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dirs.C from root tutorials-how the histograms are filled and how to access to their entries?

I checked the file “dirs.C” from root tutorials. Although I understand how the directories/folders are generated, I have many questions about the histograms generated,

  1. When checking the generated histograms, why there are two with the same name h0_0N and h0_0N;1 ? Also they are the same. What is their difference and why they are saved with those labels?
  2. Can someone explain how the histograms are filled ?
// ^^^^^^ Fill histograms
   TRandom r;
   for (i=0;i<nplanes;i++) {	// nplanes=#folder[1-9]
      cdplane[i]->cd();		// open forder [i]
      for (j=0;j<ncounters;j++) {	//  j[0-99], number of histo
         for (k=0;k<100;k++) {
            hn[i][j]->Fill(100*r.Rndm(),i+j);	
            hs[i][j]->Fill(100*r.Rndm(),i+j+k); 
         } // For over k
      }	//For over j
   }	// For over i

For example
i=0 j=0 k==[0-99], k runing from 0 to 100.
hn[0][0]->Fill(100*r.Rndm(),0);
Does this mean hn[0][0]->Fill(x value = random number, here is the weight?)
And for hs[0][0]->Fill(100*r.Rndm(),0+0+[0-99]); what is the function of k here? is that adding k in the second entry of Fill(), does hs have a larger weight?

i=0 j==1 k=[0-99]
hn[0][1]->Fill(100*r.Rndm(),1);
hs[0][1]->Fill(100*r.Rndm(),0+1+[0-99]); // [1-100]

i=1 j=[0-99] k=[0-99]
hn[1][0-99]->Fill(100*r.Rndm(),1+[0-99]); //[1-100]
hs[1][0-99]->Fill(100*r.Rndm(),1+[0-99]+[0-99]); // [1-199]

What is the function of k inside the Fill?

  1. If I want to print and get any the entry of any of the histograms, inside the planes directories how can I access to such information? I tried to print inside the k loop
    std::cout<<hn[i][j];
    to get the value of the entry k
    but it gives me the address in memory 0x55fd7b494e20.

How can I obtain the entry number and its value?


_ROOT Version: root.v6.20
Platform: Not Provided
Compiler: Not Provided


Hi,

To answer your questions:

  1. It’s just an internal detail of how data is stored in the ROOT file, they refer to the same histogram. You can access it with name h0_0N.

  2. Yes, the second parameter in Fill is the weight. The hs histograms are being filled randomly and with larger weights that the hn in the loop over k. k is used to provide that extra weight to hs (if there is a reason behind this besides demonstration purposes, I don’t know).

  3. hn[i][j] is a TH1F object, you can check here how to obtain information from it here: https://root.cern.ch/doc/master/classTH1F.html . The entry you are filling is determined by i, j and k, and the value you are filling with is the result of calling 100*r.Rndm().

@etejedor,

Thank you for your reply. However I have already had a look to TH1F documentation, without having success when I tried to obtain the values of hn. I get error like member reference base type 'TH1 *[100]' is not a structure or union. And sincerely, I do not know what it means.

By modifying the loop in the following way:

  for (int i=0;i<tnplanes;i++) {				// #### NOTE T: nplanes=#folder[0-2]
      cdplane[i]->cd();						// #### NOTE T: open forder [i]
      std::cout<<"inside loop i= "<< i << endl;
      for (int j=0;j<tncounters;j++) {		// #### NOTE T: j[0-], number of histo
          std::cout<<"inside loop j= "<< j << endl;
         for (int k=0;k<2;k++) {
            std::cout<<"---inside loop k= "<< k << endl;
            float xnentry = 100*r.Rndm();
            float xsentry = 100*r.Rndm();
            int nweight =(int)100*r.Rndm() + 1.0;
            int sweight =(int)100*r.Rndm()+ 1.0;
            hn[i][j]->Fill(xnentry,nweight);
            hs[i][j]->Fill(xsentry,sweight); 
            
