// do a lognormal fit to an histogram 


void lognormal_fit() { 


   // fit to a log normal 


TFile *ff = new TFile("mass_T4.root");
    ff->ls();


   // fit to a log normal 
   TF1 * f1 = new TF1("f1","[0]*ROOT::Math::lognormal_pdf(x,[1],[2] ) " ); 
   //TF1 * f1 = new TF1("f1","[0]*ROOT::Math::normal_pdf(x,[1],[2] ) " ); 

   // set initial parameters 
   double p[3]; 
   p[0] = h1->GetEntries()*h1->GetXaxis()->GetBinWidth(1);   // area of the histogram 
   
   // find median of histogram 
   double prob[] = {0.5}; 
   double q[1]; 
   h1->GetQuantiles(1,q,prob);
   double median = q[0];
   // find mode of histogram 
   double  mode = h1->GetBinCenter( h1->GetMaximumBin());

   std::cout << "histogram mode is " << mode  << " median is " << median << std::endl;

   if (median < 0) { 
      Error("lognormal","not valid histogram median");
      return;
   }

   // m is log(median)
   p[1] = std::log(median); 

   // s2 is  log(median) - log(mode) 

   p[2] = std::sqrt( std::log(median/mode) ); 

   f1->SetParameters(p); 
   f1->SetParName(0,"A");
   f1->SetParName(1,"m");
   f1->SetParName(2,"s");
   
   f1->Print();

   h1->Draw();
   std::cout << f1->GetNpar() << std::endl;


   h1->Fit(f1,"V");
}