tree105_tree.C

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/// \file
/// \ingroup tutorial_tree
/// \notebook
/// This example illustrates how to make a Tree from variables or arrays
/// in a C struct - without a dictionary, by creating the branches for
/// builtin types (int, float, double) and arrays explicitly.
/// See tree106_tree.C for the same example using a class with dictionary
/// instead of a C-struct.
///
/// In this example, we are mapping a C struct to one of the Geant3
/// common blocks /gctrak/. In the real life, this common will be filled
/// by Geant3 at each step and only the Tree Fill function should be called.
/// The example emulates the Geant3 step routines.
///
/// to run the example, do:
/// ~~~
/// .x tree105_tree.C   to execute with the Cling interpreter
/// .x tree105_tree.C++ to execute with native compiler
/// ~~~
/// \macro_code
///
/// \author Rene Brun
 
#include "TFile.h"
#include "TTree.h"
#include "TH2.h"
#include "TRandom.h"
#include "TCanvas.h"
#include "TMath.h"
 
const Int_t MAXMEC = 30;
 
typedef struct {
   Float_t  vect[7];
   Float_t  getot;
   Float_t  gekin;
   Float_t  vout[7];
   Int_t    nmec;
   Int_t    lmec[MAXMEC];
   Int_t    namec[MAXMEC];
   Int_t    nstep;
   Int_t    pid;
   Float_t  destep;
   Float_t  destel;
   Float_t  safety;
   Float_t  sleng;
   Float_t  step;
   Float_t  snext;
   Float_t  sfield;
   Float_t  tofg;
   Float_t  gekrat;
   Float_t  upwght;
} Gctrak_t;
 
 
void helixStep(Float_t step, Float_t *vect, Float_t *vout)
{
   // extrapolate track in constant field
   Float_t field = 20;      // magnetic field in kilogauss
   enum Evect {kX, kY, kZ, kPX, kPY, kPZ, kPP};
   vout[kPP] = vect[kPP];
   Float_t h4    = field*2.99792e-4;
   Float_t rho   = -h4/vect[kPP];
   Float_t tet   = rho*step;
   Float_t tsint = tet*tet/6;
   Float_t sintt = 1 - tsint;
   Float_t sint  = tet*sintt;
   Float_t cos1t = tet/2;
   Float_t f1 = step*sintt;
   Float_t f2 = step*cos1t;
   Float_t f3 = step*tsint*vect[kPZ];
   Float_t f4 = -tet*cos1t;
   Float_t f5 = sint;
   Float_t f6 = tet*cos1t*vect[kPZ];
   vout[kX]   = vect[kX]  + (f1*vect[kPX] - f2*vect[kPY]);
   vout[kY]   = vect[kY]  + (f1*vect[kPY] + f2*vect[kPX]);
   vout[kZ]   = vect[kZ]  + (f1*vect[kPZ] + f3);
   vout[kPX]  = vect[kPX] + (f4*vect[kPX] - f5*vect[kPY]);
   vout[kPY]  = vect[kPY] + (f4*vect[kPY] + f5*vect[kPX]);
   vout[kPZ]  = vect[kPZ] + (f4*vect[kPZ] + f6);
}
 
void tree105_write()
{
   // create a Tree file tree105.root
 
   // create the file, the Tree and a few branches with
   // a subset of gctrak
   TFile f("tree105.root", "recreate");
   TTree t2("t2", "a Tree with data from a fake Geant3");
   Gctrak_t gstep;
   t2.Branch("vect", gstep.vect, "vect[7]/F");
   t2.Branch("getot", &gstep.getot);
   t2.Branch("gekin", &gstep.gekin);
   t2.Branch("nmec", &gstep.nmec);
   t2.Branch("lmec", gstep.lmec, "lmec[nmec]/I");
   t2.Branch("destep", &gstep.destep);
   t2.Branch("pid", &gstep.pid);
 
