Re: TTrees of TTrees..?

Rene Brun (Rene.Brun@cern.ch)
Thu, 30 Oct 1997 09:13:41 +0100

William J. Deninger wrote:
>
> Hello,
>
> I'm trying to organize positron track data from E821 at BNL and have a
> question concerning the use of trees. The data is naturally divided
> into the hiarchial levels: run number, record number, fill number,
> straws hit, and finally tracks. My question is, how do I organize a
> tree such that the data is divided into the following?
>
> ROOT
> /\
> / \
> run 1 run 2 ....
> /\
> / \
> rec 1 rec 2 ...
> /\
> / \
> fill 1 fill 2 ....
> /\
> / \
> track 1 track 2 ....
> /\
> / \
> (raw) data1 data2 ..
>
> To I want to make several trees and enter them into the branches? For
> example, the first tree comprises the ROOT, the branches of the root
> contain several trees for each record, whose branches contain trees for
> fills, etc. etc. etc... , all the way to the data which is contained in
> branches themselves? Or should the raw data be kept in a TClonesArray,
> but everything else is fine? What is the best way to approach this
> organization, and how is it booked? Is it booked from Root down, or from
> data up?

Look into the example provided in $ROOTSYS/test/Event.

I suggest the following Event model:
class Event: public TObject {
EventHeader header;
RawData *rawdata;
TClonesArray *tracks;
}

with:
class EventHeader {
Int_t run;
Int_t record;
Int_t fill;
Int_t event_number;
...
}

class RawData : public TObject {
TarrayI rawTracker; // arrays of Int_t (or floats, shorts,etc..
TArrayI rawCalo; // with your original raw data
TArrayI rawMuon; // This can be a more sophisticated model !!
..
}

class Track : public TObject {
Float_t px;
Float_t py;
...
}

// Create one file per run/rec/fill
{
char filename[64];
sprintf(filename,"run%i_rec%i_fill%i",run,rec,fill);
TFile dst(filename,"recreate")
TTree T("T",filename);
EVent *event = new Event();
T.Branch("event","Event",&event,8000,1);

// Read raw data file and fill tree
while(..) {
event->ReadRawData();
event->Reconstruct();
T.Fill();
}
T.Write();
}

In this example, ROOT will create a Tree automatically splitting
the event into the following branches:
- one branch for each data member of EventHeader
- one single branch for the raw data. You can structure this part
to force one branch per detector component.
- one superbranch for the tracks. This superbranch will have one
branch
for each data member of Track.

With this structure, when you process the Tree in a Root session:
Root > TFile dst("....);
Root > T.Draw("event_number"); // reads only event_number branch
Root > T.Draw("px"); // reads only subbranch px

You can group many data bases into a logical chain)
TChain rec25("rec25","T");
rec25.Add("dst_run1_rec25_fill1");
rec25.Add("dst_run1_rec25_fill2");
rec25.Add("dst_run1_rec25_fill3");
rec25.Draw("px"); will sequentially process all 3 files

More details are given in the $ROOTSYS/test/Event example.

Rene Brun