Write one output file from several input files

ROOT
1

I would like to read in several chains, and then write out selected entries from these different chains to one output file. For example

vector<TChain*> vecChains;
//set up vecChains

TFile out("output.root", "recreate");

//loop over selected entries
{
   vecChains[chain]->GetEntry(entry);
   //write this entry to output.root
}

I’ve tried several permutations of calling CloneTree and Fill trying to write out the selected events, but I either get only events from one chain, or multiple trees in my file, not one tree with entries from each chain.

Any ideas how to do this?

Thanks,
Alan

2

Hi,

The simpliest is to chain all the TChain objects.

vector<TChain*> vecChains; TChain megaChain(treename); //loop over element of vecChains; { megaChain->Add(vecChain[i]); } // use megaChain for the selections/copy etc.
This is, of course, assuming that all the chain in vecChain contains the same type of trees.

Another alternative is to use TTree::CopyAddresses to set the branch on the chain to be copied.

Cheers,
Philippe.

3

The CopyAddresses method seems to do the trick.

Thanks a lot.

Alan