Hi Rooters,
I noticed some odd behaviour in TH2::ProjectionX/Y and TH2::ProfileX/Y. See the
example below:
TH2C* h = new TH2C("h","h",1,0,1,1,0,1)
h->Fill(0.5,-0.5)
h->Draw(); // no counts here
h->ProjectionX()->Draw(); // but there is one here
This seems a case of "what you don't see, is what you get", because the
overflow bins are used to make a projection or profile, but NOT to plot.
I can imagine that there are cases where you want to have this sort of
behaviour, but in general it seems dangerous, because you only know about
the overflow bins if you look at the statistics. It cost me a few days to
figure out why in the world my results changed in what I thought were
identical situations. I found that I was sometimes using ProfileX() and
other times ProfileX(1,h->GetNbinsY()), which I expected to yield
identical results.
The problem is in here:
TH1D *TH2::ProjectionX(const char *name, Int_t firstybin, Int_t lastybin,
Option_t
*option)
{
.... stuff removed here ...
Int_t nx = fXaxis.GetNbins();
Int_t ny = fYaxis.GetNbins();
if (firstybin < 0) firstybin = 0; <==== shouldn't this be 1
if (lastybin > ny) lastybin = ny+1; <== and this ny?
and similar pieces for the other Pro's (all versions of root I checked
have this).
Perhaps this was by design, in which case I will be more careful (and
looking for some documentation ....).
Greetings,
-- Gerco
Dr. C.J.G. Onderwater
Nuclear Physics Laboratory
401B Loomis Laboratory of Physics
University of Illinois at Urbana-Champaign
1110 West Green Street
Urbana, IL 61801-3080
Phone : (217) 244-7363
Fax : (217) 333-1215
E-mail: onderwat@uiuc.edu
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