to run the example, do to execute with native compiler:
.x tree2a.C+Note that since IO is involved, ACLiC has to be invoked to create the dictionary of class Gctrak.
Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer (Beta) from the macro found in the ROOT repository on Tuesday, January 17, 2017 at 02:44 PM.
In [1]:
%%cpp -d
#include "TROOT.h"
#include "TFile.h"
#include "TTree.h"
#include "TBrowser.h"
#include "TH2.h"
#include "TMath.h"
#include "TRandom.h"
#include "TCanvas.h"
const Int_t MAXMEC = 30;
class Gctrak : public TObject {
public:
Float_t vect[7];
Float_t getot;
Float_t gekin;
Float_t vout[7]; //! not persistent
Int_t nmec;
Int_t *lmec; //[nmec]
Int_t *namec; //[nmec]
Int_t nstep; //! not persistent
Int_t pid;
Float_t destep;
Float_t destel; //! not persistent
Float_t safety; //! not persistent
Float_t sleng; //! not persistent
Float_t step; //! not persistent
Float_t snext; //! not persistent
Float_t sfield; //! not persistent
Float_t tofg; //! not persistent
Float_t gekrat; //! not persistent
Float_t upwght; //! not persistent
Gctrak() {lmec=0; namec=0;}
ClassDef(Gctrak,1)
};
A helper function is created:
In [2]:
%%cpp -d
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);
}
A helper function is created:
In [3]:
%%cpp -d
void tree2aw()
{
//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 *gstep = new Gctrak;
t2.Branch("track",&gstep,8000,1);
//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->lmec = new Int_t[MAXMEC];
gstep->namec = new Int_t[MAXMEC];
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;
gstep->namec[l] = l+100;
}
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();
}
A helper function is created:
In [4]:
%%cpp -d
void tree2ar()
{
//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");
Gctrak *gstep = 0;
t2->SetBranchAddress("track",&gstep);
TBranch *b_destep = t2->GetBranch("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
Long64_t nentries = t2->GetEntries();
for (Long64_t i=0;i<nentries;i++) {
b_destep->GetEntry(i);
hdestep->Fill(gstep->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");
}
In [5]:
tree2aw();
tree2ar();
Draw all canvases
In [6]:
gROOT->GetListOfCanvases()->Draw()