df011_ROOTDataSource.C

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/// \file
/// \ingroup tutorial_dataframe
/// \notebook -draw
/// This tutorial illustrates how use the RDataFrame in combination with a
/// RDataSource. In this case we use a RRootDS. This data source allows to read
/// a ROOT dataset from a RDataFrame in a different way, not based on the
/// regular RDataFrame code. This allows to perform all sorts of consistency
/// checks and illustrate the usage of the RDataSource in a didactic context.
///
/// \macro_code
/// \macro_image
///
/// \date September 2017
/// \author Danilo Piparo
 
void fill_tree(const char *treeName, const char *fileName)
{
   ROOT::RDataFrame d(10000);
   auto i = 0.;
   d.Define("b1", [&i]() { return i++; }).Snapshot(treeName, fileName);
}
 
using TDS = ROOT::RDF::RDataSource;
 
int df011_ROOTDataSource()
{
   auto fileName = "df011_ROOTDataSources.root";
   auto treeName = "myTree";
   fill_tree(treeName, fileName);
 
   auto d_s = ROOT::RDF::MakeRootDataFrame(treeName, fileName);
 
   /// Now we have a regular RDataFrame: the ingestion of data is delegated to
   /// the RDataSource. At this point everything works as before.
   auto h_s = d_s.Define("x", "1./(b1 + 1.)").Histo1D({"h_s", "h_s", 128, 0, .6}, "x");
 
   /// Now we redo the same with a RDF and we draw the two histograms
   ROOT::RDataFrame d(treeName, fileName);
   auto h = d.Define("x", "1./(b1 + 1.)").Histo1D({"h", "h", 128, 0, .6}, "x");
 
   auto c_s = new TCanvas();
   c_s->SetLogy();
   h_s->DrawClone();
 
   auto c = new TCanvas();
   c->SetLogy();
   h->DrawClone();
 
   return 0;
}