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]] ------------------ backend: cpu nbytes: 144 B 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]] ------ backend: cpu nbytes: 40 B type: 5 * 1 * int64
ak.argmax(array, keepdims=True, axis=-1, mask_identity=True)
[[None], [0], [3], [None], [1]] -------- backend: cpu nbytes: 45 B 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)
Cell In[6], line 1
----> 1 array[index]
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/highlevel.py:1104, in Array.__getitem__(self, where)
675 def __getitem__(self, where):
676 """
677 Args:
678 where (many types supported; see below): Index of positions to
(...) 1102 have the same dimension as the array being indexed.
1103 """
-> 1104 with ak._errors.SlicingErrorContext(self, where):
1105 # Handle named axis
1106 (_, ndim) = self._layout.minmax_depth
1107 named_axis = _get_named_axis(self)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/_errors.py:80, in ErrorContext.__exit__(self, exception_type, exception_value, traceback)
78 self._slate.__dict__.clear()
79 # Handle caught exception
---> 80 raise self.decorate_exception(exception_type, exception_value)
81 else:
82 # Step out of the way so that another ErrorContext can become primary.
83 if self.primary() is self:
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/highlevel.py:1112, in Array.__getitem__(self, where)
1108 where = _normalize_named_slice(named_axis, where, ndim)
1110 NamedAxis.mapping = named_axis
-> 1112 indexed_layout = prepare_layout(self._layout._getitem(where, NamedAxis))
1114 if NamedAxis.mapping:
1115 return ak.operations.ak_with_named_axis._impl(
1116 indexed_layout,
1117 named_axis=NamedAxis.mapping,
(...) 1120 attrs=self._attrs,
1121 )
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:657, in Content._getitem(self, where, named_axis)
654 return out._getitem_at(0)
656 elif isinstance(where, ak.highlevel.Array):
--> 657 return self._getitem(where.layout, named_axis)
659 # Convert between nplikes of different backends
660 elif (
661 isinstance(where, ak.contents.Content)
662 and where.backend is not self._backend
663 ):
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:734, in Content._getitem(self, where, named_axis)
731 return where.to_NumpyArray(np.int64)
733 elif isinstance(where, Content):
--> 734 return self._getitem((where,), named_axis)
736 elif is_sized_iterable(where):
737 # Do we have an array
738 nplike = nplike_of_obj(where, default=None)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:649, in Content._getitem(self, where, named_axis)
640 named_axis.mapping = _named_axis
642 next = ak.contents.RegularArray(
643 this,
644 this.length,
645 1,
646 parameters=None,
647 )
--> 649 out = next._getitem_next(nextwhere[0], nextwhere[1:], None)
651 if out.length is not unknown_length and out.length == 0:
652 return out._getitem_nothing()
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/regulararray.py:722, in RegularArray._getitem_next(self, head, tail, advanced)
706 assert head.offsets.nplike is nplike
707 self._maybe_index_error(
708 self._backend[
709 "awkward_RegularArray_getitem_jagged_expand",
(...) 720 slicer=head,
721 )
--> 722 down = self._content._getitem_next_jagged(
723 multistarts, multistops, head._content, tail
724 )
726 return RegularArray(
727 down, headlength, self.length, parameters=self._parameters
728 )
730 elif isinstance(head, ak.contents.IndexedOptionArray):
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/listoffsetarray.py:441, in ListOffsetArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
435 def _getitem_next_jagged(
436 self, slicestarts: Index, slicestops: Index, slicecontent: Content, tail
437 ) -> Content:
438 out = ak.contents.ListArray(
439 self.starts, self.stops, self._content, parameters=self._parameters
440 )
--> 441 return out._getitem_next_jagged(slicestarts, slicestops, slicecontent, tail)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/listarray.py:578, in ListArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
567 nextcarry = ak.index.Index64.empty(carrylen, self._backend.nplike)
569 assert (
570 outoffsets.nplike is self._backend.nplike
571 and nextcarry.nplike is self._backend.nplike
(...) 576 and self._stops.nplike is self._backend.nplike
577 )
--> 578 self._maybe_index_error(
579 self._backend[
580 "awkward_ListArray_getitem_jagged_apply",
581 outoffsets.dtype.type,
582 nextcarry.dtype.type,
583 slicestarts.dtype.type,
584 slicestops.dtype.type,
585 sliceindex.dtype.type,
586 self._starts.dtype.type,
587 self._stops.dtype.type,
588 ](
589 outoffsets.data,
590 nextcarry.data,
591 slicestarts.data,
592 slicestops.data,
593 slicestarts.length,
594 sliceindex.data,
595 sliceindex.length,
596 self._starts.data,
597 self._stops.data,
598 self._content.length,
599 ),
600 slicer=ak.contents.ListArray(slicestarts, slicestops, slicecontent),
601 )
602 nextcontent = self._content._carry(nextcarry, True)
603 nexthead, nexttail = ak._slicing.head_tail(tail)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:295, in Content._maybe_index_error(self, error, slicer)
293 else:
294 message = self._backend.format_kernel_error(error)
--> 295 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-48/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]] -------- backend: cpu nbytes: 112 B 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]] ------------------ backend: cpu nbytes: 136 B 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]] --------- backend: cpu nbytes: 96 B type: 4 * option[var * int64]
If we compare this with simply providing an empty sublist,
array[
[
[0, 1],
[],
[],
[2],
],
]
[[10, 3], [], [], [-1]] --------- backend: cpu nbytes: 64 B 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.