Problem fitting a graph

ROOT
1

Hello! I have a TGraphErrors, i’m trying to fit with the following function:

Double_t fitf(Double_t *x, Double_t *par, Double_t *beta)
{
Int_t i;

Double_t fitval=par[1]*pow((x[0]/700),-beta[i])*pow((1+x[0]/700),-beta[i]);

return fitval;
}

I plot the points with the TGrapherrors class. The vector beta is a random Gaussian vector, so I have to fit my points 50 times, so I do:
TF1 *func = new TF1(“fit”,fitf,0.0,2500.0,3);

for (int i=0; i<Events; i++)
{
gr.Fit(&func);
gr.DrawClone(“SAME”);
}

for the fitting, but when I try to ejecute the macro, it appears the errors:

error: no matching member function for call to 'Fit’

and

candidate function not viable: no known conversion from ‘TF1 **’ to
‘const char *’ for 1st argument
virtual TFitResultPtr Fit(const char *formula ,Option_t *option="" ,O…

and I don’t know how to fix it.

Thank you in advance!

2

First: TF1 → 4 - A general C function with parameters

Then: you must set initial values for all parameters (before trying to fit).

Finally: gr.Fit(func);

3

I keep having mistakes
I have just done:

TF1 *func = new TF1(“fit”,fitf,0.0,2500.0,3);

func->SetParameters(0,1);
func->SetParNames("constant","coefficient");
func->Draw();

    
for (int i=0; i<Events; i++)
{
   gr.Fit(func);
   gr.DrawClone("SAME");
}

And the error is:

/Users/root_install/include/TF1.h:726:104: error: type ‘double (double *, double *, double )’ cannot be used prior to ‘::’
because it has no members
…ROOT::Internal::GetFunctorType<decltype(ROOT::Internal::GetTheRightOp(&F…
^
/Users/root_install/include/TF1.h:370:41: note: in instantiation of member function 'ROOT::Internal::TF1Builder<double
()(double *, double *, double )>::Build’ requested here
ROOT::Internal::TF1Builder::Build(this, f);
^
/Users/root_install/macros/signal.C:62:21: note: in instantiation of function template specialization 'TF1::TF1<double ()(double
*, double *, double )>’ requested here
TF1 func = new TF1(“fit”,fitf,0.0,2500.0,3);
^
In module ‘MathCore’:
/Users/root_install/include/Math/ParamFunctor.h:125:39: error: too few arguments to function call, expected 3, have 2
return (f)((T)x, (double)p);
~~~~ ^
/Users/root_install/include/Math/ParamFunctor.h:95:45: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::FuncEvaluator<double ()(double
*, double *, double ), int>::EvalConst’ requested here
return FuncEvaluator<Func, EvalType>::EvalConst(fFunc,x,p);
^
/Users/root_install/include/Math/ParamFunctor.h:73:4: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::operator()’ requested here
ParamFunctorHandler(const Func & fun) : fFunc(fun) {}
^
/Users/root_install/include/Math/ParamFunctor.h:304:17: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::ParamFunctorHandler’ requested
here
fImpl(new ParamFunctorHandler<ParamFunctorTempl,Func>(f) )
^
/Users/root_install/include/TF1.h:728:66: note: in instantiation of function template specialization
'ROOT::Math::ParamFunctorTempl::ParamFunctorTempl<double ()(double
*, double *, double )>’ requested here
f->fFunctor.reset(new TF1::TF1FunctorPointerImpl<Fnc_t>(ROOT::M…
^
/Users/root_install/include/TF1.h:370:41: note: in instantiation of member function 'ROOT::Internal::TF1Builder<double
()(double *, double *, double )>::Build’ requested here
ROOT::Internal::TF1Builder::Build(this, f);
^
/Users/root_install/macros/signal.C:62:21: note: in instantiation of function template specialization 'TF1::TF1<double ()(double
*, double *, double *)>’ requested here
TF1 func = new TF1(“fit”,fitf,0.0,2500.0,3);
^
In module ‘MathCore’:
/Users/root_install/include/Math/ParamFunctor.h:121:26: error: too few arguments to function call, expected 3, have 2
return (*f)(x, p);
~~~~ ^
/Users/root_install/include/Math/ParamFunctor.h:91:45: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::FuncEvaluator<double ()(double
*, double *, double ), int>::Eval’ requested here
return FuncEvaluator<Func, EvalType>::Eval(fFunc,x,p);
^
/Users/root_install/include/Math/ParamFunctor.h:73:4: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::operator()’ requested here
ParamFunctorHandler(const Func & fun) : fFunc(fun) {}
^
/Users/root_install/include/Math/ParamFunctor.h:304:17: note: in instantiation of member function
'ROOT::Math::ParamFunctorHandler<ROOT::Math::ParamFunctorTempl,
double ()(double *, double *, double )>::ParamFunctorHandler’ requested
here
fImpl(new ParamFunctorHandler<ParamFunctorTempl,Func>(f) )
^
/Users/root_install/include/TF1.h:728:66: note: in instantiation of function template specialization
'ROOT::Math::ParamFunctorTempl::ParamFunctorTempl<double ()(double
*, double *, double )>’ requested here
f->fFunctor.reset(new TF1::TF1FunctorPointerImpl<Fnc_t>(ROOT::M…
^
/Users/root_install/include/TF1.h:370:41: note: in instantiation of member function 'ROOT::Internal::TF1Builder<double
()(double *, double *, double )>::Build’ requested here
ROOT::Internal::TF1Builder::Build(this, f);
^
/Users/root_install/macros/signal.C:62:21: note: in instantiation of function template specialization 'TF1::TF1<double ()(double
*, double *, double *)>’ requested here
TF1 *func = new TF1(“fit”,fitf,0.0,2500.0,3);

