doc/v608/mp102__readNtuplesFillHistosAndFit_8C.html

/// \date January 2016
Int_t mp102_readNtuplesFillHistosAndFit()
{
   // No nuisance for batch execution
   gROOT->SetBatch();
   //---------------------------------------
   // Perform the operation sequentially
   TChain inputChain("multiCore");
   inputChain.Add("mp101_multiCore_*.root");
   TH1F outHisto("outHisto", "Random Numbers", 128, -4, 4);
   inputChain.Draw("r >> outHisto");
   outHisto.Fit("gaus");
   //---------------------------------------
   // We now go MP!
   // TProcessExecutor offers an interface to directly process trees and chains without
   // the need for the user to go through the low level implementation of a
   // map-reduce.
   // We adapt our parallelisation to the number of input files
   const auto nFiles = inputChain.GetListOfFiles()->GetEntries();
   // This is the function invoked during the processing of the trees.
   auto workItem = [](TTreeReader & reader) {
      TTreeReaderValue<Float_t> randomRV(reader, "r");
      auto partialHisto = new TH1F("outHistoMP", "Random Numbers", 128, -4, 4);
      while (reader.Next()) {
         partialHisto->Fill(*randomRV);
      }
      return partialHisto;
   };
   // Create the pool of processes
   ROOT::TProcessExecutor workers(nFiles);
   // Process the TChain
   auto sumHistogram = workers.ProcTree(inputChain, workItem, "multiCore");
   sumHistogram->Fit("gaus", 0);
   return 0;
}