Problem merging trees

ROOT
1

Dear all,

I would like to merge TTrees from different files. The code snippet below works fine. In my actual script, however, I am looping over several hundred files. Therefore I would like to include a f->Close() statement. This, however, kills the TTree in the file and I get a SegFault when trying to merge the TTrees. In order to avoid this I added the tree->SetDirectory(0) statement. The script runs without any errors but the output TTree has very weird entries (they are basically all the same). What can I do to fix this?

Thank you very much in advance!

P.S.: I cannot attach the root files because they are too large, if someone is interested I can maybe find a solution in order to provide them.

// ROOT
#include <TFile.h>
#include <TTree.h>
#include <TCanvas.h>
#include <iostream>
#include <fstream>

void merge_trees()  {


  TFile *f;
  TList * trees = new TList();
  TTree *tree = new TTree();

  f = new TFile("events_0.root");
  tree = (TTree*)f->Get("ParTree_PMBgMaker_Off");
  //tree->SetDirectory(0); 
  tree->GetEntry(0);  
  trees->AddLast(tree);
  // f->Close();

  f = new TFile("events_1.root");
  tree = (TTree*)f->Get("ParTree_PMBgMaker_Off");
  //tree->SetDirectory(0); 
  tree->GetEntry(0);  
  trees->AddLast(tree);
  //f->Close();  

  cout << "After loop" << endl;
  
  cout << "Found " << trees->GetSize() << " trees" << endl;
  

  TFile *out = new TFile("out.root","RECREATE");

  TTree *newtree = tree->CloneTree(0);

  newtree->SetName("test");
  newtree->Merge(trees);
  newtree->Draw("HillasLength[4]");
  
  
  return;
  
}
2

Hi,

Did you try to use TTree::MergeTrees? For example, in your code:

TTree *newtree = TTree::MergeTrees(trees);
Cheers, Bertrand.

3

Hi,

But you can not. Each of the TTree still need to access its data from inside the file (unless you explicitly load all of it in memory … but you are extremely unlikely to have enough memory).

[quote] In order to avoid this I added the tree->SetDirectory(0) statement. [/quote]This explicitly detach the TTree from its file and thus its data …

In order to do the merge you will have to process only a few files at the time.

Note that all of this (and more) is handled by TFileMerger are the command line executable ‘hadd’.

Cheers,
Philippe.

4

Hello,

thank you very much for your answers.

Yes, that did not even give the correct number of entries in the merged tree

Alright, thank you - I am not 100% aware how the data in TTrees is handled internally.

Cheers!