Update Entry in TTree branch

[cappelt]

Hey everyone,

I’m a bit lost with “simple” root functions. I want to multiply all entries of a branch of a TTree with 1.24. Here is what I tried:

void HNL_modifyEfficiency(const char * fileName = "HNLNtuple.root", const char * treeName = "outTree") {

float eff;

TFile file(fileName,"READ");
TTree *tt = (TTree*)file.Get(treeName);

tt->SetBranchAddress("eff", &eff);

for(int i = 0; i < tt->GetEntries(); i++){
    tt->GetEntry(i);
    eff = eff * 1.24;
    tt->Fill();
}

TFile *ff = new TFile("HNLNtuple_plus1Sigma.root","recreate");
tt->Write();
ff->Close();
}

What am I doing wrong?

Cheers

Christian

[couet]

This question was already asked on this forum, for instance here:

Multiplying a Branch with a coefficient

[quote]But, as I have read somewhere else in the forum, it is impossible (or “not recommended at all”) to do something like Q2 = Q2*A [/quote]It is possible but not recommended since the step to do so are not trivial. [quote]So, I need to find another way to do this, with as few steps as possible. [/quote]Simply create a new tree, with just the branch Q2 and make this new tree a friend of the first tree (oldtree->AddFriend(…) ). Alternatively, Clone the old tree and add a new branch (See $ROOT…

[cappelt]

Thanks for the hint.

just to document it here again how I’ve done it:

TFile file(fileName,"READ");
TTree *inTree = (TTree*)file.Get(treeName);


TFile::Open("HNLNtuple_plus1Sigma.root","RECREATE");
TTree *outTree = new TTree("outTree", "outTree");

inTree -> SetBranchAddress("eff", &eff);

outTree -> Branch("eff", &effNew);

for(int i = 0; i < inTree->GetEntries(); i++){
    inTree->GetEntry(i);
    effNew = eff * 1.24;
    outTree->Fill();
}

outTree->Write();