Advanced TFormula and TF1 usage

[Vorname]

Dear *,

I probably have a strange or crazy problems. But I try to extend my understanding

Entering the following in cint directly is successful:
TF1 f(“myf”,“x*exp(-x)”,0.10);
TF1 g(“g”,“myf(x)*TMath::BesselJ0(x)”,0,10);
TF1 hTF1(“h”,“g.Integral(0.,x)”,0,10);
TCanvas c(“c”,“c”,800,600);
h.Draw();
c.SaveAs(“func.pdf”);

Doing the same thing as ROOT macro, script, or compiled code fails with
input_line_15:2:118: error: member reference type ‘double’ is not a pointer
Double_t TFormula____id14289597942108301067(Double_t *x){ return ((x[0])*TMath::Exp(-(x[0]))*TMath::BesselJ0(x[0])).Integral(0.,x[0]) ; }
[…]

I mean I understand from the error that TFormula translates the string in direct C/C++ code. And this evaluates to the return value (double), which is not TF1 object, and thus has no Integrate() class member function. And you are fully right: There are dozens of alternatives (which I all tested and find them acceptable, too).

But more interesting for me is, why does it work under cint? Would be there a way that his can be done also as script/binary executable?
If this would work, convolutions like the following would be two lines!
TF1 glcore(“glcore”,"[0]*TMath::Landau(x,[1],[2],true)*TMath::Gaus(x,[3],[4],true)",0,512.);
TF1 gl(“gl”,"[&](double x, double p){glcore.SetParameters([0],[1],[2],x,[3]); return glcore.Integral(x-5[3],x+5[3]);}",0,512.,4);
That would be really cool!

Is there such a way?
Many thanks in advance! (And sorry, if that question already appeared and was answered somewhere else. A link to there would be very kind from you, then! My internet researches where rather successless so far.)

Cheers, Martin

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_ROOT Version:6.18
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[moneta]

Hi,

As you pointed out, the example does not work in TFormula when using in a macro, but it works fine when using in the interpreter. Also if housing an interpreted Lambda the outcome is the same, because when the TF1 object g is created in a macro is not visible to the Cling global scope.
I am not sure what is the best way to fix this. However adding this line before creating the TF1 h, fixes the problem:

gInterpreter->ProcessLine("TF1 & g = *( (TF1*) gROOT->GetFunction(\"g\"))" );
TF1 h("h1","g.Integral(0.,x)",0,10);

Lorenzo

[Vorname]

Well, that’s interesting! That might be a solution … Not that simple as I had in mind. But still, I understand it.
Many thanks!

[Vorname]

Hi again,

I guess, Lambdas together with std::function is then probably the most direct way. Here the complete example as root script.

#include "TH1D.h"
#include "TCanvas.h"
#include "TF1.h"
void test() {
  TF1 gala("glcore","[0]*TMath::Landau(x,[1],[2],true)*TMath::Gaus(x,[3],[4],true)",-100.,100.);
  std::function<double(double*,double*)> f =
    [&gala](double*x,double*p){
      gala.SetParameters(p[0],p[1],p[2],x[0],p[3]);
      return gala.Integral(x[0]-5.*p[3],x[0]+5.*p[3]);
    };
  TF1 gl("gl",f,0,512.,4);
  gl.SetParameters(1000.,130.,10.,13.);
  TH1D h("h","ADC Channels",512,0.,512.);
  h.FillRandom("gl",10000);
  TCanvas c("c","c",800,600);
  h.Draw(); // <- up to here from sample
  // Fitting now
  gl.SetParNames("norm","MPV_L","sigma_L","sigma_G");
  gl.SetParameters(1000.,100.,20.,20.);
  gl.FixParameter(0,h.GetEntries()*h.GetBinWidth(5));
  gl.SetNpx(200);
  h.Fit("gl","ML");
  c.SaveAs("GLfit.pdf");
}

Btw. as ROOT macro ({…}) the TFormula way also works. But for what I needed, I then anyway had to go with Lambdas.

{
  TF1 gala("glcore","[0]*TMath::Landau(x,[1],[2],true)*TMath::Gaus(x,[3],[4],true)",-100.,100.);
  TF1 gl("gl","[&](double*x, double*p){gala.SetParameters(p[0],p[1],p[2],x[0],p[3]); return gala.Integral(x[0]-5.*p[3],x[0]+5.*p[3]);}",0,512.,4);
  gl.SetParameters(1000.,130.,10.,13.);
  TH1D h("h","ADC Channels",512,0.,512.);
  h.FillRandom("gl",10000);
  TCanvas c("c","c",800,600);
  h.Draw(); // <- up to here from sample
  // Fitting now
  gl.SetParNames("norm","MPV_L","sigma_L","sigma_G");
  gl.SetParameters(1000.,100.,20.,20.);
  gl.FixParameter(0,h.GetEntries()*h.GetBinWidth(5));
  gl.SetNpx(200);
  h.Fit("gl","ML");
  c.SaveAs("GLfit.pdf");
}

As ROOT Macro, that’s acceptable.

Many thanks, once more!
Cheers, Martin