df011_ROOTDataSource.C File Reference

Detailed Description

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.

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;
}

Date

September 2017

Author

Danilo Piparo

Definition in file df011_ROOTDataSource.C.