Re: [ROOT] TChain Scan/Draw returning different values

From: Rene Brun (Rene.Brun@cern.ch)
Date: Tue Nov 05 2002 - 12:21:37 MET


Hi Caius,

Your second file in the chain contains a Tree with 0 entries.
This gives a problem when computing the entry offset table
in case wildcarding has been used in the chain creation.
This is effectively a nasty case for which we will have to find
a solution. Meanwhile, you can circumvent the problem by doing:
   TChain* ch_ce = new TChain("t_cse");
   ch_ce->Add("./f*.root",0);
   ch_ce->GetEntry(999999); //this will force reading of all Tree headers
   ch_ce->Draw("run:snarl", "projs==1");

Thanks for having reported this problem.

Rene Brun


On Tue, 5 Nov 2002, Caius Howcroft wrote:

> 
> Hi,
> 
> root 3.3.09
> 
> I have a set of files all with the same trees in (a t_cse and a t_filter).
> 
> Here is what happens:
> 
> root [0] TChain* ch_ce = new TChain("t_cse")
> root [1] ch_ce->Add("./f*.root")
> (Int_t)11
> root [2] ch_ce->Draw("run:snarl", "projs==1")
> <TCanvas::MakeDefCanvas>: created default TCanvas with name c1
> (Int_t)20
> root [3] ch_ce->Draw("run:snarl", "projs==1")
> (Int_t)21
> root [4]
> 
> 
> you see that I run the same Draw command twice, but the first time it
> selects 20 events and the second time it selects 21 events!, This really
> worries me if I execute the same bit of code twice I get different
> results.  This effect seems to scale with files, and in the total data set
> the difference is more than a factor of 2!!!.  This isnt any fancy tree is
> just a bog standard tree with bog standard Int_t and Float_t entries.
> 
> Exactly the same thing happens for TChain::Scan.
> 
> 
> (the files I used to do this are at
> http://www.hep.phy.cam.ac.uk/~howcroft/data_trees.tar.gz)
> 
> I'm using root 3.03/09 on redhat 7.3, gcc v3.0, i685.
> 
> Am I doing somthing really stupid?
> 
> Caius
> 
> 
> ____________________________
> CAIUS HOWCROFT
> +44 (0)1223 764 128      UK(office)
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> 



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