Momentum 4D Object is not subscriptable

[srosenzweig09]

Hi, I was wondering how to access the p4 value for an entry in a vector.obj object. I have an array of p4 values and I want to access one of them, but if I try to do this, I get an error that the Momentum 4D object is not subscriptable. To get around this, I could loop through my events and build an array of vector.obj objects but that diminishes the power of columnar analysis. I would appreciate any suggestions. Thank you very much!

Example:

import uproot
import vector

t = uproot.open("my_file.root:my_tree")
particle_p4 = vector.obj(pt=t['part_pt'], eta=t['part_eta'], phi=t['part_phi'], m=t['part_m'])
...
particle_in_interesting_event = particle_p4[evt_number]

This throws an error. Am I trying to do something that is not possible yet? Thanks!

[jpivarski]

vector.obj makes a single vector object, not an array of vectors. The square brackets/__getitem__/subscripting syntax doesn't have a meaning for a single vector.

Maybe what you mean is this:

import awkward as ak
import vector
vector.register_awkward()

pt = t["part_pt"].array()
eta = t["part_eta"].array()
phi = t["part_phi"].array()
m = t["part_m"].array()
particle_p4 = ak.Array({"pt": pt, "eta": eta, "phi": phi, "m": m}, with_name="Momentum4D")
particle_p4  # look at it

all_particles_in_interesting_event = particle_p4[evt_number]
particle_in_interesting_event = all_particles_in_interesting_event[particle_number]

or if your data aren't jagged/nested, then only

particle_in_interesting_event = particle_p4[evt_number]

would be needed. Such an object would be an Awkward Array with all the usual properties that they have, but the combination of vector.register_awkward() and with_name="Momentum4D" would also give it Vector methods.

[jpivarski]

I know that we do need better documentation on the Vector pages, but maybe this is also pointing to the fact that it also needs error messages when a construction doesn't make sense. If you managed to pass arrays as pt, eta, phi arguments to vector.obj, it would be much better if vector.obj complained that they're not numbers.

No, wait! t["part_pt"] is a TBranch object, not even an array! I'm going to edit my response above so that only arrays are passed to the ak.Array constructor.

[srosenzweig09]

My apologies for the late response. I should clarify that I load my TBranch objects as awkward tables and numpy tables so they are arrays! Apologies for the confusion.

Yes, I see what you mean, though. I think I often try to use vector.obj objects instead of ak.Array objects and I think this is where I'm going wrong. Thank you!