How to control number of bins when use createHistogram

[renqipan]

Hi experts,
I use function “createHistogram” creat a histogram from a RooAbsPdf, but the number of bins is not consistent. 7 bins was assigned, but when I got the diagram it only drawn 5 bins. Would you please me tell me how to control the number of bins.
code:
RooRealVar *mass_tt= w->var(“mass_tt”);
RooAbsPdf *atem_ytt_pdf=aterm->createProjection(*mass_tt);
TH1F* aterm_ytt_hist=(TH1F*) atem_ytt_pdf->createHistogram(“rapidity_tt”,7);
aterm_ytt_hist->Scale(85000/aterm_ytt_hist->Integral());
hist_a1d->SetLineColor(2);
aterm_ytt_hist->SetLineColor(4);
aterm_ytt_hist->Draw(“histsame”);

[renqipan]

I tried largen the second argument, for example, set it as 50 or 100, then I obtain 7bins as expected.
In Root reference guide it says: TH1 * createHistogram (const char *varNameList, Int_t xbins=0, Int_t ybins=0, Int_t zbins=0).
I found the second argument is not the number of bins, it seems it is the number of events used to create the histogram.

[StephanH]

You could try the Binning argument that is documented here:
https://root.cern.ch/doc/master/classRooAbsReal.html#a6659d2c301e5cd65b83ee8c9422c2553

[StephanH]

I also had a look at the code, but it looks correct. Could you post an example that we can run?

[renqipan]

Hi,
I found the problem. the command “createHistogram(“rapidity_tt”,7);” create a histogram with even bin width, but my PDF(the original histogram) is not even bin width. The edge of my bins is :

Double_t edges[8]={-4,-1.8,-1,-0.2,0.6,1.2,2,4};

How can I create a histogram with not even width using createHistogram?

[StephanH]

In the documentation I linked you should find an argument that’s also called Binning, but it should take the number of bins and an array with bin boundaries.

[StephanH]

What you can do is either use IntrinsicBinning() or you can create a new RooBinning, and pass it like this:

createHistogram(... Binning(RooBinning(pass your arguments here));