Reading a branch within a branch

ROOT
1

Hi experts,

I’m looking to access some leaf variables saved in a nested set of branches.
The file can be found here: /afs/cern.ch/work/a/afray/public/pool.root

Essentially, I want to access the variables found within CollectionTree->McEventCollection_p5_GEN_EVENT->m_genParticles->

I’m not quite sure how to go about accessing the variables m_px, m_py etc.

I’ve tried the standard routine of:

TFile* inputFile = new TFile("/afs/cern.ch/user/a/afray/public/PhD/SpuriousSignalSyst/JIRA_v2_copy/pool.root", "READ");

TTree* inputTree = (TTree*)inputFile->Get("CollectionTree");
        
Float_t px(0), py(0), pz(0), m(0);
Int_t pdfID(0);
inputTree->SetBranchAddress("McEventCollection_p5_GEN_EVENT/m_genParticles/m_px", &px);

but I receive the error:
‘Error in TTree::SetBranchAddress: unknown branch -> McEventCollection_p5_GEN_EVENT/m_genParticles/m_px’.

I’m aware the SetBranchAddress line is incorrect in formalism (I don’t think you can / separate the branches) but I’m not sure what the correct form should be. Is there a simple way around this?

Cheers,
Antony

2

Hi Anthony,

[quote]I’m aware the SetBranchAddress line is incorrect in formalism (I don’t think you can / separate the branches)[/quote]This is not accurate. Please review the User’s Guide chapter on TTree and reading data out of it or check the result of TTree::MakeClass, MakeSelector and MakeProxy.

"McEventCollection_p5_GEN_EVENT/m_genParticles/m_px"This is unlikely to be the correct branch name. How did you derive this name?

Since there is a trailing ‘s’ in the name of the data member I am guessing this is actually a collection. You will get more than one float per entry. I.e. you need to pass an array rather than a single variable.

Also how did you derive that m_px was of type ‘float’, this is unusual (but possible). We most often see ‘double’.

I am guessing something like this might work for you …

inputTree->SetMakeClass(true); const int kMaxNumberOfGenParticles = .....; Double_t px[kMaxNumberOfGenParticles] inputTree->SetBranchAddress("McEventCollection_p5_GEN_EVENT.m_genParticles.m_px", &(px[0]));

Or you may want to investigate using the TTreeReader instead.

Cheers,
Philippe.

3

Hi Philippe,

Thanks for the suggestions. I’ve tried the following code:

TFile* inputFile = new TFile("/afs/cern.ch/user/a/afray/public/PhD/SpuriousSignalSyst/JIRA_v2_copy/pool.root", "READ");
TTree* inputTree = (TTree*)inputFile->Get("CollectionTree");
inputTree->SetMakeClass(true);
const int kMaxNumberOfGenParticles = 1000;
Double_t px[kMaxNumberOfGenParticles];
inputTree->SetBranchAddress("McEventCollection_p5_GEN_EVENT.m_genParticle.m_px", &(px[0]));

but I receive the error

Error in TTree::SetBranchAddress: unknown branch -> McEventCollection_p5_GEN_EVENT.m_genParticles.m_px

I’m not sure why the branches are named as such. It’s an output file from running a command needed to validate a job option file for an MC production request via JIRA.

But to clarify, the file is structured such that:
CollectionTree (Tree) -> McEventCollection_p5_GEN_EVENT (Branch) -> m_genParticles (SubBranch of previous) -> m_px (Leaf variable of interest).

m_px being a float was purely a guess, using a double in retrospect is a better choice.

I will read up on TTreeReader and look at the User’s Guide TTree section, unless you have any other ideas.

Cheers,
Antony

4

[quote]but I receive the error

Error in TTree::SetBranchAddress: unknown branch -> McEventCollection_p5_GEN_EVENT.m_genParticles.m_px[/quote]The name I used was just a guess. See the result of TTree::Print or TTree::MakeSelector to have a better idea of the actual branch names. (and if m_genParticles is not split, you will need to either use the original shared library or the result of MakeProxy (or the TTreeReader might also work).

Cheers,
Philippe.

5

Did this ever get resolved…? I have the same issue:

a file containing a tree, containing a branch, containing a branch, containing a variable I need.
None of the above seems to have worked…

How does one do this?

6

Please follow up at Accessing a leaf within a branch in a branch