double myfunction(double *x)
{ 
   
   double x1 = generator->Uniform(0,1);
   double y1 = generator->Uniform(0,1);
   double w1 = x1 + y1;
   if (w1>=0.&w1<=1.){return w1;};
   else if (w1>1.&w1<=2.){return (2.-w1);};
   else {return 0.;};
}

void ex5n(){ 
auto c1 = new TCanvas("canvas","Example",200,10,600,400);
TRandom3 * generator = new TRandom3();
auto f1 = new TF1("f1",myfunction,0,2,1);    // tried doing  auto f1 = new TF1("f1",myfunction,0,2,0);  & double myfunction()           
auto h1=  new TH1F("h1","test",100,0,2);
h1->FillRandom("f1",1000000);
h1->Draw(); 

 
}