Automatic binning in 2D histogram

ROOT
1

Hi all,
I am having a problem while using automatic binning in 2D histogram. Here is the code

#include "TFile.h"
#include "TH2.h"

int main(){
  double n = 10;
  double x;
  double y;
 TFile *file = TFile::Open("test.root","recreate");
 
TH2D *test = new TH2D("test","test hist",1,0,0,1,0,0);

for (int i=0; i<n; i++  ){
  x= i*i;
  y=(i*i)*i;
  test->Fill(x,y);

 }
 file->Write();
  file->Close();
  delete file;
}

It gives an empty histogram.
Can anyone please tell me what is wrong?

Thanks in advance.

2

You do not indicate which version you use. I suggest to do the following:

[code]#include “TFile.h”
#include “TH2.h”

int main(){
double n = 10;
double x;
double y;
TFile *file = TFile::Open(“test.root”,“recreate”);

TH2D *test = new TH2D(“test”,“test hist”,10,0,1,10,0,1);
test->SetBit(TH1::kCanRebin);

for (int i=0; i<n; i++ ){
x= ii;
y=(ii)*i;
test->Fill(x,y);

}
file->Write();
file->Close();
delete file;
} [/code]

3

That works. But when the y range is large, information in the lower bins gets lost because size of the bins is increased.

4

Yes, this is obvious. Create your histogram with more bins.

Rene

5

I tried that. But in one case, i need large y range ie. minimum y value is 0.ooooo1 and maximum y value is 1000 and if the number of bins is greater than 10000, it gives memory allocation error.

6

2 solutions
-use variable bin width histograms
-histogram teh log of your variable instead of the variable itself

Rene

7

Hi,

is there a way to have a histogram with a variable range but with a fixed number of bins?

Thank you,
Oleksiy

8

oh, never mind. That is exactly what “TH1::SetBit(TH1::kCanRebin)” is doing.
Thank you.