Fast, Simple CSV Parsing in C++

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 ov…

Accepted Answer

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.

Fast, Simple CSV Parsing in C++

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