4

Hi @marguiitar,

I think that the fitf function (quoted below) should only take 2 arguments. However, I am sure @jonas can you provide a more detailed answer to your issue.

Cheers,
J.

5

Hi! But the problem is that my fitting function depends on a random vector that I have to pass as a parameter… how could I do it?

Thank you in advance!

6

TF1 → 5 - A general C++ function object (functor) with parameters
TF1 → 6 - A member function with parameters of a general C++ class

7

Thank you very much!
I have just define the random number inside the function:

Double_t fitf(Double_t *x, Double_t *par)
{
Int_t i;
TRandom3 r;

Double_t fitval=par[1]*pow((x[0]/700),-r.Gaus(2.4,0.24))*pow((1+x[0]/700),-r.Gaus(2.4,0.24));

return fitval;
}

However I have to fit my points 50 times, so I do:

TF1 *func = new TF1(“fit”,fitf,0.0,2500.0,2);

func->SetParameters(0,1.0);
func->SetParameters(1,1.0);

for (int i=0; i<50; i++)
{
   gr.Fit(func);
   gr.DrawClone("SAME");
}

It fit my points but only one time, could you tell me if are there any error?

Thank you very much

8

If you fit 50 times the same graph with the same function, why do you expect to see different results?

9

Because r.Gaus(2.4,0.24) is random, so if I call it 50 times it should be 50 different functions, isn’t it?

10

TRandom3 r(0); // make it really random

12 
// root -l trial.cxx
#include <cmath>
#include "TF1.h"
#include "TRandom3.h"
#include "TPad.h"
Double_t fitf(Double_t *x, Double_t *p) {
  return p[0] * std::pow((x[0] / 700.), -p[1]) * std::pow((1. + x[0] / 700.), -p[1]);
}
void trial(void) {
  TF1 *func = new TF1("fit", fitf, 1.0, 2500.0, 2);
  func->SetParNames("constant", "beta");
  func->SetNpx(10000);
  TRandom3 r(0); // make it really random
  for (int i = 0; i < 50; i++) {
    func->SetParameter(0, 1.0); // free parameter ("constant")
    func->FixParameter(1, r.Gaus(2.4, 0.24)); // fixed parameter ("beta")
#if 1 /* 0 or 1 */
    func->DrawClone(i ? "SAME" : "");
#else /* 0 or 1 */
    gr.Fit(func);
    gr.DrawClone(i ? "SAME" : "ALP");
#endif /* 0 or 1 */
  }
  if (gPad) gPad->SetLogy(1);
}
13

Thank you very much!!
My last question, what really means func->DrawClone(gPad ? “SAME” : “”);?
is not the same as gr.DrawClone(“SAME”)? gr is my TGraphErrors gr(…)

14

You may need something like this (also, using “i” instead of “gPad” will make it more reliable):
gr.DrawClone(i ? "SAME" : "ALP");

16

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