Re: [ROOT] TFormula parameter name parsing

From: Roland Kuhn (
Date: Thu Oct 14 2004 - 19:24:47 MEST

Hi Rene!

On Oct 13, 2004, at 8:54 AM, Rene Brun wrote:

> Hi Roland,
> I agree with your comments about TTreeFormula.
> This class is a key element of TTree::Draw, but it can also be used
> independently.
> It requires a much better documentation.
I tried to understand what's going on, and I must admit that I didn't 
really have enough brain memory to get a complete picture (at least not 
in a reasonable timeframe). From what I've seen TTreeFormula is always 
managed by a TTreeFormulaManager. I have already found out that the 
main use of this class is to manage the (up to four) expressions in the 
varexp of TTree::Draw plus the selection criterion, but from the code 
it is not immediately clear to me what the semantics are. What do I 
need to get it working reliably with a TChain?

I'm currently looping with a custom MakeClass-generated class, filling 
my own arbirarily dimensioned histograms. Now I want to include a cut 
criterion, which should be parsed like a selection to TTree::Draw. I 
figure I need something a la

in constructor:
manager=new TTreeFormulaManager;

in Loop():
cut=new TTreeFormula("some_name",cut,tree);
for(;;) {
   Double_t cut_result=cut->EvalInstance();

I suppose I should also add manager->UpdateFormulaLeaves() to my 
::Notify(), right? Is there something else which is needed?

Thanks very much for your help!


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