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
Author: Enrico Guiraud, Danilo Piparo (CERN), Massimo Tumolo (Politecnico di Torino)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Thursday, March 23, 2023 at 10:44 AM.
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
Draw all canvases
gROOT->GetListOfCanvases()->Draw()