How to access each Tree in a TChain

ROOT
1

Dears experts,

I have several trees, one for each sample (backgrounds + signal), I want to be able to put all of them in a chain like the TChain does, but then I want to be able to access independently each tree sample in the .C file, so that in one run I can make separate histogram for backgrounds and signal. For example, in one run I want to be able to:
loop over background 1 -> Fill histogram bkg1 -> save root file 1
loop over background 2 -> Fill histogram bkg2 -> save root file 2
…
loop over background n -> Fill histogram bkgn -> save root file n

As I understood the TChain will add all the samples in one Tree, so I can´t access independently each tree. Does anyone have an idea how I can do that with root?

Best regards

2

If you want to access individual ROOT files, do NOT create a TChain. Simply loop over your files, and analyse one at a time.

3

Dear Coyote,

but can I still make a MakeClass on one tree and then do not specify this tree to be read in the header, but instead to make a loop on all the tree directly in the .C file?

Best

4

If you look at the skeleton source code generated by TTree::MakeClass, you’ll notice that the class constructor has one parameter “(TTree *tree)”. You can use your own “tree” (you open ROOT files yourself and pass a pointer to an existing “tree” to the class constructor).
Of course, you can also “enhance” the source code, adding additional parameters here and there and/or adding class methods (e.g. which would allow you to “open” and “close” files one by one on demand) and so on.

5

Dear Coyote,

  • I was wondering if I couldn´t use the Tree friends as well for that? Is there some constraint? did I misundertood the Tree friends function?
  • Actually I want to know if ROOT provide a code to analyse all the NTuples in parallel so that I do not need to modify the initial code.

Best

6

Well, you can simply run multiple independent ROOT sessions simultaneously (e.g. start one “root” per each ROOT file).
Otherwise, see PROOF and PROOF: Parallel Processing, and in particular PROOF-Lite.
Note: you will need to use TTree::MakeSelector or TTree::MakeProxy (I don’t know which one is preferred) instead of TTree::MakeClass.

7

Dear Coyote,

thank you very much for you help, I managed to make what I needed with one of the idea you proposed.

Best