zanmato1984 commented on code in PR #44394:
URL: https://github.com/apache/arrow/pull/44394#discussion_r1823743699
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cpp/src/arrow/compute/api_vector.h:
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@@ -705,5 +736,56 @@ Result<std::shared_ptr<Array>> PairwiseDiff(const Array&
array,
bool check_overflow = false,
ExecContext* ctx = NULLPTR);
+/// \brief Return the reverse indices of the given indices.
+///
+/// For indices[i] = x, reverse_indices[x] = i. And reverse_indices[x] = null
if x does
+/// not appear in the input indices. For indices[i] = x where x < 0 or x >=
output_length,
+/// it is ignored. If multiple indices point to the same value, the last one
is used.
+///
+/// For example, with indices = [null, 0, 3, 2, 4, 1, 1], the reverse indices
is
+/// [1, 6, 3] if output_length = 3,
+/// [1, 6, 3, 2, 4, null, null] if output_length = 7.
Review Comment:
It is particularly useful for cases where the indices are only a subset of
the output (in other words, the output contains "holes"/`null`s). Consider an
input `[3, 0]` (imagine that row `1` and `2` are evaluated to `null` in the
`if_else` special form's condition so they go to neither `true` nor `false`
branch, thus having no slots in the input to `reverse_indices`/`permute`), it
is desired to have the output as `[1, null, null, 0]` (as opposed to a
presumable length-2 output `[1, null]`).
This is also noted in my comments in one of the tests:
https://github.com/apache/arrow/pull/44394/files#diff-ee27abfb87a9105ebbdb8bdcdd26aaec752f2631181a078e3ef5a07b4f00d1ceR995-R1000
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