How to filter with arrays containing missing values#
import awkward as ak
import numpy as np
Indexing with missing values#
In Building an awkward index, we looked building arrays of integers to perform awkward indexing using ak.argmin() and ak.argmax(). In particular, the keepdims argument of ak.argmin() and ak.argmax() is very useful for creating arrays that can be used to index into the original array. However, reducers such as ak.argmax() behave differently when they are asked to operate upon empty lists.
Let’s first create an array that contains empty sublists:
array = ak.Array(
[
[],
[10, 3, 2, 9],
[4, 5, 5, 12, 6],
[],
[8, 9, -1],
]
)
array
[[], [10, 3, 2, 9], [4, 5, 5, 12, 6], [], [8, 9, -1]] --------------------- type: 5 * var * int64
Awkward reducers accept a mask_identity argument, which changes the ak.Array.type and the values of the result:
ak.argmax(array, keepdims=True, axis=-1, mask_identity=False)
[[-1], [0], [3], [-1], [1]] ------------------- type: 5 * 1 * int64
ak.argmax(array, keepdims=True, axis=-1, mask_identity=True)
[[None], [0], [3], [None], [1]] -------------------- type: 5 * 1 * ?int64
Setting mask_identity=True yields the identity value for the reducer instead of None when reducing empty lists. From the above examples of ak.argmax(), we can see that the identity for the ak.argmax() is -1: What happens if we try and use the array produced with mask_identity=False to index into array?
As discussed in Indexing with argmin and argmax, we first need to convert at least one dimension to a ragged dimension
index = ak.from_regular(
ak.argmax(array, keepdims=True, axis=-1, mask_identity=False)
)
Now, if we try and index into array with index, it will raise an exception
array[index]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/highlevel.py:948, in Array.__getitem__(self, where)
947 with ak._errors.SlicingErrorContext(self, where):
--> 948 out = self._layout[where]
949 if isinstance(out, ak.contents.NumpyArray):
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:537, in Content.__getitem__(self, where)
536 def __getitem__(self, where):
--> 537 return self._getitem(where)
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:590, in Content._getitem(self, where)
589 elif isinstance(where, ak.highlevel.Array):
--> 590 return self._getitem(where.layout)
592 # Convert between nplikes of different backends
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:665, in Content._getitem(self, where)
664 elif isinstance(where, Content):
--> 665 return self._getitem((where,))
667 elif is_sized_iterable(where):
668 # Do we have an array
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:582, in Content._getitem(self, where)
575 next = ak.contents.RegularArray(
576 this,
577 this.length,
578 1,
579 parameters=None,
580 )
--> 582 out = next._getitem_next(nextwhere[0], nextwhere[1:], None)
584 if out.length is not unknown_length and out.length == 0:
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/regulararray.py:692, in RegularArray._getitem_next(self, head, tail, advanced)
677 self._handle_error(
678 self._backend[
679 "awkward_RegularArray_getitem_jagged_expand",
(...)
690 slicer=head,
691 )
--> 692 down = self._content._getitem_next_jagged(
693 multistarts, multistops, head._content, tail
694 )
696 return RegularArray(
697 down, headlength, self._length, parameters=self._parameters
698 )
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/listoffsetarray.py:420, in ListOffsetArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
417 out = ak.contents.ListArray(
418 self.starts, self.stops, self._content, parameters=self._parameters
419 )
--> 420 return out._getitem_next_jagged(slicestarts, slicestops, slicecontent, tail)
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/listarray.py:488, in ListArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
479 assert (
480 outoffsets.nplike is self._backend.index_nplike
481 and nextcarry.nplike is self._backend.index_nplike
(...)
