Get Data by TChain

[anwseris42]

Hi Rooters,

I tried to get data from many root files by using TChain but I failed and can’t figure out how to do.

First I write :

Chain ch("T");
ch.Add("/data/*.root");

Then should I use TFile::Open? By a loop?
And how should I use GetEntry?

Thanks!

[eguiraud]

Hi,
you have to pass ch to the TTreeReader constructor:

TTreeReader r(&ch);
TTreeReaderValue<int> x(r, "x"); // proxy to the `x` column
while(r.Next())
  std::cout << *x  << std::endl; // print all values of x

If you have access to ROOT v6.10, you might want to also take a look at TDataFrame for a higher level interface to read and manipulate ROOT data.