Is there a way to write std::vector<std::vector<float>> in a tree with uproot (or a reasonable alternative structure)?

[g-matteucci]

Hi everyone! 👋

I’m working with uproot4 (*) and ran into a bit of a roadblock. In my project, I need to store a variable amount of data across a variable number of channels, so I’m trying to use a nested std::vector structure. The issue arises when I try to write this data to a TTree in a ROOT file.

For example, I have the following structure in Python:

data = {    'dynamic_wfs': awkward.Array([
                    [ [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]],
                    [ [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]],
                    [ [0, 0, 0],    [0, 0, 0],    [0, 0, 0],    [0, 0, 0],    [0, 0, 0]],
                ])}

When I try to write this to a ROOT tree, I encounter the following exception:

TypeError: cannot write Awkward Array type to ROOT file:

    var * var * int64

It is now my understanding that this data structures are off-limits in uproot at the moment.

Question:
Is there a way to write variable-sized std::vector<std::vector<float>> structures, or a resonable alternative / workaround?
Any advice or suggestions would be greatly appreciated. Thanks in advance for any help you can offer!

(*) but the following seems to be true for uproot5, which I plan on upgrading to in the future

[jpivarski]

In short, no. Writing doubly nested variable-length lists is a fundamentally different prospect from singly nested variable-length lists in TTree, and it's beyond our scope to attempt it.

(This is one of the reasons that we're looking forward to RNTuple: there aren't limitations like this.)

To get this to work, you'll need ROOT. I'd suggest running your Awkward Array through ak.to-rdataframe and then build the std::vector<std::vector<int64_t>> in a short C++ string.

[jpivarski]

https://awkward-array.org/doc/main/reference/generated/ak.to_rdataframe.html

https://awkward-array.org/doc/main/user-guide/how-to-convert-rdataframe.html