// example of fitting a 3D function 


void exampleFit3D() { 
   
   const int n = 1000; 
   double x[n], y[n], z[n], v[n]; 
   double ev = 0.1;

   // generate the data
   TRandom2 r; 
   for (int i = 0; i < n; ++i) { 
      x[i] = r.Uniform(0,10);
      y[i] = r.Uniform(0,10);
      z[i] = r.Uniform(0,10); 
      v[i] = sin(x[i] ) + cos(y[i]) + z[i] + r.Gaus(0,ev);         
   }

   // fill the fit data
   ROOT::Fit::BinData data(n,3); 
   double xx[3];
   for(int i = 0; i < n; ++i) {
      xx[0] = x[i]; 
      xx[1] = y[i]; 
      xx[2] = z[i]; 
      data.Add(xx, v[i], ev); 
   }

   TF3 * f3 = new TF3("f3","[0] * sin(x) + [1] * cos(y) + [2] * z",0,10,0,10,0,10);
   f3->SetParameters(2,2,2);
   ROOT::Fit::Fitter fitter;
   ROOT::Math::WrappedMultiTF1 wf(*f3,3);
   fitter.SetFunction(wf); 
   bool ret = fitter.Fit(data); 
   if (ret) { 
      ROOT::Fit::FitResult & res = fitter.Result(); 
      res.Print(std::cout);
   }
   else 
      cout << "Fit Failed" << endl; 
}