Sum of user defined and/or TFormula based functions

[canigia]

Hi,

is there a (simple) possibility to combine (sum up) multiple TF1 implemented as user defined C++ functions? Combining just TFormula based functions is working well, but I get a crash when doing the same with user functions. The code below demonstrates the problem.

double sig(double *x, double *p) 
{ return p[0]*exp(-0.5*(x[0]-p[1])*(x[0]-p[1])/(p[2]*p[2])); }
double bkg(double *x, double *p) 
{ return p[0]+x[0]*p[1]+x[0]*x[0]*p[2]; }

void testsumfcn()
{
  // --> This works well <--
  //TF1 *fsig = new TF1("fsig", "[0]*TMath::Gaus(x,[1],[2])", 0, 10);
  //TF1 *fbkg = new TF1("fbkg", "[0]+x*[1]+x*x*[2]", 0, 10);

  // --> This gives a crash/lot's of errors <--
  TF1 *fsig = new TF1("fsig", sig, 0, 10, 3);  
  TF1 *fbkg = new TF1("fbkg", bkg, 0, 10, 3);

  fsig->SetParNames("A","#mu","#sigma");
  fbkg->SetParNames("a0","a1","a2");
  
  TF1 *fsum = new TF1("fsum","fsig+fbkg", 0, 10);
  
  fsum->SetParameter("A",10);
  fsum->SetParameter("#mu",5);
  fsum->SetParameter("#sigma",1);
  fsum->SetParameter("a0",1);
  fsum->SetParameter("a1",0.5);
  fsum->SetParameter("a2",-0.01);
  
  fsum->Draw();
  fsum->Print();
}

Ideally I’d also like to be able to combine/mix user functions with TFormula based TF1. I already tried with lambda functions as well, but without success.

Is there any other way to do it properly?

Klaus

ROOT Version: 6.18-04
Platform: Debian Linux
Compiler: Not Provided

---

[oshadura]

@moneta could you please take a look? Thank you in advance!

[canigia]

Has nobody an idea, how this is supposed to work?

Best, Klaus

[moneta]

Hi,
What you are doing cannot work for C++ based functions, but only for TFormula based ones.
With C++ function you can combine them using C++ (e.g. lambda functions):

TF1 * fsum = new TF1("sum",[&](double *x, double *p){ return fsig->EvalPar(x,&p[0]) + fbkg->EvalPar(x,&p[3]); }, 0, 10, 6);

Lorenzo