[ROOT] Automatic constructor with bin centers beeing interger values.

From: ANCIANT E. (eric.anciant@sodern.fr)
Date: Tue Nov 23 2004 - 11:20:37 MET

Hello rooters,

I often create histograms whose axis correspond to integer values,
(for example a Number of tracks per event)
When doing so, I like to have the "integer" values centered in the bin
(for exemple TH1F hNbTrack("NbTracks","Number of Tracks",100,-0.5,99.5);
Because :
- I find it visually more clear (not ambiguous)
- I am always scared that some manipulation (to name it TAxis::SetRange
) ends up recomputing Mean using center of bin, thus I feel more relaxed
if the centers of the bins are the real "integer" value (see below
little exemple of possible problem ):
TH1F hNbTrackCentered("NbTracksCentered","Number of Tracks
TH1F hNbTrackNotCentered("NbTracksNotCentered","Number of Tracks not

TCanvas canvas;

// now zoom graphically on X axis arround bin 50 and look at displayed
Mean ..

This is a known 'feature', already discussed in this mailing list ...

I wonder if it would not be worth adding to the TH1 ( 2 / 3 ) classes,
dedicated constructors for this special
Case (possibly inline) something like :

TH1(const char* name, const char* title, Int_t firstBin, Int_t lastBin )
   if (nbins <= 0) nbins = 1;
   if (lastBin < firstBin ) { }; // Don't know what would be best in
that case ..
   TH1(lastBin - firstBin + 1,((Axis_t)firstBin
   fNcells = fXaxis.GetNbins()+2;

Or this property could be implemented in TAxis::Set( ...) ...

This would allow fast, easy, bias free creation of histograms whose axis
are meant as integer values ...
Another advantage I see is that I remember seing some people confused in
creating "integer"/discrete axis bin
Histograms, like the typical :

root [3] TH1D hTypeOfSomething("type","type",10,1,10);
root [4] hTypeOfSomething.GetBinWidth(1)
(const Axis_t)9.00000000000000020e-001

The automatic bin creation would prevent some mistakes ..


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