Hi. Is there a simple way of adding a branch to an existing tree (with branches)? I have a branch that has the values of oncoming particle energies, and I would like, based on those, to compute the cross section and include it in a separate branch. I realize I could just create a new tree, copy the branches from the old tree, and include a branch with cross section values…however is there a simpler way to do this?
Thanks!
You can do something like:
void upd() {
TFile *f = new TFile("hs.root","update");
TTree *T = (TTree*)f->Get("ntuple");
float px,py;
float pt;
TBranch *bpt = T->Branch("pt",&pt,"pt/F");
T->SetBranchAddress("px",&px);
T->SetBranchAddress("py",&py);
Long64_t nentries = T->GetEntries();
for (Long64_t i=0;i<nentries;i++) {
T->GetEntry(i);
pt = TMath::Sqrt(px*px+py*py);
bpt->Fill();
}
T->Print();
T->Write();
delete f;
}
Rene
Rene,
Million thanks! It worked.
With one correction, though: px and py need to be defined as Double_t.
Using float at best results in compilation error, and at worst in wrong
values.
of course you must be coherent in the declaration of the variables.
Rene
Rene, I would like to understand better what you mean by being coherent.
The original root file was produced by geant4 code, and the variable which was used in the tree->Branch("",) declaration was a G4double. Are you saying that root’s Double_t corresponds to G4double? Or is this just a general issue of using a 64bit memory for a 64bit number?
By “coherent” I meant that you must use the correct data type declaration. G4Double is “double” or “Double_t”, but not a float, Float_t
Rene
Thanks again!
Hi,
I would like to emphasize the
line in the code at the beginning. He is filling the branch not the tree, I was getting messed up results because I tried to call
tree->Fill();
