Trouble calculating the ratio of two histograms

[amschertz]

Hi all,
I’m trying to use root to calculate the ratio between two histograms (trying to find the acceptance of my experiment). I’ve been following the tutorial shown here, but when I try to apply the code to my histograms, it doesn’t work. As far as I can tell, the problem is with Clone() and/or Divide().
Here’s the code I wrote. I’m creating the canvas as a diagnostic more than anything else. The numerator and denominator histogram will display fine, but the ratio histogram won’t, both before and after I take the ratio. If anyone sees something I’m doing wrong, that would be so helpful.

void testratio() {
  //Open ROOT files
  TFile *fnum = TFile::Open("hist_pomegapi_30496.acc.root");
  TFile *fden = TFile::Open("hist_thrown.root");
  
  //Read the 3 Pion Mass histograms
  TH1I *locHist_3PiMass_num = (TH1I*)fnum->Get("3PiMass_Measured");
  TH1I *locHist_3PiMass_den = (TH1I*)fden->Get("3PiMass_thrown");

  //Define a canvas
  TCanvas *cc = new TCanvas("cc", "cc", 600, 600);
  cc->Divide(2,2);

  //Draw the 3 Pion Mass histograms
  cc->cd(1);
  locHist_3PiMass_num->Draw();
  cc->cd(2);
  locHist_3PiMass_den->Draw();

  //Take their ratio and draw it
  TH1I *locHist_ratio = (TH1I*)locHist_3PiMass_num->Clone();
  cc->cd(3);
  locHist_ratio->Draw();
  locHist_ratio->Divide(locHist_3PiMass_den);
  cc->cd(4);
  locHist_ratio->Draw();
}

[Wile_E_Coyote]

Try to “subtract” your histograms (“division” works fine for TH1D and TH1F but not really for TH1I):

cc->cd(3);
TH1I *locHist_ratio = new TH1I(*locHist_3PiMass_num);
locHist_ratio->SetNameTitle("locHist_ratio", "measured - thrown");
locHist_ratio->Add(locHist_3PiMass_den, -1.0);
locHist_ratio->Draw();