void rand(){
  TDirectory *cdir = gDirectory;
  TFile *f = new TFile("k_rap-pt.root");
  cdir->cd();
  TH2F *h2=(TH2F*)f ->Get("kplus");

  for ( Int_t i=0;i<500;i++){

    Float_t pt; 
    Float_t rap;  
    h2->GetRandom2(pt,rap);
    printf("pt = %g, rap = %g\n",pt,rap); 
  }
  h2->Draw();

}