exampleFunctor.C

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// Tutorial illustrating how creating a TF1 class using functor or class member functions 
//
//  can be run with:
//  root > .x exampleFunctor.C
//  root > .x exampleFunctor.C+ with ACLIC
//
//Author: Lorenzo Moneta

#include "TF1.h"
#include "TMath.h"
#include "TLegend.h"


double MyFunc (double *x, double *p ) { 
   return TMath::Gaus(x[0],p[0],p[1] );
}

// function object (functor) 
struct MyDerivFunc { 
   MyDerivFunc(TF1 * f): fFunc(f) {}
   double operator() (double *x, double * )  const { 
      return fFunc->Derivative(*x);
   }
   TF1 * fFunc; 
};
// function class with a member function
struct MyIntegFunc { 
   MyIntegFunc(TF1 * f): fFunc(f) {}
   double Integral (double *x, double * ) const { 
      double a = fFunc->GetXmin();
      return fFunc->Integral(a, *x);
   }
   TF1 * fFunc; 
};



void exampleFunctor() { 

   double xmin = -10;  double xmax = 10; 

   // create TF1 using a free C function 
   TF1 * f1 = new TF1("f1",MyFunc,xmin,xmax,2);
   f1->SetParameters(0.,1.);
   f1->SetMaximum(3);   f1->SetMinimum(-1);
   f1->Draw(); 

   // Derivative function 
   // example to create TF1 using a functor 

   // in order to work with interpreter the function object must be created and lived all time for all time 
   // of the TF1. In compiled mode, the function object can be passed by value (reccomended) and there 
   // is also no need to specify the type of the function class. Example is as follow: 
   // TF1 * f2 = new TF1("f2",MyDerivFunc(f1), xmin, xmax,0); // only for C++ compiled mode 
   
   MyDerivFunc * deriv = new MyDerivFunc(f1);
   TF1 * f2 = new TF1("f2",deriv, xmin, xmax, 0, "MyDerivFunc"); 

   f2->SetLineColor(kBlue); 
   f2->Draw("same");


   // integral function 
   // example to create a TF1 using a member function of a user class 
  
   // in order to work with interpreter the function object must be created and lived all time for all time 
   // of the TF1. In compiled mode there is no need to specify the type of the function class and the name 
   // of the member function
   // TF1 * f3 = new TF1("f3",intg,&MyIntegFunc::Integral,xmin,xmax, 0); // only for C++ compiled mode 

   MyIntegFunc * intg = new MyIntegFunc(f1);
   TF1 * f3 = new TF1("f3",intg,&MyIntegFunc::Integral, xmin, xmax, 0, "MyIntegFunc","Integral"); 

   f3->SetLineColor(kRed); 
   f3->Draw("same");

   TLegend * l = new TLegend(0.78, 0.25, 0.97 ,0.45);
   l->AddEntry(f1, "Func");
   l->AddEntry(f2, "Deriv.");
   l->AddEntry(f3, "Integral");
   l->Draw();


}

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