Simple event loop on NanoAOD

simple_loop.cc

//Compile with:
//c++ `root-config --cflags --libs --evelibs` -Wno-unsequenced -O3 code/simple_loop.cc -o simple_loop

#include <iostream>
#include <map>
#include <string>

#include <TFile.h>
#include <TROOT.h>
#include <TH1D.h>
#include <TTreeReader.h>
#include <TTreeReaderValue.h>
#include <TTreeReaderArray.h>
#include <TTreeReaderArray.h>
#include <TStopwatch.h>

using namespace std;

//Do a simple array-based loop on NanoAOD and saves the output to a TFile in out.root
//More info on NanoAOD is located here: https://cms-nanoaod-integration.web.cern.ch/integration/master/mc94X_doc.html
void loop_plain(TTreeReader& reader) {

    reader.Restart();
    
    TStopwatch sw;
    
    const unsigned int nbytes = 0;
   
    //As usual in C++ ROOT, we have to predefine every branch and this can get annoying
    //You can find the list of all branches with description here: https://cms-nanoaod-integration.web.cern.ch/integration/master/mc94X_doc.html
    TTreeReaderValue<unsigned int> nJet(reader, "nJet");
    TTreeReaderArray<float> Jet_pt(reader, "Jet_pt");
    TTreeReaderArray<float> Jet_eta(reader, "Jet_eta");
    TTreeReaderArray<float> Jet_phi(reader, "Jet_phi");
    TTreeReaderArray<float> Jet_mass(reader, "Jet_mass");

    unsigned int nevents = 0;
    TFile out("out.root", "RECREATE");
    TH1D h_sumpt("sumpt", "sumpt", 100, 0, 1000);

    sw.Start();

    while (reader.Next()) {
        double sum_pt = 0.0;

        //Simple numbers like nJet can be accesssed using *nJet 
        for (unsigned int _nJet = 0; _nJet < *nJet; _nJet++) {
            //Arrays can be accessed like this
            const auto pt = Jet_pt[_nJet];
            const auto eta = Jet_eta[_nJet];
            const auto phi = Jet_phi[_nJet];
            const auto mass = Jet_mass[_nJet];
            sum_pt += pt;
        }
        h_sumpt.Fill(sum_pt);
        nevents += 1;
    }

    sw.Stop();
    const auto cpu_time = sw.CpuTime();
    const auto real_time = sw.RealTime();

    const auto speed = (double)nevents / real_time / 1000.0;
    cout << "h_sumpt " << h_sumpt.GetEntries() << " " << h_sumpt.GetMean() << endl;

    cout << "loop_plain " << "cpu_time=" << cpu_time << ",real_time=" << real_time << ",speed=" << speed << " kHz" << endl;

    out.Write();
    out.Close();
}

int main( int argc, char *argv[]) {
    gROOT->SetBatch(true); 
    
    //Check command line arguments
    if (argc != 2) {
        cerr << "simple_loop /path/to/nano/input.root" << endl;
        return 0;
    }

    TFile tf(argv[1]);
    TTreeReader reader("Events", &tf);
 
    //Run the event loop   
    loop_plain(reader);
    return 0;
}