Hi Vincent,
When the Tree has been created with a top level branch,
you must set the address of this top level branch when you read the
Tree. You do not have to set the address of individual branches.
By default TTree::GetEntry will rebuilt in memory your TEvent class.
If you are not interested by the TEvent object, but only by one or a few
branches, you have to indicate this intended behaviour to the tree by
calling TTree::SetMakeClass. The call to SetMakeClass is automatically
generated when generating the analysis code via TTree::MakeClass.
Note that you cannot mix the two modes. If you use SetMakeClass(1),
then decide to move to the normal analysis mode (via TBrowser for
example), you must call tree.SetMakeClass(0).
Rene Brun
On Mon, 19 May
2003, Vincent [iso-8859-15] Régnard wrote:
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> Hi rooters,
> I have been seekink for a bug in my code for hours and I finally found the
> solution in the roottalkdigest ;) , but would like to know more about it.
> I have some classes similar to the the exemple TEvent. I built a tree with it
> and now want to access the branches. Drawing, cutting and using the
> treeviewer did worked pefectly. I am now using the possibility to only read
> one branch as specified in the documentation:
>
> - ------- my tree names input_tree
> // only get kin energy branch
> input_tree->SetBranchStatus("*",0);// disbles all branches
> input_tree->SetBranchStatus("Ek",1);// enables kin energy branche
>
> TBranch * Ek_branch=input_tree->GetBranch("Ek");// or get
> Ek_branch->SetBasketSize(16000);
> Ek_branch->SetAddress(&EvKinEnergy);
> if (!Ek_branch) cout << "Kinetic energy branch undefined" << endl;
> if (!input_tree->GetBranchStatus("Ek")) cout << "Kinetic energy branch not
> active" << endl;
> cout << "Entries in event branch " << Ek_branch->GetEntries() << endl;
> cout << "Get entry err code : " << Ek_branch->GetEntry(5) << endl;
> cout << "Energy = " << EvKinEnergy << endl;
> - -----
>
> when doing that I always had the same value for EvKinEnergy (its initial
> value) instead of the value stored in the tree. When calling the
> TBranch::GetEntries() succeded, but reading the values went wrong. I saw that
> one is to invoque the TTree::SetMakeClass() method to get it work properly:
> input_tree->SetMakeClass(1);
> but I do not find information on this method and do not hunderstand what it
> does exactly. Could you please tell me more about the role of this method.
>
> Thanks a lot for your help.
>
> - --
> V Régnard
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> La vente forcée de logiciels est illégale en France, comment l'empêcher:
> http://www.linuxfrench.net/oem/communique/communique1.html
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