Hi Andre,
There are currently two ways to loop over a Tree
- via TTree::Draw
- via code/class generated by TTree::MakeClass/MakeSelector
The second option is more general as it allows you to:
- loop on more than one Tree at the same time
- fill more than one histogram
- use complex selections
It would look natural for your application to use the second form.
However, I can also propose an alternative solution that implies writing
a minimal piece of code, ie using Tree Friends.
In your first pass, you compute one or several new variables (in your case
only a new variable "final_discriminant"). You fill a new tree with this
single variable, eg Tree named "Final" in file "Final.root"
In a second pass, you can do:
data_events->AddFriend("Final","Final.root");
data_events->Draw("mass","final_discriminant>0.9");
Note that in this case, the second pass may be in a different ROOT session.
Also, you can add as many friends as you like.
Rene Brun
Andre Holzner wrote:
>
> Hello rooters,
>
> I have some TTree of events. For each event, I'd like to perform
> a complex calculation (e.g. calculate the value of a pdf based
> on a variable in the event or combine multiple pdf values).
>
> I currently have a method in this calculation class to which
> I can give a TTree and it plots the distribution of this value
> (possibly after some cuts).
>
> However, I would find it much more convenient, if I could
> use such a calculation directly in TTree::Draw (as a value,
> as a cut would also be useful !).
>
> I'm thinking of the following example:
>
> TTree *signal_events, *background_events, *data_events;
> ...
>
> MyCalculation *calc = new MyCalculation;
> calc->calculate_pdfs_from_mc(signal_events,background_events);
> calc->SetName("final_discriminant");
> ...
> data_events->Draw("final_discriminant");
> data_events->Draw("mass","final_discriminant>0.9");
>
> So here's my question: Is it possible in root
> to have a class (which e.g. inherits from TTreeFormula ??)
> which can be registered in some dictionary and then can be
> used in TTree::Draw in an expression ?
>
> thanks for the advice,
>
> André
>
>
>
> --
> ------------------+----------------------------------
> Andre Holzner | +41 22 76 76750
> Bureau 32 2-C13 | Building 32
> CERN | Office 2-C13
> CH-1211 Geneve 23 | http://wwweth.cern.ch/~holzner/
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