edponce commented on a change in pull request #11159:
URL: https://github.com/apache/arrow/pull/11159#discussion_r709831886
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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));
Review comment:
You can preallocate the output buffer:
`builder->Reserve(list_array.length());`
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