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tree2.C File Reference

Detailed Description

View in nbviewer Open in SWAN 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 tree2a.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 tree2.C to execute with the Cling interpreter
.x tree2.C++ to execute with native compiler
#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 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;
//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
//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
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");
//create one histogram
TH1F *hdestep = new TH1F("hdestep","destep in Mev",100,1e-5,3e-5);
//read only the destep branch for all entries
for (Long64_t i=0;i<nentries;i++) {
//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);
// Allow to use the TTree after the end of the function.
void tree2() {
#define f(i)
Definition RSha256.hxx:104
#define e(i)
Definition RSha256.hxx:103
int Int_t
Definition RtypesCore.h:45
const Bool_t kFALSE
Definition RtypesCore.h:92
bool Bool_t
Definition RtypesCore.h:63
long long Long64_t
Definition RtypesCore.h:73
float Float_t
Definition RtypesCore.h:57
const Bool_t kTRUE
Definition RtypesCore.h:91
@ kRed
Definition Rtypes.h:66
int nentries
R__EXTERN TRandom * gRandom
Definition TRandom.h:62
#define gPad
virtual void SetFillColor(Color_t fcolor)
Set the fill area color.
Definition TAttFill.h:37
virtual void SetMarkerColor(Color_t mcolor=1)
Set the marker color.
Definition TAttMarker.h:38
A TTree is a list of TBranches.
Definition TBranch.h:89
virtual Int_t GetEntry(Long64_t entry=0, Int_t getall=0)
Read all leaves of entry and return total number of bytes read.
Definition TBranch.cxx:1652
virtual void SetAddress(void *add)
Set address of this branch.
Definition TBranch.cxx:2620
The Canvas class.
Definition TCanvas.h:23
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition TFile.h:54
1-D histogram with a float per channel (see TH1 documentation)}
Definition TH1.h:575
virtual TFitResultPtr Fit(const char *formula, Option_t *option="", Option_t *goption="", Double_t xmin=0, Double_t xmax=0)
Fit histogram with function fname.
Definition TH1.cxx:3892
virtual Int_t Fill(Double_t x)
Increment bin with abscissa X by 1.
Definition TH1.cxx:3350
virtual Double_t Gaus(Double_t mean=0, Double_t sigma=1)
Samples a random number from the standard Normal (Gaussian) Distribution with the given mean and sigm...
Definition TRandom.cxx:274
virtual Double_t Rndm()
Machine independent random number generator.
Definition TRandom.cxx:552
A TTree represents a columnar dataset.
Definition TTree.h:79
virtual TBranch * GetBranch(const char *name)
Return pointer to the branch with the given name in this tree or its friends.
Definition TTree.cxx:5275
virtual Long64_t GetEntries() const
Definition TTree.h:460
virtual void Draw(Option_t *opt)
Default Draw method for all objects.
Definition TTree.h:428
virtual void ResetBranchAddresses()
Tell all of our branches to drop their current objects and allocate new ones.
Definition TTree.cxx:8047
return c1
Definition legend1.C:41
TF1 * f1
Definition legend1.C:11
Double_t Sqrt(Double_t x)
Definition TMath.h:691
Short_t Abs(Short_t d)
Definition TMathBase.h:120
auto * l
Definition textangle.C:4
#define snext(osub1, osub2)
Definition triangle.c:1167
Rene Brun

Definition in file tree2.C.