Issues when trying to store nested STL vectors (3 dimensions) in TTree

[Dom]

Hi everyone,

when trying to store a vector<vector<vector<Float_t> > > into a TTree according to the example code attached to this post (run via $ root -l run_example.cpp), entries are not properly stored. To be more precise, the vectors stored seem to correspond to arrays with dimensions [10][5][1] (according to TTree::Scan) while the code should result in [10][5][10]. 2D nested vector I/O works fine. According to this thread, I/O with 3D nested vectors should be possible, so I would like to ask if there is something wrong with the way I try to store the nested vectors or if there is something else to consider.

run_example.cpp (143 Bytes)
mk_nested.cpp (1.3 KB)

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ROOT Version: 6.14/02 (from tag v6-14-02)
Platform: Ubuntu 16.04.5 LTS
Compiler: ACLiC / g++ (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609

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[pcanal]

The data is correctly stored:

root [0] 
Attaching file nested_example.root as _file0...
(TFile *) 0x7fbe52c00060
root [1] gInterpreter->GenerateDictionary("vector<vector<vector<Float_t> > >");
root [2] vector<vector<vector<Float_t> > > *ptr = nullptr;
root [3] tree->SetBranchAddress("nested",&ptr)
(int) 0
root [4] tree->GetEntry(0)
(int) 2250
root [5] ptr->size()
(unsigned long) 10
root [6] ptr->back().size()
(unsigned long) 5
root [7] ptr->back().back().size()
(unsigned long) 10

However TTree::Scan does not handle this case well.

[Dom]

Thanks, my mistake (unsuccessfully tried TTree::Draw and TTree:Scan without trying to access the data directly). I suppose that there is no way to make TTree::Draw work with nested vectors of dimension 3 or larger yet?

[pcanal]

That is correct. And instead we are going to offer this feature via RDataFrame.