Using MakeClass and Chains

Hi everyone! I’ve been trying to run a macro over multiple root files. I used MakeClass on one of these trees to generate a template and then tried to change it to read in a Chain. When I try to run the macro though, it says there are zero events and produces empty histograms. I can’t find where I’ve gone wrong though, I’d really appreciate any help! This is my code:

    #define MakeHistograms_cxx
#include "MakeHistograms.h"
#include <TH2.h>
#include <TStyle.h>
#include <TCanvas.h>
#include <iostream>
#include "TFileCollection.h"
#include "THashList.h"
#include "TList.h"
void MakeHistograms::Loop()
  //Stop ROOT opening every canvas as it's created

  //Auto-generated stuff
  //Long64_t nentries = fChain->GetEntriesFast();

  Long64_t nbytes = 0, nb = 0;

  Long64_t N =  fChain->GetEntries();
  //Print the number of events
  std::cout << "Number of entries is " <<  N << std::endl;

  // Create write histogram before creating histograms 

  TFile hfile("hfile.root","RECREATE");
  //Make useful histograms, jet pt and NTrack
  TH1F *h = new TH1F("h", "Jet pT", 100, 50, 2500.);
  TH1F *g = new TH1F("g", "NTrack", 100, 0, 120);

  //Number of pT slices
  const Int_t nHists = 9;

  std::cout << "STAGE 2 Number of entries is " <<  N << std::endl;

  Double_t edges[nHists+1] = {400,500,650,800,1000,1200,1500,2000,3000,6000};

      //Array of histograms
  TH1F *m[nHists];

      //Use these to give the histograms unique names       
  char name[20];
  char title[100];
  char buffer[50];

  for (Int_t i=0;i<nHists;i++) {
    sprintf(title,"m%d pT between %5.0f and %5.0f GeV",i,edges[i],edges[i+1]);
    m[i] = new TH1F(name,title,120,-0.5,119.5);

 std::cout << "STAGE 2 Number of entries is " <<  N << std::endl;

  //Fill useful histograms
  for (Long64_t i = 0; i < N; i++) {
    //cout << i << endl;
    for (unsigned long j = 0; j <jet_pt->size(); j++){
      for(int k=0; k<nHists;k++){
        double minPt = edges[k];
        double maxPt = edges[k+1];
        if (jet_pt->at(j)<maxPt){



  //Print other useful info
  Double_t Min = h->GetMinimum();
  Double_t Max = h->GetMaximum();
  std::cout << "Maximum value is " << Max  << std::endl;
  std::cout << "Minimum value is " << Min << std::endl;


/* To run use root -l, .L MakeHistograms.C, run() */
int run(){

  /*TTree *tree;
  TFile *f = (TFile*)gROOT->GetListOfFiles()->FindObject("T.root");
  if (!f || !f->IsOpen()) {
    f = new TFile("T.root");


  // name of tree is the argument
  TChain *tree = new TChain("outTree");
  // New is a place holder.... 
  TFileCollection fc("New");

  // add the names of the trees to files_signal.txt

   MakeHistograms m(tree);
   return 0;

And I have three files in files_signal.txt containing only the three lines:


Removing everything but the last three lines in run() gives a macro that works fine on a single file. Thanks again for any help! :slight_smile:

Hi @KatR,

So when you print

  std::cout << "Number of entries is " <<  N << std::endl;

It says the chain has zero entries?

Can you try to construct the chain differently? Instead of using AddFileInfoList, can you try adding one by one the files to the chain with:

and check immediately the number of entries?

1 Like

That works!!! Yes, previously that line was returning 0 entries, but making the change to AddFile fixed it. Thank you, I was so stuck with that!