How to Read a Tree


There are 2 ways of reading a Tree and to make selections. Both ways will be illustrated by using the file generated in the previous section How to Create and Fill a Tree.

The simple way via TTree::Draw

Use this function if you want to quickly histogram a few variables with a very simple selection mechanism. The loop on the Tree is automatically done by ROOT.

void Draw(Text_t *varexp, Text_t *selection, Option_t *option,Int_t nevents, Int_t firstevent);

where the parameters have the following meaning:

By default the temporary histogram created is called htemp. If varexp contains >>hnew, the new histogram created is called hnew and it is kept in the current directory. Example:
    tree.Draw("sqrt(x)>>hsqrt","y>0")
will draw sqrt(x) and save the histogram as "hsqrt" in the current directory.

By default, the specified histogram is reset. To continue to append data to an existing histogram, use "+" in front of the histogram name.

    tree.Draw("sqrt(x)>>+hsqrt","y>0")
will not reset hsqrt, but will continue filling.

Look at the tutorial macro ntuple1 for a more detailed example.

The kind of expressions you can specify in this function is restricted on purpose (see class TFormula). Our previous experience with the query processor of PAW indicates that we better restrict this kind of selection mechanism to trivial cases if we want our users to trust their results.

Note that this method does not require access to the original class code, if you only want to perform the kind of operations described above.

The general way

Again, at the difference of PAW, we believe that the ROOT user must be responsible to organize himself the loop on all events (rows) of a Tree. In a loop, in general, one wants to generate many histograms, or correlate variables from one Tree with variables of another Tree, etc. We reproduce below a slightly simplified version of the macro eventb. This macro illustrates:
{
   gROOT->Reset();

//   Connect file generated in $ROOTSYS/test
   TFile f("Event.root");

//   Create an histogram
   TH1F *hnseg = new TH1F("hnseg","Number of segments for selected tracks",5000,0,5000);

//   Start main loop on all events
   TClonesArray *tracks = 0;
   Event *event = new Event();   //object must be created before setting the branch address

   TBranch *bntrack = T.GetBranch("fNtrack");
   TBranch *branch  = T.GetBranch("event");
   branch->SetAddress(&event);
   Int_t nevent = T.GetEntries();
   Int_t nselected = 0;
   for (Int_t i=0;i<nevent;i++) {
      bntrack->GetEvent(i);
      if (event->GetNtrack() > 587)continue;
      nselected++;
      T.GetEvent(i);                  //read complete accepted event in memory
      hnseg->Fill(event->GetNseg());  //Fill histogram with number of segments
      tracks = event->GetTracks();    //get pointer to the TClonesArray object
      tracks->Clear();                //clear it
   }
//  Delete ClonesArray and histogram objects
   event->Finish();

//  Draw the histogram
   hnseg->Draw();
}

This method requires access to the class implementation when one of the branches is an object.

Note that in the above example, the object of class Event must be created via the new operator. It cannot be a static object because a new object will be recreated for each event.

ROOT includes a function TTree::MakeCode to automatically generate the code for a skeleton analysis function.


Rene Brun, Fons Rademakers
Last update 26/04/97 by Rene Brun