Read in list of files, TCHain

ROOT
1

Hi all,

so far I read in one file and looped through all events and did my plots.
That works fine.
so like this and then a loop
TFile* f = TFile::Open(file);

TTree* t_truth = (TTree*)f->Get("Truth");
TTree* t_indet = (TTree*)f->Get("InDet");

/* loop over all events */

Now I need to read in say 4 input files (same structure just more events then).
so I tried the above with TChain
TChain* f = new TChain(“test”);
for (std::vector::const_iterator itr = files.begin() ; itr != files.end() ; ++itr)
{
chain.Add(itr); // add filenames to chain
}
TTree t_truth = (TTree*)f->Get(“Truth”);
TTree* t_indet = (TTree*)f->Get(“InDet”);

so “f” is a chain pointer now. That does not work however. I guess TChain is a sort of TTree already but I dont know how to use it now …

Or, is TChain the wrong thing for that prupose?

Thanks a lot!
Andi

2

Hi,
a TChain behaves like a TTree, only the TTree spans over multiple files. See root.cern.ch/root/html/TChain#TChain:TChain on how to use it. In your case you need two chains, one for each TTree, even though both TChains use the same set of files.
Axel.

3

Hi,

damn am I stupid! I was on the page you mentioned before I posted on the forum. But I did not read the line:

TChain ch(“T”); creates a chain to process a Tree called “T”

I called the chain “test” instead of the name of a tree …

Thanks it works now :slight_smile:
Andi

4

Hi,

the problem now is it only works if I add only 1 file to the chain …

When I add a second one it recognises it and shows twice the amount of events but all variables are 0 now …

Giving the two files separately it is ok, but not one after the other.

TChain* t_btag = new TChain(“BTag”);

std::vector<TString>::const_iterator firstFile = files.begin();
for (std::vector<TString>::const_iterator itr = firstFile ; itr != files.end() ; ++itr)
{
  std::cout << "Adding to chain: " << *itr << std::endl;
  if (t_btag!=0) t_btag->Add(*itr);
}

and later the event cycle. So if I add more than 1 file it does not work anymore.

What am I doing wrong now?

Thanks a lot!
Andi

5

Hi,

that might depend on your event loop. If you use SetBranchAddress you’ll have to call it each time the chain loads a new tree. Please post how you’re trying to get the data out of the trees.

Axel.

6

Hi,

I do something like this

t_btag->GetBranch(“nJets”)->SetAddress(&nJets);

and perevent I write:
t_btag->GetEvent(evt);

Cheers
Andi

7

Hi Andi,
Use TChain::SetBranchAddress(“nJets”, &nJets);
Axel.

8

Thanks a lot!
it works now

I have a problem now with histos and mem leak but I will open a separate thread about it.

Cheers
Andi