486 and self._stops.nplike is self._backend.index_nplike
487 )
--> 488 self._handle_error(
489 self._backend[
490 "awkward_ListArray_getitem_jagged_apply",
491 outoffsets.dtype.type,
492 nextcarry.dtype.type,
493 slicestarts.dtype.type,
494 slicestops.dtype.type,
495 sliceindex.dtype.type,
496 self._starts.dtype.type,
497 self._stops.dtype.type,
498 ](
499 outoffsets.data,
500 nextcarry.data,
501 slicestarts.data,
502 slicestops.data,
503 slicestarts.length,
504 sliceindex.data,
505 sliceindex.length,
506 self._starts.data,
507 self._stops.data,
508 self._content.length,
509 ),
510 slicer=ak.contents.ListArray(slicestarts, slicestops, slicecontent),
511 )
512 nextcontent = self._content._carry(nextcarry, True)
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:281, in Content._handle_error(self, error, slicer)
280 else:
--> 281 raise ak._errors.index_error(self, slicer, message)
IndexError: cannot slice ListArray (of length 5) with [[-1], [0], [3], [-1], [1]]: index out of range while attempting to get index -1 (in compiled code: https://github.com/scikit-hep/awkward/blob/awkward-cpp-15/awkward-cpp/src/cpu-kernels/awkward_ListArray_getitem_jagged_apply.cpp#L43)
The above exception was the direct cause of the following exception:
IndexError Traceback (most recent call last)
Cell In[6], line 1
----> 1 array[index]
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/highlevel.py:947, in Array.__getitem__(self, where)
518 def __getitem__(self, where):
519 """
520 Args:
521 where (many types supported; see below): Index of positions to
(...)
945 have the same dimension as the array being indexed.
946 """
--> 947 with ak._errors.SlicingErrorContext(self, where):
948 out = self._layout[where]
949 if isinstance(out, ak.contents.NumpyArray):
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/_errors.py:56, in ErrorContext.__exit__(self, exception_type, exception_value, traceback)
53 try:
54 # Handle caught exception
55 if exception_type is not None and self.primary() is self:
---> 56 self.handle_exception(exception_type, exception_value)
57 finally:
58 # Step out of the way so that another ErrorContext can become primary.
59 if self.primary() is self:
File ~/micromamba-root/envs/awkward-docs/lib/python3.10/site-packages/awkward/_errors.py:66, in ErrorContext.handle_exception(self, cls, exception)
64 self.decorate_exception(cls, exception)
65 else:
---> 66 raise self.decorate_exception(cls, exception)
IndexError: cannot slice ListArray (of length 5) with [[-1], [0], [3], [-1], [1]]: index out of range while attempting to get index -1 (in compiled code: https://github.com/scikit-hep/awkward/blob/awkward-cpp-15/awkward-cpp/src/cpu-kernels/awkward_ListArray_getitem_jagged_apply.cpp#L43)
This error occurred while attempting to slice
<Array [[], [10, 3, 2, 9], ..., [], [8, 9, -1]] type='5 * var * int64'>
with
<Array [[-1], [0], [3], [-1], [1]] type='5 * var * int64'>
From the error message, it is clear that for some sublist(s) the index -1 is out of range. This makes sense; some of our sublists are empty, meaning that there is no valid integer to index into them.
Now let’s look at the result of indexing with mask_identity=True.
index = ak.argmax(array, keepdims=True, axis=-1, mask_identity=True)
Because it contains an option type, index already satisfies rule (2) in Building an awkward index, and we do not need to convert it to a ragged array. We can see that this index succeeds:
array[index]
[[None], [10], [12], [None], [9]] ---------------------- type: 5 * var * ?int64
Here, the missing values in the index array correspond to missing values in the output array.
Indexing with missing sublists#
Ragged indexing also supports using None in place of empty sublists within an index. For example, given the following array
array = ak.Array(
[
[10, 3, 2, 9],
[4, 5, 5, 12, 6],
[],
[8, 9, -1],
]
)
array
[[10, 3, 2, 9], [4, 5, 5, 12, 6], [], [8, 9, -1]] --------------------- type: 4 * var * int64
let’s use build a ragged index to pull out some particular values. Rather than using empty lists, we can use None to mask out sublists that we don’t care about:
array[
[
[0, 1],
None,
[],
[2],
],
]
[[10, 3], None, [], [-1]] ----------------------------- type: 4 * option[var * int64]
If we compare this with simply providing an empty sublist,
array[
[
[0, 1],
[],
[],
[2],
],
]
[[10, 3], [], [], [-1]] --------------------- type: 4 * var * int64
we can see that the None value introduces an option-type into the final result. None values can be used at any level in the index array to introduce an option-type at that depth in the result.