df025_RNode.C File Reference

Detailed Description

Manipulate RDF objects in functions, loops and conditional branches.

Each RDataFrame object has its own type. It helps with performance, but sometimes it gets in the way of writing simple code that manages RDF objects. Luckily, every RDF object can be converted to the generic RNode type. This tutorial shows how to take advantage of RNode to easily manipulate RDataFrames.

 
/// A generic function that takes an RDF object and applies a string filter
ROOT::RDF::RNode AddFilter(ROOT::RDF::RNode node, string_view filterStr)
{
   return node.Filter(filterStr);
}
 
void df025_RNode()
{
   ROOT::RDataFrame df(8);
 
   // Using the generic AddFilter helper function defined above: RNode in, RNode out
   auto f1 = AddFilter(df, "rdfentry_ > 0");
   auto f2 = f1.Filter([](ULong64_t e) { return e > 1; }, {"rdfentry_"});
 
   // Conditionally applying a filter is simple with ROOT::RDF::RNode
   bool someCondition = true;
   auto maybe_filtered = ROOT::RDF::RNode(f2);
   if (someCondition)
      maybe_filtered = maybe_filtered.Filter("rdfentry_ > 3");
 
   // Adding new columns with Define in a loop is simple thanks to ROOT::RDF::RNode
   auto with_columns = ROOT::RDF::RNode(maybe_filtered);
   for (auto i = 0; i < 3; ++i)
      with_columns = with_columns.Define("x" + std::to_string(i), "42");
 
   // RNodes can be used exactly like any other RDF object
   std::cout << "Entries passing the selection: " << with_columns.Count().GetValue() << std::endl;
}

 
Entries passing the selection: 4

Date

June 2020

Authors

Danilo Piparo, Enrico Guiraud (CERN)

Definition in file df025_RNode.C.