Hi David,
I think you didn't get my point.
I'm NOT talking about the number of entries of a histogram -
this "feature" I know already from PAW.
What I'm talking about are the MEAN AND RMS.
When you subtract two identical plots, it doesn't make any sense
to get a mean or rms different from 0. (see again my example below)
If you subtract two histograms, their values for sumw (stats[0]) and sumwx
(stats[2]) are added for the resulting histogram instead of being
subtracted.
This will thus result in a wrong mean value.
The way ROOT behaves right now, you can get two identical histograms
having different MEAN and RMS.
I tried this for ROOT-versions
3.01/06 29 August 2001 and
2.24/05 25 May 2000.
If there's a good reason for this, I'd really like to know.
Cheers,
Marc-Andre.
P.S.: Here again my example...
form1 = new TFormula("form1","abs(sin(x)/x)");
sqroot = new TF1("sqroot","x*gaus(0) + [3]*form1",0,10);
sqroot->SetParameters(10,4,1,20);
h1f = new TH1F("h1f","Test random numbers",200,0,10);
h1f->SetFillColor(45);
h1f->FillRandom("sqroot",10000);
h1f->Draw();
h1f->Add(h1f, -1.);
h1f->Draw();
-----------------------------------------------------------------------------
Marc-Andre Pleier Tel. : +0049 6221 516506
MPI Kernphysik Fax : +0049 6221 516603
Postfach 103980 Email: Marc-Andre.Pleier
D-69029 Heidelberg @mpi-hd.mpg.de
-----------------------------------------------------------------------------
On Thu, 6 Sep 2001, David d'Enterria wrote:
> Hi Marc-Andre,
>
> Marc-Andre Pleier wrote:
>
> > I just found out there seems to be a bug in ROOT when you subtract histograms:
> > Instead of subtracting the values in the stats array (sumw, sumwx), they are
> > added
>
> This is a known old "feature" of ROOT :-)
> http://root.cern.ch/root/roottalk/roottalk99/1883.html
> http://root.cern.ch/root/roottalk/roottalk01/2560.html
>
> Best regards,
>
> --
> David D'ENTERRIA - Marie-Curie PostDoc Fellow - TAPS·PHENIX·ALICE
> Groupe Photons SUBATECH tel: +33 2 51 85 85 60
> 4, A. Kastler BP 20722 · 44307 Nantes fax: +33 2 51 85 84 24
> http://www-subatech.in2p3.fr/~photons mailto:enterria@in2p3.fr
> ---------------------------------------------------------------------
> Phenix Group (Bld. 510C)
> Brookhaven National Lab. Upton, NY 11973, USA fax: +1 631 344 3253
> http://www.phenix.bnl.gov/~enterria mailto:denterri@bnl.gov
> ---------------------------------------------------------------------
>
>
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