swh:1:snp:af87cd67498ef4fe47c76ed3e7caffe5b61facaf
Tip revision: 3e0f60454f9d2d1a444bbd3737ff66a160942db6 authored by Fons Rademakers on 22 September 2011, 08:54:09 UTC
tag patch release v5-30-02.
tag patch release v5-30-02.
Tip revision: 3e0f604
tree2.C
#include "TFile.h"
#include "TTree.h"
#include "TBrowser.h"
#include "TH2.h"
#include "TRandom.h"
#include "TCanvas.h"
// This example illustrates how to make a Tree from variables or arrays
// in a C struct. Use of C structs is strongly discouraged and one should
// use classes instead. However support for C structs is important for
// legacy applications written in C or Fortran.
// see tree2a.C for the same example using a class 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 tree2.C to execute with the CINT interpreter
// .x tree2.C++ to execute with native compiler
//
// Author: Rene Brun
const Int_t MAXMEC = 30;
// PARAMETER (MAXMEC=30)
// COMMON/GCTRAK/VECT(7),GETOT,GEKIN,VOUT(7),NMEC,LMEC(MAXMEC)
// + ,NAMEC(MAXMEC),NSTEP ,PID,DESTEP,DESTEL,SAFETY,SLENG
// + ,STEP ,SNEXT ,SFIELD,TOFG ,GEKRAT,UPWGHT
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 tree2w()
{
//create a Tree file tree2.root
//create the file, the Tree and a few branches with
//a subset of gctrak
TFile f("tree2.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,"getot/F");
t2.Branch("gekin",&gstep.gekin,"gekin/F");
t2.Branch("nmec",&gstep.nmec,"nmec/I");
t2.Branch("lmec",gstep.lmec,"lmec[nmec]/I");
t2.Branch("destep",&gstep.destep,"destep/F");
t2.Branch("pid",&gstep.pid,"pid/I");
//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 tree2r()
{
//read the Tree generated by tree2w and fill one histogram
//we are only interested by the destep branch.
//note that we use "new" to create the TFile and TTree objects !
//because we want to keep these objects alive when we leave
//this function.
TFile *f = new TFile("tree2.root");
TTree *t2 = (TTree*)f->Get("t2");
static Float_t destep;
TBranch *b_destep = t2->GetBranch("destep");
b_destep->SetAddress(&destep);
//create one histogram
TH1F *hdestep = new TH1F("hdestep","destep in Mev",100,1e-5,3e-5);
//read only the destep branch for all entries
Int_t nentries = (Int_t)t2->GetEntries();
for (Int_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
TCanvas *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]");
if (gROOT->IsBatch()) return;
// invoke the x3d viewer
gPad->GetViewer3D("x3d");
}
void tree2() {
tree2w();
tree2r();
}