Variable binning (irregular bin size) 1 D histogram filling

ROOT
1

Hi,

I would like to get help for filling 1 D histogram with variable binning & irregular bin size from another 1D histogram’s bin contents with regular binning.
More specifically,
I have 2D scattering plot and get 1 D histogram with FitSlicesY. Now this 1 D histogram with regular bin size has to be converted into another histogram with desired variable (irregular) bin size within the range.

Can anybody please point me an example?
Thanks

2

see documentation of TH1::Rebin

// -case 2 xbins!=0 // a new histogram is created (you should specify newname). // The parameter is the number of variable size bins in the created histogram. // The array xbins must contain ngroup+1 elements that represent the low-edge // of the bins. // If the original histogram has errors stored (via Sumw2), the resulting // histograms has new errors correctly calculated. // // examples: if h1 is an existing TH1F histogram with 100 bins // Double_t xbins[25] = {...} array of low-edges (xbins[25] is the upper edge of last bin // h1->Rebin(24,"hnew",xbins); //creates a new variable bin size histogram hnew

Rene

3

Hi Rene,
Thanks. But, upto creating the histogram with desired binning is fine but filling correctly is a problem for me.

double xbin[9]={20,30,40,50,75,100,200,350,2000};
int nbins = LeptonInvPtResVPtGen_Scatter4_2->GetNbinsX();
TH1F* GR = new TH1F("","" ,8, xbin );

HERE, I need HELP. It is not correct what I am trying to do but recreates the original.

for (int ibin = 1; ibin <= nbins; ibin++) {
double GR_bin_4 = LeptonInvPtResVPtGen_Scatter4_2->GetBinContent(ibin);
double GR_bin_4_error = LeptonInvPtResVPtGen_Scatter4_2->GetBinError(ibin);

GR->SetBinContent(ibin,GR_bin_4);
GR->SetBinError(ibin, GR_bin_4_error);

GR->Draw();
}

4

adding to my previous reply.

TH1F *hnew = LeptonInvPtResVPtGen_Scatter4_2->Rebin(8,“hnew”,xbin);
hnew->Draw();

New histogram gives greater value than the original value in the range (even from the highest bin) for a modified bin size.
eg.
Original histo has 4% -4.5% for 200-360 in 4 bins
Modified histo has 10.5 % for 200-350 in 1 bin

That’s where I see the problem when I simply do rebin. Could you please clarify it?
Thanks

5

Could you post a small root file containing your histogram before rebinning?

Rene

6

Here is my root files attached.
Please have a look.

test.root has 2D scattering plot of name LeptonInvPtResVPtGen_Scatter4
Then I FitSlicesY and get 1D LeptonInvPtResVPtGen_Scatter4_2 (saved as test_sigma.root)
It has to be rebinned (irregular bin size).

Thanks
test_sigma.root (15.5 KB)
test.root (647 KB)

7

I do not understand your rebinning!
The slice “LeptonInvPtResVPtGen_Scatter4_2” has a bin-width of 40, the first bin is from 20->60 and the second from 60->100.
Now you try to rebin into a new histogram where the 1st bin is from 20->30 and the second from 30->40. The rebinning makes sense only when the bin edges are identical. For example you could define the 1st bin going from 20->100 and the 2nd from 100->400.
In your comparison you should also take into account the normalization, ie divide by the bin-width.

Rene

8

Thanks.
Finally, I defined the required binning in the original script so that my original root file has the histogram filled accordingly.
No rebinning …
Thanks again