RE: [ROOT] TFormula parameter name parsing

From: Philippe Canal (pcanal@fnal.gov)
Date: Thu Oct 14 2004 - 21:21:49 MEST 

Hi,

TTreeFormulaManager is in charge of 'synchronizing' the array indices of
several TTreeFormula object. 
If you have only one TTreeFormula, you do not need to worry about the
TTreeFormulaManager (a default one is provided).

So you current code could be simplified with 

cut=new TTreeFormula("some_name",cut,tree);
for(;;) {
   ii=LoadTree(i);
   fChain->GetEntry(ii);
   Double_t cut_result=cut->EvalInstance();
   ...
}
delete cut;

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

Yes (or cut->UpdateFormulaLeaves()).  The TTreeFormula object need to be
poked any time the branch objects are changes (i.e. when the chain opens a
new file).

Also note that if your cut has arrays in it (or any other collection that
has more than one value per entry), you might have to use something like:

   int ndata = cut->GetNdata();
   for(int i = 0; i<ndata; ++i) {
      cut_result = cut->EvalInstance(i);
   }

Cheers,
Philippe.

-----Original Message-----
From: owner-roottalk@pcroot.cern.ch [mailto:owner-roottalk@pcroot.cern.ch]
On Behalf Of Roland Kuhn
Sent: Thursday, October 14, 2004 12:25 PM
To: Rene Brun
Cc: roottalk@pcroot.cern.ch
Subject: Re: [ROOT] TFormula parameter name parsing

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);
manager->Add(cut);
manager->Sync();
for(;;) {
   ii=LoadTree(i);
   fChain->GetEntry(ii);
   Double_t cut_result=cut->EvalInstance();
   ...
}
manager->Remove(cut);
manager->Sync();

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!

Ciao,
					Roland

--
TU Muenchen, Physik-Department E18, James-Franck-Str. 85747 Garching Telefon
089/289-12592; Telefax 089/289-12570
--
A mouse is a device used to point at
the xterm you want to type in.
Kim Alm on a.s.r.

This archive was generated by hypermail 2b29 : Sun Jan 02 2005 - 05:50:10 MET