edponce commented on a change in pull request #11159:
URL: https://github.com/apache/arrow/pull/11159#discussion_r709801049
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File path: cpp/src/arrow/compute/kernels/scalar_nested.cc
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@@ -62,6 +62,57 @@ const FunctionDoc list_value_length_doc{
"Null values emit a null in the output."),
{"lists"}};
+template <typename Type, typename IndexType>
+Status ListElement(KernelContext* ctx, const ExecBatch& batch, Datum* out) {
+ using ListArrayType = typename TypeTraits<Type>::ArrayType;
+ using IndexScalarType = typename TypeTraits<IndexType>::ScalarType;
+ ListArrayType list_array(batch[0].array());
+ const auto& index_scalar = batch[1].scalar_as<IndexScalarType>();
+ auto index = index_scalar.value;
+ if (ARROW_PREDICT_FALSE(index < 0)) {
+ return Status::Invalid("Index ", index,
+ " is out of bounds: should be greater than or equal
to 0");
+ }
+ std::unique_ptr<ArrayBuilder> builder;
+ RETURN_NOT_OK(MakeBuilder(ctx->memory_pool(), list_array.value_type(),
&builder));
+ for (int i = 0; i < list_array.length(); ++i) {
+ if (list_array.IsNull(i)) {
+ RETURN_NOT_OK(builder->AppendNull());
+ continue;
+ }
+ std::shared_ptr<arrow::Array> value_array = list_array.value_slice(i);
+ auto len = value_array->length();
+ if (ARROW_PREDICT_FALSE(index > len)) {
+ return Status::Invalid("Index ", index, " is out of bounds: should be in
[0, ", len,
+ "]");
+ }
+ ARROW_ASSIGN_OR_RAISE(auto scalar, (value_array->GetScalar(index)));
+ RETURN_NOT_OK(builder->AppendScalar(*scalar));
+ }
+ ARROW_ASSIGN_OR_RAISE(auto result, builder->Finish());
+ out->value = result->data();
+ return Status::OK();
+}
+
+template <typename InListType, typename IndexType>
+void AddListElement(ScalarFunction* func) {
+ ScalarKernel
kernel{KernelSignature::Make({InputType::Array(InListType::type_id),
+
InputType::Scalar(IndexType::type_id)},
+ OutputType{ListValuesType},
+ /*is_varargs=*/false),
+ ListElement<InListType, IndexType>};
+ kernel.null_handling = NullHandling::COMPUTED_NO_PREALLOCATE;
+ kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
+ DCHECK_OK(func->AddKernel(std::move(kernel)));
+}
+
+const FunctionDoc list_element_doc(
+ "Compute elements using of nested list values using an index",
+ ("`lists` must have a list-like type.\n"
+ "For each value in each list of `lists`, the element at `index`\n"
+ "is emitted."),
+ {"lists", "index"});
Review comment:
In the long description, you can add a note about `null` behavior: "Null
values emit a null in the output."
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