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/envs/awkward-docs/lib/python3.10/site-packages/awkward/highlevel.py:950, in Array.__getitem__(self, where)
949 with ak._errors.SlicingErrorContext(self, where):
--> 950 out = self._layout[where]
951 if isinstance(out, ak.contents.NumpyArray):
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:510, in Content.__getitem__(self, where)
509 def __getitem__(self, where):
--> 510 return self._getitem(where)
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:563, in Content._getitem(self, where)
562 elif isinstance(where, ak.highlevel.Array):
--> 563 return self._getitem(where.layout)
565 # Convert between nplikes of different backends
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:638, in Content._getitem(self, where)
637 elif isinstance(where, Content):
--> 638 return self._getitem((where,))
640 elif is_sized_iterable(where):
641 # Do we have an array
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:555, in Content._getitem(self, where)
548 next = ak.contents.RegularArray(
549 this,
550 this.length,
551 1,
552 parameters=None,
553 )
--> 555 out = next._getitem_next(nextwhere[0], nextwhere[1:], None)
557 if out.length is not unknown_length and out.length == 0:
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/regulararray.py:708, in RegularArray._getitem_next(self, head, tail, advanced)
693 self._maybe_index_error(
694 self._backend[
695 "awkward_RegularArray_getitem_jagged_expand",
(...)
706 slicer=head,
707 )
--> 708 down = self._content._getitem_next_jagged(
709 multistarts, multistops, head._content, tail
710 )
712 return RegularArray(
713 down, headlength, self._length, parameters=self._parameters
714 )
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/listoffsetarray.py:409, in ListOffsetArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
406 out = ak.contents.ListArray(
407 self.starts, self.stops, self._content, parameters=self._parameters
408 )
--> 409 return out._getitem_next_jagged(slicestarts, slicestops, slicecontent, tail)
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/listarray.py:544, in ListArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
535 assert (
536 outoffsets.nplike is self._backend.index_nplike
537 and nextcarry.nplike is self._backend.index_nplike
(...)
542 and self._stops.nplike is self._backend.index_nplike
543 )
--> 544 self._maybe_index_error(
545 self._backend[
546 "awkward_ListArray_getitem_jagged_apply",
547 outoffsets.dtype.type,
548 nextcarry.dtype.type,
549 slicestarts.dtype.type,
550 slicestops.dtype.type,
551 sliceindex.dtype.type,
552 self._starts.dtype.type,
553 self._stops.dtype.type,
554 ](
555 outoffsets.data,
556 nextcarry.data,
557 slicestarts.data,
558 slicestops.data,
559 slicestarts.length,
560 sliceindex.data,
561 sliceindex.length,
562 self._starts.data,
563 self._stops.data,
564 self._content.length,
565 ),
566 slicer=ak.contents.ListArray(slicestarts, slicestops, slicecontent),
567 )
568 nextcontent = self._content._carry(nextcarry, True)
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/contents/content.py:271, in Content._maybe_index_error(self, error, slicer)
270 message = self._backend.format_kernel_error(error)
--> 271 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-17/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/envs/awkward-docs/lib/python3.10/site-packages/awkward/highlevel.py:949, in Array.__getitem__(self, where)
520 def __getitem__(self, where):
521 """
522 Args:
523 where (many types supported; see below): Index of positions to
(...)
947 have the same dimension as the array being indexed.
948 """
--> 949 with ak._errors.SlicingErrorContext(self, where):
950 out = self._layout[where]
951 if isinstance(out, ak.contents.NumpyArray):
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/_errors.py:63, in ErrorContext.__exit__(self, exception_type, exception_value, traceback)
60 try:
61 # Handle caught exception
62 if exception_type is not None and self.primary() is self:
---> 63 self.handle_exception(exception_type, exception_value)
64 finally:
65 # `_kwargs` may hold cyclic references, that we really want to avoid
66 # as this can lead to large buffers remaining in memory for longer than absolutely necessary
67 # Let's just clear this, now.
68 self._kwargs.clear()
File ~/micromamba/envs/awkward-docs/lib/python3.10/site-packages/awkward/_errors.py:78, in ErrorContext.handle_exception(self, cls, exception)
76 self.decorate_exception(cls, exception)
77 else:
---> 78 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-17/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.