TChain and generating histograms

[ssol]

Hi,
I have a .root file with four TTrees and each of them has a branch with the name ‘InvaraintMass’. I want to make a single histogram with all the four InvariantMass overlaying on each other. I do not know how to make a single histogram from many TTrees.

Kindly help me with this.

Thank you very much!

[Graipher]

What do you mean when you say “overlaying”? Do you need to distinguish which came from which file?

If not, you can just use a TChain:

auto chain = new TChain("tree_name");
chain->Add("file_name1.root");
...
chain->Add("file_nameN.root");

chain->Draw("InvariantMass");

If you do want to be able to distinguish them, you probably want to put them into a THStack:

std::vector<std::string> file_names = {"file_name1.root", ..., "file_nameN.root"};
TFile * f;
TTree * tree;
auto histStack = new THStack();
for (auto file_name : file_names) {
    f = TFile::Open(file_name.c_str());
    tree = (TTree*) f->Get("tree_name");
    tree->Draw("InvariantMass>>h(100, 0, 120)");  // Add correct binning here
    histStack->Add((TH1*) gROOT->FindObject("h"));
}
histStack->Draw("PLC");

[eguiraud]

Hi,
probably something like this would work:

TChain c;
c.Add("file.root/tree1");
c.Add("file.root/tree2");
c.Add("file.root/tree2");
TTreeReader r(&c);
TTreeReaderValue<double> v(r, "InvariantMass");
while (r.Next())
  std::cout << *v << std::endl;