            std::cout<<"i: "<< i << "   j: "<< j << "   k: " << k << "\txnentry: "<< xnentry << "   nweight: "<< nweight << "   xsentry: "<< xsentry << "    sweight: " << sweight << endl;
            std::cout<<"hN[i "<< i << " ] [j "<< j << " ]: "<<  hn[i][j]->GetBinContent(k);
            std::cout<<"\thS[i "<< i << " ] [j "<< j << " ]: "<<  hs[i][j]->GetBinContent(k)<<endl;
         } // #### NOTE T: For over K
         std::cout<<"exit k loop "<< endl;
      }	// #### NOTE T: For over J
      std::cout<<"exit j loop "<< endl;
   }	// #### NOTE T: For over i  
   std::cout<<"exit i loop "<< endl;

Which gives me in the terminal:

inside loop i= 0
inside loop j= 0
---inside loop k= 0
i: 0   j: 0   k: 0	xnentry: 22.073   nweight: 44   xsentry: 43.0809    sweight: 56
hN[i 0 ] [j 0 ]: 0	hS[i 0 ] [j 0 ]: 0
---inside loop k= 1
i: 0   j: 0   k: 1	xnentry: 25.7467   nweight: 79   xsentry: 36.8463    sweight: 60
hN[i 0 ] [j 0 ]: 0	hS[i 0 ] [j 0 ]: 0
exit k loop 
exit j loop 
inside loop i= 1
inside loop j= 0
---inside loop k= 0
i: 1   j: 0   k: 0	xnentry: 86.0942   nweight: 48   xsentry: 37.6793    sweight: 86
hN[i 1 ] [j 0 ]: 0	hS[i 1 ] [j 0 ]: 0
---inside loop k= 1
i: 1   j: 0   k: 1	xnentry: 91.954   nweight: 26   xsentry: 66.818    sweight: 40
hN[i 1 ] [j 0 ]: 0	hS[i 1 ] [j 0 ]: 0
exit k loop 
exit j loop 
inside loop i= 2
inside loop j= 0
---inside loop k= 0
i: 2   j: 0   k: 0	xnentry: 84.3097   nweight: 11   xsentry: 53.0921    sweight: 34
hN[i 2 ] [j 0 ]: 0	hS[i 2 ] [j 0 ]: 0
---inside loop k= 1
i: 2   j: 0   k: 1	xnentry: 16.5796   nweight: 53   xsentry: 85.8411    sweight: 57
hN[i 2 ] [j 0 ]: 0	hS[i 2 ] [j 0 ]: 0
exit k loop 
exit j loop 
exit i loop 

I can see the values, e.g. h2_0N has two entries(corresponding to k=0,1): xentry(k=0)=84.3097, xentry(k=1)16.5796 with wight (y axis value center in ): nweight(k=0)=11, nweight(k=1)=53 respectively. But I want to obtain the x value with GetBinCenter or the y value GetBinContent bur it gives 0.5 and 0, when I am expecting to obtain the xnentry and the nweight. Do you know the correct way to obtain such values?

Hi,

@moneta can perhaps shed some light on this last question.

I used

std::cout<<"\t Xaxis, GetMaximumBin "<<  hn[i][j]->GetMaximumBin()<<endl; 
                           std::cout<<"\t Xaxis, GetBinCenter "<<  hn[i][j]->GetBinCenter( hn[i][j]->GetMaximumBin() )<<endl; 
                           std::cout<<"\t Xaxis, FinBin "<<  hn[i][j]->FindBin( hn[i][j]->GetBinCenter( hn[i][j]->GetMaximumBin() ))<<endl; 
                            std::cout<<"\t Yaxis, GetBinError "<<  hn[i][j]->GetBinError(hn[i][j]->GetMaximumBin())<<endl; 
                             std::cout<<"\t Yaxis, content "<<  hn[i][j]->GetBinContent( hn[i][j]->GetMaximumBin() )<<endl;

And I got x, y.

Hi,

I am not sure I have understood well your question. Please explain it better.

Lorenzo