   // Initialize particle parameters at first point
   Float_t px, py, pz, p, charge = 0;
   Float_t vout[7];
   Float_t mass  = 0.137;
   Bool_t newParticle = kTRUE;
   gstep.step    = 0.1;
   gstep.destep  = 0;
   gstep.nmec    = 0;
   gstep.pid     = 0;
 
   // transport particles
   for (Int_t i=0; i<10000; i++) {
      // generate a new particle if necessary
      if (newParticle) {
         px = gRandom->Gaus(0, .02);
         py = gRandom->Gaus(0, .02);
         pz = gRandom->Gaus(0, .02);
         p  = TMath::Sqrt(px * px + py *py + pz * pz);
         charge = 1;
         if (gRandom->Rndm() < 0.5)
            charge = -1;
         gstep.pid    += 1;
         gstep.vect[0] = 0;
         gstep.vect[1] = 0;
         gstep.vect[2] = 0;
         gstep.vect[3] = px / p;
         gstep.vect[4] = py / p;
         gstep.vect[5] = pz / p;
         gstep.vect[6] = p * charge;
         gstep.getot   = TMath::Sqrt(p * p + mass * mass);
         gstep.gekin   = gstep.getot - mass;
         newParticle = kFALSE;
      }
 
      // fill the Tree with current step parameters
      t2.Fill();
 
      // transport particle in magnetic field
      helixStep(gstep.step, gstep.vect, vout); // make one step
 
      // apply energy loss
      gstep.destep  = gstep.step*gRandom->Gaus(0.0002, 0.00001);
      gstep.gekin  -= gstep.destep;
      gstep.getot   = gstep.gekin + mass;
      gstep.vect[6] = charge*TMath::Sqrt(gstep.getot * gstep.getot - mass * mass);
      gstep.vect[0] = vout[0];
      gstep.vect[1] = vout[1];
      gstep.vect[2] = vout[2];
      gstep.vect[3] = vout[3];
      gstep.vect[4] = vout[4];
      gstep.vect[5] = vout[5];
      gstep.nmec    = (Int_t)(5 * gRandom->Rndm());
      for (Int_t l=0; l<gstep.nmec; l++)
         gstep.lmec[l] = l;
      if (gstep.gekin < 0.001)
         newParticle = kTRUE;
      if (TMath::Abs(gstep.vect[2]) > 30)
         newParticle = kTRUE;
   }
   // save the Tree header. The file will be automatically closed
   // when going out of the function scope
   t2.Write();
}
 
void tree105_read()
{
   // read the Tree generated by tree105_write and fill one histogram
   // we are only interested by the destep branch.
 
   // note that we create the TFile and TTree objects on the heap
   // because we want to keep these objects alive when we leave
   // this function.
   auto f = TFile::Open("tree105.root");
   auto t2 = f->Get<TTree>("t2");
   static Float_t destep;
   TBranch *b_destep = t2->GetBranch("destep");
   b_destep->SetAddress(&destep);
 
   // create one histogram
   auto hdestep = new TH1F("hdestep", "destep in Mev", 100, 1e-5, 3e-5);
 
   // read only the destep branch for all entries
   Long64_t nentries = t2->GetEntries();
   for (Long64_t i=0; i<nentries; i++) {
      b_destep->GetEntry(i);
      hdestep->Fill(destep);
   }
 
   // we do not close the file
   // we want to keep the generated histograms
   // we fill a 3-d scatter plot with the particle step coordinates
   auto c1 = new TCanvas("c1", "c1", 600, 800);
   c1->SetFillColor(42);
   c1->Divide(1, 2);
   c1->cd(1);
   hdestep->SetFillColor(45);
   hdestep->Fit("gaus");
   c1->cd(2);
   gPad->SetFillColor(37);
   t2->SetMarkerColor(kRed);
   t2->Draw("vect[0]:vect[1]:vect[2]");
 
   // Allow to use the TTree after the end of the function.
   t2->ResetBranchAddresses();
}
 
void tree105_tree()
{
   tree105_write();
   tree105_read();
}