Nick van Eijndhoven wrote:
>
> Dear friends,
> I have two TH1F* objects h1 and h2 and would like to get the ratio
> of the two histos into a third one with the correct error calculation
> (i.e. Sumw2() option).
>
> If I do :
>
> TH1F h3=(*h1);
> h3.Sumw2();
> h3.Divide(h1,h2,1,1);
> h3.Draw();
>
> I get the correct result (i.e. h3 drawn with correct error bars).
>
> However, now I would like to use the convenient 'histo operators' and do :
>
> TH1F h3=(*h1)/(*h2);
> h3.Draw();
>
> But now h3 is drawn as a histo without error bars, giving me the impression
> that the Sumw2() option was not used.
> When I do h3.Draw("e1p") I see that indeed the errors are incorrect.
>
> Q : How can I achieve correct error calculation using the 'histo operators' ?
>
> Note : A default won't work, since sometimes in h3=(*h1)+(*h2) one would like
> to add without Sumw2() and sometimes with Sumw2() set.
>
I suggest the following sequence:
TH1F h3 = (*h1)
h3.Sumw2()
h3 = h3/(*h2)
and you will get the correct error bars.
Unless I have objections, I will systematically activate
the creation/storage of error bars in case of multiplication
and divisions.
> Another point is the following :
>
> I would like to have h3 also defined as TH1F*, but clearly
>
> TH1F* h3=(*h1)/(*h2) will fail.
>
> How to achieve this ?
Do not use operators, but TH1::Divide directly
TH1F *h3 = new TH1F("h3",....) //same params as h1
h3->Divide(...)
Rene Brun
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