GetBinLowEdge() for 2D and 3D histograms

ROOT
1

Dear all,

this seems like an awfully simple question but I just can’t find the answer: I want to process a 2D histogram bin by bin for which I need to know the bin boundaries at each bin. In 1D I do that with GetBinLowEdge(ix) and GetBinWidth(ix). What are the equivalents for 2D and 3D histograms? I.e. something like GetBinLowEdgeX(ix,iy), GetBinLowEdgeY(ix,iy) etc.
I guess I can reconstruct them from the overall histogram information, but there’s got to be a simpler way…?

Many thanks,

Thomas

2

h->GetXaxis()->GetBinLowEdge(i); //for 1,2 and 3-d h->GetYaxis()->GetBinLowEdge(i); //for 2 and 3-d h->GetZaxis()->GetBinLowEdge(i); //for 3-d
Rene

3

Sorry Rene, my mind operates in a different rest frame than yours! I don’t get the expected results with your prescription. Maybe my interpretation of bin/global bin is incorrect. What’s wrong with the following example:

{
  Int_t nBinX = 5;
  Int_t nBinY = 5;
  Double_t xMin = 0.;
  Double_t xMax = 1.;
  Double_t yMin = 0.;
  Double_t yMax = 1.;

  TH2D* hist = new TH2D("hist","hist", nBinX, xMin, xMax, nBinY, yMin, yMax);
  hist->FillRandom("expo",10000);

  for (Int_t ibx = 1; ibx <= nBinX; ibx++) {
    for (Int_t iby = 1; iby <= nBinY; iby++) {
      Double_t xBinMin = hist->GetXaxis()->GetBinLowEdge(hist->GetBin(ibx,iby));
      Double_t xBinMax = hist->GetXaxis()->GetBinUpEdge(hist->GetBin(ibx,iby));
      Double_t yBinMin = hist->GetYaxis()->GetBinLowEdge(hist->GetBin(ibx,iby));
      Double_t yBinMax = hist->GetYaxis()->GetBinUpEdge(hist->GetBin(ibx,iby));
      Double_t nBinCont = hist->GetBinContent(ibx,iby);

      cout << " bin " << ibx << " " << iby << " (global bin: " << hist->GetBin(ibx,iby) << ")" << endl;
      cout << " xBin: " << xBinMin << " - " << xBinMax;
      cout << ", yBin: " << yBinMin << " - " << yBinMax;
      cout << ", content: " << nBinCont << endl;
    }
  }
  hist->Draw("box");
}

As output I would expect

bin 1 1 (global bin: 1)
xBin: 0.0 - 0.2, yBin: 0.0 - 0.2, content: whatever
bin 1 2 (global bin: 2)
xBin: 0.0 - 0.2, yBin: 0.2 - 0.4, content: whatever’

Instead, I get:

bin 1 1 (global bin: 8)
xBin: 1.4 - 1.6, yBin: 1.4 - 1.6, content: 271
bin 1 2 (global bin: 15)
xBin: 2.8 - 3, yBin: 2.8 - 3, content: 255

Thanks again,

Thomas

4

Simply do:

{
  Int_t nBinX = 5;
  Int_t nBinY = 5;
  Double_t xMin = 0.;
  Double_t xMax = 1.;
  Double_t yMin = 0.;
  Double_t yMax = 1.;

  TH2D* hist = new TH2D("hist","hist", nBinX, xMin, xMax, nBinY, yMin, yMax);
  hist->FillRandom("expo",10000);

  for (Int_t ibx = 1; ibx <= nBinX; ibx++) {
    for (Int_t iby = 1; iby <= nBinY; iby++) {
      Double_t xBinMin = hist->GetXaxis()->GetBinLowEdge(ibx);
      Double_t xBinMax = hist->GetXaxis()->GetBinUpEdge(ibx);
      Double_t yBinMin = hist->GetYaxis()->GetBinLowEdge(iby);
      Double_t yBinMax = hist->GetYaxis()->GetBinUpEdge(iby;
      Double_t nBinCont = hist->GetBinContent(ibx,iby);

      cout << " bin " << ibx << " " << iby << " (global bin: " << hist->GetBin(ibx,iby) << ")" << endl;
      cout << " xBin: " << xBinMin << " - " << xBinMax;
      cout << ", yBin: " << yBinMin << " - " << yBinMax;
      cout << ", content: " << nBinCont << endl;
    }
  }
  hist->Draw("box");
}

Rene

5

D’oh!! Of course!! I should have tried that.
Thank you for your patience,

Thomas