how to understand name a record directly with name of a tuple?

[ZhenxuanZhang-Jensen]

Hi, experts
I meet a code like this name = ("a","b"), events[name] = ak.Array([*]), I don't understand what this kind of expression means, how to save a tuple as a name in an awkward array? what is this mean? thanks for the help.

[agoose77]

tuple keys in NumPy and Awkward usually refer to dimensions, e.g.

>>> import numpy as np
>>> matrix = np.identity(3)
>>> matrix[(0, 1)]
0

Awkward Array extends this to support another kind of structural feature: fields. Awkward Array let's you nest fields, e.g.

ak.count(x.y.z)

and this is the same as writing

ak.count(x[('y', 'z')])

Due to how indexing works, it's also the same if you didn't write any parentheses

ak.count(x['y', 'z'])

as Python constructs a tuple for the __getitem__ call.

So, to answer your question, events[("a", "b")] = ... is setting the field b of events.a (the RecordArray at field a) to a given value. You might think that you can just write

events.a.b = ...

But due to how Awkward Array works, this won't actually update events, because events.a is not a reference to the field a of events, but rather a shallow copy.

[ZhenxuanZhang-Jensen]

okay, cool, this is very clear, thanks a lot

[agoose77]

I should add - the order of fields and dimensions in the index is interchangeable, see https://awkward-array.readthedocs.io/en/latest/_auto/ak.Array.html#projection