> Nick van Eijndhoven wrote:
> >
> > Dear friends,
> > In ROOT I have 3 histos (1 dimensional), h1, h2 and h3.
> > h1 I fit with some function f and now I would like to subtract
> > h3 from h2 in which I weight each bin of h3 with the function value of
> > f in that bin. I.e. result= h3 - f(x)*h2.
> > Could anyone of you tell me the most effective (c.q. elegant) way to
> > do this, such that I don't have to change all the code when changing the
> > function f ?
>
> On Tue, 3 Jun 1997, Rene Brun wrote:
>
> Below is a script illustrating a possible solution:
>
> Rene Brun
>
> {
> gROOT->Reset();
> TFile f("hsimple.root"); //assume a file with histogram named hpx
> Int_t ncx = hpx->GetNbinsX();
> TH1F hnew = *hpx; //copy hpx to hnew
> hnew.Reset();
> TFormula f1("f1","x*x*x");
> for (Int_t bin=1;bin<=ncx;bin++) { //fill hnew with function values
> Float_t x = hnew.GetBinCenter(bin);
> hnew.Fill(x, f1.Eval(x));
> }
> TH1F hresult = hnew*(*hpx); //make result histogram
> //you could also do directly
> TH1F hresult = h1 -hnew*(*hpx);
> hresult.Draw();
> }
>
This looks like a candidate for a member function (or method) of TH1F,
perhaps one should be able to say something like (once a histo hnew and a
function f1 have been defined):
root> hnew.Sample("f1")
which would run the algorithm shown in Rene's script, i.e. sample the
function f1 at the bin centres of hnew and fill the results into hnew.
A word of caution though with function sampling, quite often a simple
sampling at the bin centres is not what you want, you would rather want
your function *integrated* over the bin range to be able to compare a
theory prediction with the experimental contents of a possibly wide bin.
Now, if somebody implements the "Sample" method for root histo objects
then a version which numerically integrates a function over the bin
range (and divides by the bin width) should perhaps also be provided. This
could be a very powerful tool.
cheers, Stefan
---Stefan Kluth------------CERN PPE Fellow---------------------/-\/\----
- CERN PPE Division - phone: +41 22 767 2975 - \_/--\ -
- CH-1211 Geneva 23 - fax: +41 22 782 1974 - |_|| -
---Switzerland-------------e-mail: stefan.kluth@cern.ch--------|--|__---
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