocity.close();

And that worked really well :) If anyone has a more efficient solution, I would be very glad to see! :)