// ## Preparation
// A simple helper function to fill a test tree: this makes the example
// stand-alone.
void fill_tree(const char *treeName, const char *fileName)
{
   ROOT::RDataFrame d(10);
   int i(0);
   d.Define("Xic_p_ProbNNp", [&i]() { return (double)i; })
      .Define("Xic_p_ProbNNpr",
              [&i]() {
                 auto j = i * i;
                 ++i;
                 return j;
              })
      .Snapshot(treeName, fileName);
}
int df001_modified()
{
   // We prepare an input tree to run on
   auto fileName = "df001_introduction.root";
   auto treeName = "myTree";
   fill_tree(treeName, fileName);
   ROOT::RDataFrame d(treeName, fileName, {"Xic_p_ProbNNp","Xic_p_ProbNNpr"});
   const auto cns = d.GetColumnNames();
   for(const auto& cn : cns)
       std::cout << cn.data() << std::endl;
   auto entries2 = d.Filter("Xic_p_ProbNNp < 5. && Xic_p_ProbNNpr < 5.").Count();
   std::cout << *entries2 << " entries passed the string filter" << std::endl;
   return 0;
}