void fitexpbinned(){
  //generate binned data
  const int nbin = 100;
  TH1F *hdata = new TH1F("data", "exp distribution", nbin, 0., 10.);
  TRandom2 r;

  for(int ievent=0; ievent<100000; ievent++){
    hdata->Fill(r.Exp(4.));
  }
  hdata->Draw();

  //fit binned data
  //A is the normalization and K is the mean of the exponential to fit 
  double A, K;
  double A_start = 100, A_step = 100., K_start = 1.0, K_step = 0.1;
  double ll_max = -10000000., K_max = -10000000., A_max = -10000000.;
  double n[nbin],x[nbin];

  //get the bin centers and bin contents in two arrays
  for (int ibin=1; ibin<nbin; ibin++){
    x[ibin] = hdata->GetBinCenter(ibin);
    n[ibin] = hdata->GetBinContent(ibin);
  } 

  //maximization 
  for (int iA=0; iA<100; iA++){
    A = A_start + iA*A_step;
    for (int iK=0; iK<100; iK++){
      K = K_start + iK*K_step;
      double ll = 0.;
      for (int ibin=1; ibin<nbin; ibin++){
	double lambda = A*exp(-x[ibin]/K);
	ll += -lambda + n[ibin]*log(lambda);
        if (ll>ll_max) {
	  ll_max = ll;
          K_max  = K;
	  A_max  = A;
	}
      } 
      //cout<<"A= "<<A<<" K= "<<K<<" ll= "<<ll<<endl;
    }
  }

  //result of maximization
  cout<<"A_max= "<<A_max<<" K_max= "<<K_max<<" ll_max= "<<ll_max<<endl;
  TF1 *f1 = new TF1("expfit","2500.0*exp(-0.25*x)",0.,10.);
  f1->Draw("same");
}