TChain with TTrees that has friends

ROOT
1

Hi all,

I’m using ROOT 5.26b or 5.26e or 5.28h to read datafiles that contains 3 TTree called “T1”,“T2” and “T3”

I use to make them friends:

pT1->AddFriend("T2"); //or  pT1->AddFriend(pT2) after getting T2 from file
pT1->AddFriend("T3"); //or  pT1->AddFriend(pT3) after getting T3 from file

I noticed that on some new datafile the friendship relation is ALREADY SAVED in the root file (In other words, I discovered that the friendship relation is preserved when TTree are written in the root file).
For such files I can just TFile::Get(“T1”) and pT1 has access to T2 and T3 variables and pT1->GetListOfFriends()->Print() correctly reports that T2 and T3 are friends.

Now the weird thing is when I make a TChain of several root files with the above mentioned structure:

TChain ch1("T1") ;
ch1.Add("file1.root") ;

The chain get access to T2 and T3 variables without specifying again the friendship, but:

  • ch1.GetListOfFriends() is null.
  • ch1.Print(“all”) printout T1 variables, then T2, T3, then AGAIN T3 and finally again T2

Is it intended? Is it affecting the way Draw makes plots?
What happen if I anyway specify again the friendship:

TChain ch2("T2") ; TChain ch3("T3") ; 
ch1.AddFriend(&ch2) ;
ch1.AddFriend(&ch3) ;
ch2.Add("file1.root") ; ch3.Add("file1.root") ; //etc

?

Thanks a lot for your clarification.

2

[quote]Is it intended?
[/quote]Not really, but I can see why it would be that way. (The TChain itself has no direct friends).

[quote]Is it affecting the way Draw makes plots?[/quote]No, it should not be affected.

[quote]What happen if I anyway specify again the friendship:[/quote]The TTrees would then be friend twice leading to possible name ambiguity.

Cheers,
Philippe.

3

Hi Philippe,

thanks for your reply.
I’d like to ask more on this:

What does name ambiguity imply? It is especially important to me understand if, e.g. with a TTree::Draw() or TTree::Scan(), just the first matching branch is taken and values are not processed twice, in particular when branches contain indexed values like vector or vector<vector>.
I made some tests and it seems that no duplications are made, but I’d like to ask for a confirmation also from experts since I didn’t try all possible varexpr/cut combinations.

Thanks a lot
Matteo

4

[quote]What does name ambiguity imply? It is especially important to me understand if, e.g. with a TTree::Draw() or TTree::Scan(), just the first matching branch is taken and values are not processed twice[/quote]That is correct.

Cheers,
Philippe.

5

Actually, yes.
If you do MakeProxy on the chain with automatically added friends, these friends does not appear in the generated class, while if you add them manually, they appear.