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:1105, in Array.__getitem__(self, where)
676 def __getitem__(self, where):
677 """
678 Args:
679 where (many types supported; see below): Index of positions to
(...)
1103 have the same dimension as the array being indexed.
1104 """
-> 1105 with ak._errors.SlicingErrorContext(self, where):
1106 # Handle named axis
1107 (_, ndim) = self._layout.minmax_depth
1108 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:1113, in Array.__getitem__(self, where)
1109 where = _normalize_named_slice(named_axis, where, ndim)
1111 NamedAxis.mapping = named_axis
-> 1113 indexed_layout = prepare_layout(self._layout._getitem(where, NamedAxis))
1115 if NamedAxis.mapping:
1116 return ak.operations.ak_with_named_axis._impl(
1117 indexed_layout,
1118 named_axis=NamedAxis.mapping,
(...)
1121 attrs=self._attrs,
1122 )
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:710, in RegularArray._getitem_next(self, head, tail, advanced)
694 assert head.offsets.nplike is index_nplike
695 self._maybe_index_error(
696 self._backend[
697 "awkward_RegularArray_getitem_jagged_expand",
(...)
708 slicer=head,
709 )
--> 710 down = self._content._getitem_next_jagged(
711 multistarts, multistops, head._content, tail
712 )
714 return RegularArray(
715 down, headlength, self._length, parameters=self._parameters
716 )
718 elif isinstance(head, ak.contents.IndexedOptionArray):
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/listoffsetarray.py:443, in ListOffsetArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
437 def _getitem_next_jagged(
438 self, slicestarts: Index, slicestops: Index, slicecontent: Content, tail
439 ) -> Content:
440 out = ak.contents.ListArray(
441 self.starts, self.stops, self._content, parameters=self._parameters
442 )
--> 443 return out._getitem_next_jagged(slicestarts, slicestops, slicecontent, tail)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/listarray.py:567, in ListArray._getitem_next_jagged(self, slicestarts, slicestops, slicecontent, tail)
556 nextcarry = ak.index.Index64.empty(carrylen, self._backend.index_nplike)
558 assert (
559 outoffsets.nplike is self._backend.index_nplike
560 and nextcarry.nplike is self._backend.index_nplike
(...)
565 and self._stops.nplike is self._backend.index_nplike
566 )
--> 567 self._maybe_index_error(
568 self._backend[
569 "awkward_ListArray_getitem_jagged_apply",
570 outoffsets.dtype.type,
571 nextcarry.dtype.type,
572 slicestarts.dtype.type,
573 slicestops.dtype.type,
574 sliceindex.dtype.type,
575 self._starts.dtype.type,
576 self._stops.dtype.type,
577 ](
578 outoffsets.data,
579 nextcarry.data,
580 slicestarts.data,
581 slicestops.data,
582 slicestarts.length,
583 sliceindex.data,
584 sliceindex.length,
585 self._starts.data,
586 self._stops.data,
587 self._content.length,
588 ),
589 slicer=ak.contents.ListArray(slicestarts, slicestops, slicecontent),
590 )
591 nextcontent = self._content._carry(nextcarry, True)
592 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-44/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.