mt102_readNtuplesFillHistosAndFit.C File Reference

Detailed Description

Read n-tuples in distinct workers, fill histograms, merge them and fit.

Knowing that other facilities like TProcessExecutor might be more adequate for this operation, this tutorial complements mc101, reading and merging. We convey another message with this tutorial: the synergy of ROOT and STL algorithms is possible.

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Int_t mt102_readNtuplesFillHistosAndFit()
{

   // No nuisance for batch execution
   gROOT->SetBatch();

   // Perform the operation sequentially ---------------------------------------
   TChain inputChain("multiCore");
   inputChain.Add("mt101_multiCore_*.root");
   TH1F outHisto("outHisto", "Random Numbers", 128, -4, 4);
   inputChain.Draw("r >> outHisto");
   outHisto.Fit("gaus");

   // We now go MT! ------------------------------------------------------------

   // The first, fundamental operation to be performed in order to make ROOT
   // thread-aware.
   ROOT::EnableThreadSafety();

   // We adapt our parallelisation to the number of input files
   const auto nFiles = inputChain.GetListOfFiles()->GetEntries();

   // We define the histograms we'll fill
   std::vector<TH1F> histograms;
   auto workerIDs = ROOT::TSeqI(nFiles);
   histograms.reserve(nFiles);
   for (auto workerID : workerIDs) {
      histograms.emplace_back(TH1F(Form("outHisto_%u", workerID), "Random Numbers", 128, -4, 4));
   }

   // We define our work item
   auto workItem = [&histograms](UInt_t workerID) {
      TFile f(Form("mt101_multiCore_%u.root", workerID));
      TNtuple *ntuple = nullptr;
      f.GetObject("multiCore", ntuple);
      auto &histo = histograms.at(workerID);
      for (auto index : ROOT::TSeqL(ntuple->GetEntriesFast())) {
         ntuple->GetEntry(index);
         histo.Fill(ntuple->GetArgs()[0]);
      }
   };

   TH1F sumHistogram("SumHisto", "Random Numbers", 128, -4, 4);

   // Create the collection which will hold the threads, our "pool"
   std::vector<std::thread> workers;

   // Spawn workers
   // Fill the "pool" with workers
   for (auto workerID : workerIDs) {
      workers.emplace_back(workItem, workerID);
   }

   // Now join them
   for (auto &&worker : workers)
      worker.join();

   // And reduce with a simple lambda
   std::for_each(std::begin(histograms), std::end(histograms),
                 [&sumHistogram](const TH1F &h) { sumHistogram.Add(&h); });

   sumHistogram.Fit("gaus", 0);

   return 0;
}

Date

January 2016

Author

Danilo Piparo

Definition in file mt102_readNtuplesFillHistosAndFit.C.