RDataFrame creation without giving nentries in advance

ROOT
1

Dear Users,

I am interested in the possibility to create a RDataFrame without knowing in advance the number of entries.
So I would like to know wich is the constructor form.
Thank you in advance

ROOT Version: 6.32.00
Platform: Ubuntu 22.04
Compiler: gcc 11.4.0

2

Try maybe with the default constructor. RDataframe d;
And then call d.Fill(…) every time you want to add an entry

3

Thank you for this suggestion.
I tried with the following declaration:
ROOT::RDataFrame test;
and it yields me the following error:
error: no matching function for call to ‘ROOT::RDataFrame::RDataFrame()’

In addition, I would be interested in the process to construct/insert values in the RDataFrame created this way.
To my knowledge both Define and Foreach automatically iterate on the nentries which are supposed to be known in advance.
Thanks

4

Maybe try setting 0 as number of entries, and then call Fill ?

5

Thanks, creating RDataFrame test(0); works.
But I dont understand how to use command Fill, since it has not been created as a ttree.
Define and Foreach loop over 0 entries.

Could you provide a snippet of code?

Thank

6

Ohh, I see, I was wrong. The Fill method is for filling eg a histogram from the already created RDataFrame.

I am not an expert on this class, but I think RDataFrame is just an interface to a TTree or another data source (CSV file, Numpy array, etc.). You need to “fill” the Datasource (ROOT: ROOT::RDF::RDataSource Class Reference), not the RDataFrame. @vpadulan might confirm.

So I guess in your case you would need to do:

TTree t;
t.Fill(your_event1);
t.Fill(your_event2);
RDataFrame r(t.GetEntries(), …);