Function ColumnarTableToVector

cpp/examples/arrow/row_wise_conversion_example.cc:121–173

}

arrow::Result<std::vector<data_row>> ColumnarTableToVector(
    const std::shared_ptr<arrow::Table>& table) {
  // To convert an Arrow table back into the same row-wise representation as in the
  // above section, we first will check that the table conforms to our expected
  // schema and then will build up the vector of rows incrementally.
  //
  // For the check if the table is as expected, we can utilise solely its schema.
  std::vector<std::shared_ptr<arrow::Field>> schema_vector = {
      arrow::field("id", arrow::int64()), arrow::field("components", arrow::int64()),
      arrow::field("component_cost", arrow::list(arrow::float64()))};
  auto expected_schema = std::make_shared<arrow::Schema>(schema_vector);

  if (!expected_schema->Equals(*table->schema())) {
    // The table doesn't have the expected schema thus we cannot directly
    // convert it to our target representation.
    return arrow::Status::Invalid("Schemas are not matching!");
  }

  // As we have ensured that the table has the expected structure, we can unpack the
  // underlying arrays. For the primitive columns `id` and `components` we can use the
  // high level functions to get the values whereas for the nested column
  // `component_costs` we need to access the C-pointer to the data to copy its
  // contents into the resulting `std::vector<double>`. Here we need to be careful to
  // also add the offset to the pointer. This offset is needed to enable zero-copy
  // slicing operations. While this could be adjusted automatically for double
  // arrays, this cannot be done for the accompanying bitmap as often the slicing
  // border would be inside a byte.

  auto ids = std::static_pointer_cast<arrow::Int64Array>(table->column(0)->chunk(0));
  auto components =
      std::static_pointer_cast<arrow::Int64Array>(table->column(1)->chunk(0));
  auto component_cost =
      std::static_pointer_cast<arrow::ListArray>(table->column(2)->chunk(0));
  auto component_cost_values =
      std::static_pointer_cast<arrow::DoubleArray>(component_cost->values());
  // To enable zero-copy slices, the native values pointer might need to account
  // for this slicing offset. This is not needed for the higher level functions
  // like Value(…) that already account for this offset internally.
  const double* ccv_ptr = component_cost_values->raw_values();
  std::vector<data_row> rows;
  for (int64_t i = 0; i < table->num_rows(); i++) {
    // Another simplification in this example is that we assume that there are
    // no null entries, e.g. each row is fill with valid values.
    int64_t id = ids->Value(i);
    int64_t component = components->Value(i);
    const double* first = ccv_ptr + component_cost->value_offset(i);
    const double* last = ccv_ptr + component_cost->value_offset(i + 1);
    std::vector<double> components_vec(first, last);
    rows.push_back({id, component, components_vec});
  }

  return rows;
}

arrow::Status RunRowConversion() {
  std::vector<data_row> original_rows = {