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GH-49674: [C++][Array] Preserve ordered flag for DictionaryType in MakeEmptyArray #49798

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GH-49674: [C++][Array] Preserve ordered flag for DictionaryType in MakeEmptyArray #49798

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21 changes: 21 additions & 0 deletions cpp/src/arrow/array/array_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -4433,4 +4433,25 @@ TEST_F(TestDayTimeIntervalBuilder, TestConstructors) {
ASSERT_TRUE(builder3.type()->Equals(type));
}

TEST(ArrayTest, MakeEmptyPreservesDictionaryOrderedFlagInArray) {
auto dict_type = arrow::dictionary(arrow::int32(), arrow::utf8(), /*ordered=*/true);
auto schema = arrow::schema({arrow::field("col", dict_type)});

ASSERT_OK_AND_ASSIGN(auto batch, arrow::RecordBatch::MakeEmpty(schema));

ASSERT_EQ(batch->num_rows(), 0);
ASSERT_EQ(batch->num_columns(), 1);

// Schema should be correct
auto schema_type =
std::static_pointer_cast<arrow::DictionaryType>(batch->schema()->field(0)->type());
// Ensure array type preserves the ordered flag
auto array_type =
std::static_pointer_cast<arrow::DictionaryType>(batch->column(0)->type());

ASSERT_TRUE(schema_type->ordered());
ASSERT_TRUE(array_type->ordered()) << "MakeEmpty() lost ordered flag in array type";
ASSERT_TRUE(array_type->Equals(*schema_type));
}

} // namespace arrow
7 changes: 6 additions & 1 deletion cpp/src/arrow/array/util.cc
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Expand Up @@ -924,7 +924,12 @@ Result<std::shared_ptr<Array>> MakeEmptyArray(std::shared_ptr<DataType> type,
std::unique_ptr<ArrayBuilder> builder;
RETURN_NOT_OK(MakeBuilder(memory_pool, type, &builder));
RETURN_NOT_OK(builder->Resize(0));
return builder->Finish();
ARROW_ASSIGN_OR_RAISE(auto result, builder->Finish());
// Preserve original DictionaryType (including ordered flag)
if (type->id() == Type::DICTIONARY) {
result->data()->type = type;
}
return result;
}

namespace internal {
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