TH2D::Integral()

From: Anil Singh <anil79_at_fnal.gov>
Date: Tue, 2 Jun 2009 18:22:47 +0500

Dear Rooters,
I have a TH2D histo, filled with random numbers in range (0,1) distributed flat in either of axes. The integral is ofcourse equal to the number of entries filled. But if I divide this x-y plane into four boxes (by cutting simultaneously on X and Y axis). Now I calculate the integral for each box, and add those values. Contrary to my expectations, the value does not turn out to be same as that obtained by integrating full histo.
//==================

Full Integral: 10000

Box1: 2551
Box2: 2546
Box3: 5053
Box4: 2546

Sum of all Boxes: 12696
//====================

I am also appending my code below.
Anil Singh

//===========================================================

float BoxIntegral(TH2D* h1, float xmin, float xmax, float ymin, float ymax) {
h1->GetXaxis()->SetRangeUser(xmin,xmax); h1->GetYaxis()->SetRangeUser(ymin,ymax); return h1->Integral();

}

void experiment(){

TH2D* h2=new TH2D("h2","test",100,0,1,100,0,1); TRandom3 r(0);

for (int i=0;i<10000;i++) {
  float r1 = r.Rndm();
  float r2 = r.Rndm();
  h2->Fill(r1,r2);
}
float all=h2->Integral();

float x_cut=0.5;
float y_cut=0.5;

// Box1

float box1=BoxIntegral(h2,0,x_cut,0,y_cut);

//Box2

float box2=BoxIntegral(h2,0,x_cut,y_cut,1);

//Box3

float box3=BoxIntegral(h2,x_cut,1,0,y_cut);

//Box4

float box4=BoxIntegral(h2,x_cut,1,0,y_cut);

std::cout<<"Full Integral: "<<all<<endl;
std::cout<<"Box1: "<<box1<<endl;
std::cout<<"Box2: "<<box2<<endl;
std::cout<<"Box3: "<<box3<<endl;
std::cout<<"Box4: "<<box4<<endl;
std::cout<<"Sum of all Boxes: "<<box4+box3+box2+box1<<endl;

}

//========================================================
Received on Tue Jun 02 2009 - 15:22:51 CEST