Question

I am trying to parse a simple CSV file, with data in a format such as:

20.5,20.5,20.5,0.794145,4.05286,0.792519,1
20.5,30.5,20.5,0.753669,3.91888,0.749897,1
20.5,40.5,20.5,0.701055,3.80348,0.695326,1

So, a very simple and fixed format file. I am storing each column of this data into a STL vector. As such I’ve tried to stay the C++ way using the standard library, and my implementation within a loop looks something like:

string field;
getline(file,line);
stringstream ssline(line);

getline( ssline, field, ',' );
stringstream fs1(field);
fs1 >> cent_x.at(n);

getline( ssline, field, ',' );
stringstream fs2(field);
fs2 >> cent_y.at(n);

getline( ssline, field, ',' );
stringstream fs3(field);
fs3 >> cent_z.at(n);

getline( ssline, field, ',' );
stringstream fs4(field);
fs4 >> u.at(n);

getline( ssline, field, ',' );
stringstream fs5(field);
fs5 >> v.at(n);

getline( ssline, field, ',' );
stringstream fs6(field);
fs6 >> w.at(n);

The problem is, this is extremely slow (there are over 1 million rows per data file), and seems to me to be a bit inelegant. Is there a faster approach using the standard library, or should I just use stdio functions? It seems to me this entire code block would reduce to a single fscanf call.

Thanks in advance!

Answer 1

Using 7 string streams when you can do it with just one sure doesn’t help wrt. performance.
Try this instead:

string line;
getline(file, line);

istringstream ss(line);  // note we use istringstream, we don't need the o part of stringstream

char c1, c2, c3, c4, c5;  // to eat the commas

ss >> cent_x.at(n) >> c1 >>
      cent_y.at(n) >> c2 >>
      cent_z.at(n) >> c3 >>
      u.at(n) >> c4 >>
      v.at(n) >> c5 >>
      w.at(n);

If you know the number of lines in the file, you can resize the vectors prior to reading and then use operator[] instead of at(). This way you avoid bounds checking and thus gain a little performance.