df023_aggregate.C File Reference

Detailed Description

Use the Aggregate action to specify arbitrary data aggregations.

This tutorial shows how to use the Aggregate action to evaluate the product of all the elements of a column. This operation may be performed using a Reduce action, however aggregate is used for the sake of the tutorial

 
void df023_aggregate()
{
 
   // Column to be aggregated
   const std::string columnName = "x";
 
   ROOT::EnableImplicitMT(2);
   auto rdf = ROOT::RDataFrame(5);
   auto d = rdf.Define(columnName, "rdfentry_ + 1.");
 
   // Aggregator function. It receives an accumulator (acc) and a column value (x). The variable acc is shared among the
   // calls, so the function has to specify how the value has to be aggregated in the accumulator.
   auto aggregator = [](double acc, double x) { return acc * x; };
 
   // If multithread is enabled, the aggregator function will be called by more threads and will produce a vector of
   // partial accumulators. The merger function performs the final aggregation of these partial results.
   auto merger = [](std::vector<double> &accumulators) {
      auto size = accumulators.size();
      for (int i = 1; i < size; ++i) {
         accumulators[0] *= accumulators[i];
      }
   };
 
   // The accumulator is initialized at this value by every thread.
   double initValue = 1.;
 
   // Multiplies all elements of the column "x"
   auto result = d.Aggregate(aggregator, merger, columnName, initValue);
 
   std::cout << *result << std::endl;
}

 
120

Date

July 2018

Authors

Enrico Guiraud, Danilo Piparo (CERN), Massimo Tumolo (Politecnico di Torino)

Definition in file df023_aggregate.C.