Sum of TGraph integrals

Dears rooters,

I have 2 TGraph that represent the fluxes. I would like to have the total flux doing the sum of the integral of the 2 curves… is it possbile? how can i do it?

thanks a lot in advance!! :slight_smile:

You can compute the two TGraph Integrals and sum them.

Dears couet and Willie_E_Coyote,

Thanks a lot for your reply!!
Actually in fact the 2 TGraph don’t have the same number of points, or same x.
I understood that TGraph::Integral is a pseudo-integral between a close polygon, so for that I can add a points in order to do it. But is not clear how can I sum the 2 integrals.
At the end I would like to obtain a function coming from the sum of the intrgral, in order to know the y values at specific x values.

thanks a lot in advance :slight_smile:

Try to play with:

  // create a test graph
  TGraph *g = new TGraph(100);
  for (Int_t i = 0; i < g->GetN(); i++) g->SetPoint(i, 0.1 * i, TMath::Sin(0.1 * i));
  // in ROOT 6, you can easily make a TF1 from the TGraph::Eval function
  // g->Sort(); // sort points along x axis (points must be in increasing order)
  TF1 *f = new TF1("f",
                   [&](double*x, double *p){ return (((x[0] >= g->GetX()[0]) && (x[0] <= g->GetX()[(g->GetN() - 1)])) ? g->Eval(x[0]) : 0.0); },
                   g->GetX()[0], g->GetX()[(g->GetN() - 1)], 0);
  f->SetNpx(3 * g->GetN());
  std::cout << "pseudo-integral = " << g->Integral() << std::endl;
  std::cout << "integral = " << f->Integral(f->GetXmin(), f->GetXmax()) << std::endl;

BTW. Depending on the “shapes” of your graphs, you may face problems with ROOT’s “integrators”:

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