void test_random(void)
{
// 
// plot the number of fits to a cdc segment that result in prob > prob_cut (currently set to 0.01).
//

#include <TMath.h>
#include <TRandom.h>

gROOT->Reset();
//TTree *Bfield = (TTree *) gROOT->FindObject("Bfield");
gStyle->SetPalette(1,0);
gStyle->SetOptStat(kTRUE);
gStyle->SetOptFit(kTRUE);
// gStyle->SetOptFit(1111);
gStyle->SetPadRightMargin(0.15);
gStyle->SetPadLeftMargin(0.15);
gStyle->SetPadBottomMargin(0.15);
gStyle->SetFillColor(0);
//

   char string[256];
   char filename[80];
   Int_t j,jj;
   
   #define npts 100000;

   // define histograms

   nbins=100;
   TH1F *uniform = new TH1F("uniform","uniform distribution",nbins,0,1);
   TH1F *decay = new TH1F("decay","exponential distribution",nbins,0,40);
   TH1F *bell = new TH1F("bell","bell distribution",nbins,-10,10);
   TH1F *counts= new TH1F("counts","Poisson distribution",nbins/10,0,10);

   //
   TCanvas *c1 = new TCanvas("c1","c1 Test random",200,10,700,700);
   c1->SetBorderMode(0);
   c1->SetFillColor(0);
   // c1->SetGridx();
   // c1->SetGridy();
   // c1->SetLogy();

   Double_t xmin=-1;
   Double_t xmax=1;

   // plot pulse function

   Double_t tau=4.;    // unis of 1/x
   Double_t mean=3;    // unis of 1/x
   Double_t sigma=2;     // units of x 
   Double_t Pmean=3;   // Poisson mean

   TRandom1 *r = new TRandom1();

   for (j=0;j<npts;j++) {
       Double_t x = r->Uniform();
       Double_t y = r->Exp(tau);
       Double_t z = r->Gaus(mean,sigma);
       // Int_t n = r->Gaus(Pmean);
       Int_t n = r->Poisson(Pmean);

       uniform->Fill(x);
       decay->Fill(y);
       bell->Fill(z);
       counts->Fill((Double_t) n);
       }

   c1->Divide(2,2);
   c1->cd(1);
   c1_1->SetBorderMode(0);
   c1_1->SetFillColor(0);;
   uniform->GetYaxis()->SetRangeUser(0,2*npts/nbins);
   uniform->Fit("pol1");
   uniform->Draw();

   c1->cd(2);
   c1_2->SetBorderMode(0);
   c1_2->SetFillColor(0);
   decay->Fit("expo");
   decay->Draw();

   c1->cd(3);
   c1_3->SetBorderMode(0);
   c1_3->SetFillColor(0);
   bell->Fit("gaus");
   bell->Draw();

   c1->cd(4);
   c1_3->SetBorderMode(0);
   c1_3->SetFillColor(0);
   counts->Fit("gaus");
   counts->Draw();

}