{
  // define a function f1, a sum of 2 gaussians f(x) = Gaus1(x) + Gaus2(x)
  TF1* f1 = new TF1("f1", "[0]*TMath::Gaus(x,[1],[2])+[3]*TMath::Gaus(x,[4],[5])", 0, 10);

  // give the parameters for parameters [0], [1] ... [5], in the same order
  f1->SetParameters(0.3, 3,0.7, 0.5, 5,0.7);

  // draw the function
  f1->Draw();

 
  // define a random number, with initial random seed 1234
  TRandom3 rnd(1234);

  // define a histogram from 0 to 1.5
  TH1D* h1 = new TH1D("h1", "h1", 150,0,1.5);

  // fill the histogram from the random number range from (0, 1)
  for (int i = 0; i<10000; i++){
    // fill the random number, repeat 10000 times, and fill them one-by-one into the histogram h1
    h1->Fill(rnd.Rndm());
  }

  // draw the histogram
  h1->Draw();

  // define another histogram to store the generated random number x
  TH1D* h2 = new TH1D("h2", "h2", 100,0,8);

  // generate random numbers x
  for (int i=0; i<10000; i++){
    // first generate random number x, range from 0 to 1
    double x = rnd.Rndm();
    // scale random x from (0,1) ==> to (0, 8) 
    x = x*8; 

    // second, generate another random number y
    double y = rnd.Rndm();
    // scale it to 0 to 0.51
    y = y*0.51;

    // get the function value at x
    double fx = f1->Eval(x);

    // check if y < fx, if yes, fill the random number x to h2 
    if ( y < fx ) {
      h2->Fill(x);
    }
  }
  
  // draw h2  
  h2->Draw("e1");


  // transformation method for exp function
  
  // define the (1/t)*exp(-x/t) function
  TF1* f2 = new TF1("f2", "1/[0]*exp(-x/[0])", 0,5);
  // give parameter [0] the value 0.8
  f2->SetParameter(0, 0.8);
  
  // define another histogram to store the histogram
  TH1D* h3 = new TH1D("h3", "h3", 50, 0, 5);

  // generate random number 
  for( int i=0; i<10000; i++) {
    //generate random r from 0 to 1
    double r = rnd.Rndm();
    // calculate random variable x
    double x = -0.8*log(1-r);
    // fill it into histogram
    h3->Fill(x);

  }

  // draw h3
  h3->Draw();



  



}