Wrapping a Object in a ROOT::Math::Funtor

Roofit and RooStats
1

Hello all,
I am trying to minimize a chi square function using ROOT::Math::Minimizer. The chi square function is defined in a class as the override of the () operator. This is then wrapped as a ROOT::Math::Functor. However, I keep running in to the trouble that the () operator is never actually called. I found some code in the documentation about how to do this and it shows the same problem. I have include a small example (altered a bit from the documentation) which illustrates the problem.

#include "Math/Functor.h"
#include <iostream>
 
class MyFunction { 
 public: 
  double operator()(const vector<double> x) const { 
    cout<<"Hello.\n";
    return x[0]+x[1]; 
   } 
};
 
int function(){
   // test directly calling the function object
   MyFunction myf;
    
   // test from function object
   ROOT::Math::Functor f1(myf,2); 
 
   double x[] = {1,2};
 
   cout << f1(x) << endl;
    
   return 0;
}

When I run this I would expect the output to be:

However what I get is:

So it looks like the () operator is never called! Overwise the cout statement would appear and the correct value of 3 would be displayed. Has anyone encountered a similar issue?

On a side note, is ROOT::Math::Minimization the most up to date way to do function minimization in root?

thanks everyone!

Nicholi Shiell.

2

Hi Nicholi,

I see a small error in your code. The operator() in MyFunctor must have a specific signature in order to work. It does not work with STL vectors.

  double operator()(const double* x) const { ... }

After making this change, I ran your code and it works fine for me. Try making this change and let me know if it fixed the issue.

Regarding minimization, I recommend you check out this page: http://root.cern.ch/drupal/content/numerical-minimization.

Cheers,
Gabriel

3

Hey Gabriel,
Thanks for the response. I tried the change you suggested the operator now looks like:

double operator()(const double* x) const { 
    cout<<"Hello.\n";
    return x[0]+x[1]; 
 }

however I got the following error:

if I change the definition of the functor to

 ROOT::Math::Functor f1(&myf,2);

I get the old output back:

Could it be the version of root I am running? I have version 5.28/00f. Also I check the root includes directory and Functor.h is there so I have the right root packages installed.

Any other ideas? What version are you running?

thanks again!

4

Hi,

You should compile your macro with ACLIC (recommended) or you can use this syntax to work in interpreter mode:

ROOT::Math::Functor f1(&myf,2,"MyFunction");

Lorenzo

5

Thanks Lorenzo, it works now. However, should I use the previous declaration if I am not using CINT?

6

Hi,

The one without the class name works with both, the one without works only without CINT

Lorenzo