I am working with a root file (array of arrays). When I load the array into python, I get an awkward array since this is an array of arrays of varying sizes. I would like to learn how to convert this to a numpy array of arrays of the same size, by populating empty elements with NaNs. How can I convert an awkward array of varying size to a numpy array?
Suppose that you have an array of variable-length lists a:
>>> import numpy as np
>>> import awkward as ak
>>> a = ak.Array([[0, 1, 2], [], [3, 4], [5], [6, 7, 8, 9]])
>>> a
<Array [[0, 1, 2], [], ... [5], [6, 7, 8, 9]] type='5 * var * int64'>
The function that makes all lists have the same size is ak.pad_none. But first, we need a size to pad it to. We can get the length of each list with ak.num and then take the np.max of that.
>>> ak.num(a)
<Array [3, 0, 2, 1, 4] type='5 * int64'>
>>> desired_length = np.max(ak.num(a))
>>> desired_length
4
Now we can pad it and convert that into a NumPy array (because it now has rectangular shape).
>>> ak.pad_none(a, desired_length)
<Array [[0, 1, 2, None], ... [6, 7, 8, 9]] type='5 * var * ?int64'>
>>> ak.to_numpy(ak.pad_none(a, desired_length))
masked_array(
data=[[0, 1, 2, --],
[--, --, --, --],
[3, 4, --, --],
[5, --, --, --],
[6, 7, 8, 9]],
mask=[[False, False, False, True],
[ True, True, True, True],
[False, False, True, True],
[False, True, True, True],
[False, False, False, False]],
fill_value=999999)
The missing values (None) are converted into a NumPy masked array. If you want a plain NumPy array, you can ak.fill_none to give them a replacement value.
>>> ak.to_numpy(ak.fill_none(ak.pad_none(a, desired_length), 999))
array([[ 0, 1, 2, 999],
[999, 999, 999, 999],
[ 3, 4, 999, 999],
[ 5, 999, 999, 999],
[ 6, 7, 8, 9]])