Error: redefinition of ' variable '

[lm17n000032]

Hi ROOTers,

I have this code:

{

gROOT->Reset();
gSystem->Load("/.../libPhysics.so");

TLorentzVector target(0.0, 0.0, 0.0, 0.938);
TLorentzVector beam(0.0, 0.0, .65, .65);
TLorentzVector W = beam + target;

Double_t masses[3] = { 0.938, 0.139, 0.139} ;

TGenPhaseSpace event;
event.SetDecay(W, 3, masses);

TH2F h2("h2","h2", 50,1.1,1.8, 50,1.1,1.8);

for (Int_t n=0;n<100000;n++) {
	Double_t weight = event.Generate();

	TLorentzVector *pProton = event.GetDecay(0);

	TLorentzVector *pPip = event.GetDecay(1);
	TLorentzVector *pPim = event.GetDecay(2);


	TLorentzVector pPPip = *pProton + *pPip;
	TLorentzVector pPPim = *pProton + *pPim;

	h2.Fill(pPPip.M2() ,pPPim.M2() ,weight);
}

h2.SetXTitle("(m_{#pi^{+}}+m_{p})^{2}");
h2.SetYTitle("(m_{#pi^{-}}+m_{p})^{2}");
h2.Draw("cont");

}

I receve this output:

/home/lm17n000032/Scrivania/./PhaseSpace.C:9:16: error: redefinition of 'target'
TLorentzVector target(0.0, 0.0, 0.0, 0.938);
               ^
/home/lm17n000032/Scrivania/./PhaseSpace.C:9:16: note: previous definition is here
TLorentzVector target(0.0, 0.0, 0.0, 0.938);

I don’t understand why this code doesn’t work. Someone can help me please?
Target is definited only once.

---

_ROOT Version: 6.13/03
_Platform: Xubuntu 18.04
_Compiler: gcc version 7.3.0

---

[lm17n000032]

Why happened this? Some times run sometimes not.

[Axel]

Indeed, ROOT6’s support of the combination of gROOT->Reset() and unnamed macros is limited.

Please try this:

void PhaseSpace() {
  TLorentzVector target(0.0, 0.0, 0.0, 0.938);
  TLorentzVector beam(0.0, 0.0, .65, .65);
  TLorentzVector W = beam + target;

  Double_t masses[3] = { 0.938, 0.139, 0.139} ;

  TGenPhaseSpace event;
  event.SetDecay(W, 3, masses);

  TH2F h2("h2","h2", 50,1.1,1.8, 50,1.1,1.8);

  for (Int_t n=0;n<100000;n++) {
	Double_t weight = event.Generate();

	TLorentzVector *pProton = event.GetDecay(0);

	TLorentzVector *pPip = event.GetDecay(1);
	TLorentzVector *pPim = event.GetDecay(2);


	TLorentzVector pPPip = *pProton + *pPip;
	TLorentzVector pPPim = *pProton + *pPim;

	h2.Fill(pPPip.M2() ,pPPim.M2() ,weight);
  }

  h2.SetXTitle("(m_{#pi^{+}}+m_{p})^{2}");
  h2.SetYTitle("(m_{#pi^{-}}+m_{p})^{2}");
  h2.DrawCopy("cont");
}

[lm17n000032]

The error vanish,and the code work,but i dont understand why if I change .drawCOpy with draw the code generate a withe canvas.

[Axel]

Because it’s a function-local histogram: the liftetime of h2 ends at the end of the function, and it will get removed from the canvas. By using DrawCopy(), a copy gets drawn and stays on the canvas even when the lifetime of h2 ends.

[lm17n000032]

Thank you so much. Now i understand the reason.