Rebin Histogram and change X-Axis range

ROOT
1

Hello,
I am trying to change the X-Axis Range and then rebin the data. I want the x-axis range to be from [-120,0] and use Rebin (12), for 10 bins. I have the following lines of code below, the rebin still considers the original x-axis ranges from [-200, 0] and does not work -
Warning in TH1F::Rebin: ngroup=12 is not an exact divider of nbins=200.

Am I missing something here ?

  TFile *f = new TFile("output.18.root"); 
  TH1F * h1 = (TH1F*)f->Get("h_KLFitterLL");
  h1->SetAxisRange(-120., 0., "X");
  h1->Rebin(12);
  h1->Scale(1/h1->Integral());

Thank you
Shreya

2

It looks like you want to rebin only a part of the histogram. Seems to me that can not work… What will happen to the bins between -200 and -120 ? they will stay un-rebinned ?
Seems to me you should rebin the whole histogram and set the Rebin parameter such as you get the number of bins you need between -120 and 0.

3

Hello,
Thank you for your reply. I tried this option as well as shown below, it gives me ~ 8 bins from [-120,0]. I am then adding the two histograms to calculate the difference in the area under the histograms as shown in the code below and this part is problematic…

  TFile *f = new TFile("output.18.root"); 
  TH1F * h1 = (TH1F*)f->Get("h_KLFitterLL");
  h1->Rebin(10);                                          //gives 20 bins from [-200, 0]
  h1->SetAxisRange(-120.,0.,"X");                //gives 8 bins from [-120, 0]
  h1->Scale(1/h1->Integral());

  TFile *f1 = new TFile("output.1.root");  
  TH1F * h2 =(TH1F*)f1->Get("h_KLFitterLL");
  h2->Rebin(10);
  h2->SetAxisRange(-120.,0.,"X");
  h2->Scale(1/h2->Integral());
  
//Calculate overlap area 
TH1F *hnew = new TH1F("hnew","subtracted_histo",8,-120,0);   // creating a histogram with 8 bins between -120, 0. 
hnew->Add(h1,h2,1.,-1.);

TH1F *h3 = new TH1F(*h1);
  h3->SetNameTitle("h3", "min(h1, h2)");
  h3->Reset("M");
  for (int i = 1; i <= h3->GetNbinsX(); i++)
    h3->Fill(h3->GetBinCenter(i),
	     ((h1->GetBinContent(i) < h2->GetBinContent(i)) ? h2->GetBinContent(i)- h1->GetBinContent(i):h1->GetBinContent(i)- h2->GetBinContent(i)));
  std::cout << h3->Integral() << std::endl;
  std::cout <<"width_integral"<<" "<< h3->Integral("width") << std::endl;

Error in <TH1F::Add>: Attempt to add histograms with different number of bins : nbins h1 = 10 , nbins h2 =  20**

I do not understand why nbins are different in h1 and h2, when I am doing the same rebinning and same X-axis range…
Thank you !