float getExpRndmNumber(int max){
  //Implementation of the function.
  float x = 0;
  float y = 0;
  do {
    x = max * gRandom->Uniform(0,1);
    y = gRandom->Uniform(0,1);}
  while (y> TMath::Exp(-x));
  return x;
}

int ex12_frwl(){
  TH1F*  h1 = new TH1F("h1", "Exponentialverteilung", 100, 0, 10);
  const int iterations = 10000;

  for (int ii = 0; ii < 10000; ii++){
    h1->Fill(getExpRndmNumber(10));
  }

  h1->Draw();

  TF1* myExp = new TF1("myExp", "[0]*TMath::Exp(-x)", 0,10);

  // Normalisation has to be #iterations divided by #bins multiplied by range.
  myExp->SetParameter(0, iterations/10.); 
  myExp->Draw("same");
}