Branch with collection

ROOT
1

Dear ROOT developers and users,

I am using ver4.04 on Linux.
I found that there is a TTree::Branch() fuction to create a branch holding a collection of an object:

Int_t Branch(TCollection *list, Int_t bufsize, Int_t splitlevel, const char *name),

and wondered if this is a more elegant way than using variable length array of simple data types.
This function is also mentioned a little in the users manual
but I could not find any example.
Could some one show me how to use and intended usage of this function?

–
Kame

2

Hi,

if at all possible you should use a TClonesArray of objects, not arrays of simple data types. This is widely documented, e.g. in the Event tutorial.

An “under-documented” method often means it’s not meant to be used if you don’t know what it’s about :slight_smile: The Branch method you’re referring to can be used to create a branch containing a collection (e.g. a TList) of objects. But this is far less powerful than using the regular TClonesArray approach, and it is replaced by more general methods like
template TBranch *Branch(const char *name, T **addobj, Int_t bufsize=32000, Int_t splitlevel=99) (which finds out by itself what kind of object is has to store).

Cheers, Axel.

3

Instead of having a top level class like
class Event {
A *a;
B *b;
C *d; etc
where A, B, C, etc are your objects derived from TObject,
you can built a collection, eg
TList *myevent = new TList();
myevent->Add(a);
myevent->Add(b);
myevent->Add©; etc
and create your top level branch with
tree.Branch(myevent,32000,99);
This is more dynamic than the first solution. In a large collaboration,
people can easily add a new object (or collection of objects)
in the list. There is no need to modify the function creating
the Tree and the branch.

Rene

4

Hi Rere and Axel

Thank you both for the repiles.

I suppose the intended usage of the Branch method is diffrerent from what I expected.
I thought it can be used for writing variable length data event by event.
I tried a code like

struct Track {
  Track() {}
  Track(double ty_, double x_) : tx(tx_), ty(ty_) {}
  Float_t tx, ty;
};

int main()
{
  TFile f("test.root", "recreate");
  
  TObjArray *trks = new TObjArray;
  trks->SetOwner(true);
  TTree tree("tree", "P152 BaseTrack");
  tree.Branch(trks);

  ifstream is("input_data_file");
  int data_length;
  while (is >> data_length) {
    trks.Clear();
    // Read track data with variable length
    Float_t tx, ty;
    for (int i = 0; i < data_length; ++i) { 
      is >> tx >> ty;
      trks->Add(new BaseTrack(tx, ty));
    }
    tree.Fill();
  }
  f.Write();
  return 0;
}

But no data member was accecssible via Tree::Draw() method.

Thank you again.

Kame

5

Proceed as indicated by Axel using a TClonresArray.
see $ROOTSYS/test/MainEvent.cxx

Rene