Hot to use TMinuit2

Newbie
1

Dear experts

I want to use TMinuit2 to fit some straight tracks (I assume it’s better than TMinuit), but all legacy code I can get are using TMinuit instead of TMinuit2, so here I have two questions:

  1. Is TMinuit2 better than TMinuit? (even only in some cases)
  2. Is there any example for how to use TMinuit2 comparing with doing the same thing with TMinuit?
2

Hi,

Thanks for the post.
Minuit served us very well for a long time. However, ROOT is transitioning to the Minuit2 minimizer, that builds on the experience of Minuit and, only because of that, aims to be better than that.
I would perhps start by pointing you to ROOT: Minuit2

Cheers,
Danilo

3

Thank you for pointing this page, I have read the page and there are two calls:

minimum->SetMaxFunctionCalls(1000000); //for Minuit/Minuit2
minimum->SetMaxIterations(10000); //for GSL

What are the difference of them? And Do I need to set the second one when using TMinuit2?

4

And I have another question, in the Fitting function I need to use some extra information instead of fitting parameters, I made it a class and Fitting function is a member function so that it can access the member parameter in the class, but when write ROOT::Math::Functor functionRT(&FittingFunctionRT, 4); I got this:

error: must explicitly qualify name of member function when taking its address

then I added the namespace ROOT::Math::Functor functionRT(&TrackFitMinimizer::FittingFunctionRT, 4); I got this error:

Functor.h:121:14: error: called object type 'double (TrackFitMinimizer::*)(const double *)' is not a function or function pointer
      return fFunc(x);

What should I do?

5

Hi,

I think the error is not related to ROOT: perhaps one can have a look to pages like this one? c++ - Function Pointer (Within Class) - Stack Overflow

Cheers,
D

6

Thank you, I understood about the error, and can you please tell more details about the functions I mentioned before? What are the difference of them? And Do I need to set the second one when using TMinuit2?

7

Hi,

For creating the Functor class from a non-static member function, you need to pass a pointer to the class itself, in order to get its data, and the member function pointer. Here is an example of using the Functor class from Mathematical libraries - ROOT

#include "Math/Functor.h"

class MyFunction {
public:
    double operator()(const double *x) const
    {
        return x[0]+x[1];
    }
    double Eval(const double * x) const
    {
        return x[0]+x[1];
    }
};

double freeFunction(const double * x ) {
    return x[0]+x[1];
}

int main() {
    // Test directly calling the function object.
    MyFunction myf;

    // Test from a free function pointer.
    ROOT::Math::Functor f1(&freeFunction,2);

    // Test from function object.
    ROOT::Math::Functor f2(myf,2);

    // Test from a member function.
    ROOT::Math::Functor f3(&myf,&MyFunction::Eval,2);
    double x[] = {1,2};
    cout << f1(x) << endl;
    cout << f2(x) << endl;
    cout << f3(x) << endl;
    return 0;
}

Then for doing the minimization you should not use the TMinuit2 class. This is a legacy and deprecated class, that has a signature similar to TMinuit. For minimising a function, you should use the minimiser interface, as described in this tutorial: ROOT: tutorials/fit/NumericalMinimization.C File Reference.

Concerning your previous question:

minimum->SetMaxFunctionCalls(1000000); //for Minuit/Minuit2
minimum->SetMaxIterations(10000); //for GSL

Minuit and Minuit2 do not have a limit on the maximum number of iterations but on the number of function calls. Normally, if not computing the Hessian but computing numerical derivatives you have something like:

number_of_function_calls = number_of_iterations * (number_of_parameters * 2 + 2)

Teh GSL minimiser instead can control the number of iterations. In general you can always use the default values and change them only if needed.

Lorenzo