Copy a TTree from a TFile to another TFile

ROOT
1

In ROOT 6, what is the canonical way to read a TTree from a TFile and write it entirely to another TFile? Using TTree() or TTreeReader() functionality?

2

Hi,

The simplest way is to use the rootcp utility.

Cheers,
Danilo

3

Hi,

From C++, you would use the TTree::CloneTree method.

Cheers,
Philippe.

4

Thanks for the help.

I don’t know what rootcp is and don’t know how to use/find it.

I tried this in C++:

TFile* rootfile_in = new TFile("in.root", "READ"); TFile* rootfile_out = new TFile("out.root", "RECREATE"); TTree* roottree_in = (TTree*)rootfile_in->Get("tree"); TTree* roottree_out = roottree_in->CloneTree(0); roottree_out->SetNameTitle("name", "title"); roottree_out->CopyEntries(roottree_in); roottree_out->Write(); rootfile_out->Close(); rootfile_in->Close();
Works fine. When I try it in Python:

rootfile_in = TFile('in.root', 'READ') rootfile_out = TFile('out.root', 'RECREATE') roottree_in = rootfile_in.Get('tree') roottree_out = roottree_in.CloneTree(0) roottree_out.SetNameTitle('name', 'title') roottree_out.CopyEntries(roottree_in) roottree_out.Write() rootfile_out.Close() rootfile_in.Close()
I get two TTrees in the output file, one properly filled (cycle 3), the other one only half filled (cycle 2). It works with small trees, but for large TTrees (~ 40000 events), I see this behavior. What am I doing wrong?

5

Hi,

rootcp is a utility that we provide as part of the latest releases (v6.06).

Nothing. This is the expected behavior (See the User’s guide for more details), the ‘cycle 2’ is a backup snapshot of the TTree meta-data to reduce the risk of data loss (in case the program filling the tree unexpectedly terminate) [It does not take much of space on the file]

Cheers,
Philippe.

6

Hi,

just to put a reference to the new root commandline utils (which include rootcp): root.cern.ch/how/how-quickly-in … ntent-file
In any case, it is just enough to type “rootcp” once ROOT is installed, at least version 6.06.

Cheers,
Danilo

7

Thank you very much. All this information is very helpful.

I tried just typing rootcp, but my ROOT installation was apparently not fresh enough.