Allow to skip lines from beginning

[jschueller]

This adds possibility to skip lines from the beginning of the file.
Apparently its faster to use std::ignore instead of std::getline but I could not make it work.

What do you think @d99kris ?

[d99kris]

Hi @jschueller thanks for suggesting a new functionality (and providing a PR). Have you seen the LabelParams argument? It might already provide the functionality you are looking for.

You can use LabelParams(n, -1) which will skip lines until line n (zero-based) and then read the column labels from that line (if there are no labels you'd specify n - 1). For a CSV file with say 3 leading lines to ignore like this:

Open,High,Low,Close,Volume,Adj Close
64.529999,64.800003,64.139999,64.620003,21705200,64.620003
64.419998,64.730003,64.190002,64.620003,20235200,64.620003
64.330002,64.389999,64.050003,64.360001,19259700,64.360001
64.610001,64.949997,64.449997,64.489998,19384900,64.489998
64.470001,64.690002,64.300003,64.620003,21234600,64.620003

You can use n = 3

  rapidcsv::Document doc("examples/colhdr.csv", rapidcsv::LabelParams(3, -1));
  std::vector<float> col = doc.GetColumn<float>("Close");
  std::cout << "Read " << col.size() << " values." << std::endl;

If the data has no column labels you'd use n = 2.

Let me know if this would work or if I am missing some aspect that the PR handles?

There may be some room for improvement in the documentation for this though, I will try to look into that separately.

[jschueller]

indeed this seems to work