Re: Plotting a tree's current entry vs. its previous entry

Hi Philippe,

Philippe Canal <pcanal_at_fnal.gov> writes:

> The simplest solution is to do the following:
>
> TFile *offsetFile = TFile::Open("offset.root","RECREATE");
> TTree *offseter = tree->CloneTree( 1 ) ; // 1 is the offset
> offsetFile->Write();
>
> TChain *chain = new TChain(tree->GetName());
> chain->Add("offset.root");
> chain->Add(realfilename);
>
> chain->AddFriend(tree);

Thanks. I get what you are saying. This is a nice way to avoid having to duplicate the tree in two files.

In trying to implement this idea there are a few details that may be useful for the record:

• One should add an alias when calling AddFriend(tree,alias) so that subsequent Draw() formula can distinguish between the original and offset trees.
• It's personal preference, but I swapped the chain/tree when adding the friend. Having the chain be the master means comparing one event into the future. Having the tree be the master means comparing one event into the past.
• In my case, my tree's have degenerate names in the file and must be distinguished by their full paths. To handle this, TChain::AddFile() is used instead of Add() as it allows specifying a full path. This path can be fully reconstructed from TTree meta data.

In case anyone finds it useful, attached is the Python module I wrote while testing things.

-Brett.