zip

[jpivarski]

This is an operation that combines a list (std::vector) of arrays (std::shared_ptr<Content>) into a RecordArray if any member of that list is one-dimensional, and a ListArray or RegularArray of RecordArray for each dimension that all arrays share.

It existed in old Awkward, though the implementation will be easier now. Here's what that looked like:

>>> import awkward
>>> first = awkward.fromiter([[1, 2, 3], [], [4, 5]])
>>> second = awkward.fromiter([[1.1, 2.2, 3.3], [], [4.4, 5.5]])
>>> awkward.JaggedArray.zip(first, second)
<JaggedArray [[(1, 1.1) (2, 2.2) (3, 3.3)] [] [(4, 4.4) (5, 5.5)]] at 0x7fb20a91c590>

Those 2-tuples are a RecordArray in which the first field is from first and the second field is from second. Since first and second are both ListType with the same subarray lists for each element, the output is a ListType of RecordType (e.g. ListOffsetArray of RecordArray), in which the shared offsets are used in the single ListOffsetArray that wraps the RecordArray.

Perhaps this was too implicit in the past—perhaps there should be a zipdepth parameter that explicitly controls how many levels of list nesting to expect to be identical for all arguments (throwing an error if they're not).

zip is the way to make particle records from CMS NanoAOD, for instance. (It's useful.)

[jpivarski]

Note: it's not just tuples.

>>> awkward.JaggedArray.zip({"x": first, "y": second}).tolist()
[[{'x': 1, 'y': 1.1}, {'x': 2, 'y': 2.2}, {'x': 3, 'y': 3.3}],
 [],
 [{'x': 4, 'y': 4.4}, {'x': 5, 'y': 5.5}]]

[jpivarski]

The ak.zip([array1, array2, array3]) and ak.zip({"key1": array1, "key2": array2, "key3": array3}) functions should probably take a depthlimit parameter to indicate how many levels of jaggedness to try to combine.

depthlimit=0 would be a synonym for the RecordArray constructor, which creates top-level fields with whatever jaggedness the fields have unreconciled. depthlimit=None should go all the way down, broadcasting if possible. depth=1 would broadcast the first levels together, resulting in a ListArray(RecordArray(...)), depth=2 would produce ListArray(ListArray(RecordArray(...))), etc.

[jpivarski]

I had intended to create a PR for this, but accidentally committed to master (too distracted). So instead of linking to a PR, here's the one commit that finished it: 3800552

[jpivarski]

The test passed, so I'm closing this.