[GitHub] [arrow] tianchen92 commented on a change in pull request #7887: ARROW-9304: [C++] Add "AppendEmpty" builder APIs for use inside StructBuilder::AppendNull

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tianchen92 commented on a change in pull request #7887:
URL: https://github.com/apache/arrow/pull/7887#discussion_r484177957



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File path: cpp/src/arrow/pretty_print.cc
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@@ -339,6 +339,17 @@ class ArrayPrinter : public PrettyPrinter {
     children.reserve(array.num_fields());
     for (int i = 0; i < array.num_fields(); ++i) {
       children.emplace_back(array.field(i));
+      // set field array null at index which parent struct is null,
+      // because StructBuilder#AppendNull dosen't set child null bitmap.
+      auto field_null_buf = array.field(i)->null_bitmap();
+      if (field_null_buf != nullptr) {
+        uint8_t* field_null_data = field_null_buf->mutable_data();

Review comment:
       Thanks for review, I have some questions, correct me if I am wrong:
   1 now seems AppendNull/AppendNulls set null bitmap and append empty value using data_builder_, if AppendEmpty/AppendEmpties also do this, what is the difference between this two API?
   2 From http://arrow.apache.org/docs/format/Columnar.html#struct-layout that there is no need for the child arrays to be null when the top-level struct slot is null, so I thought only 'empty' value should be appended (without setting null bitmap)。So in PrettyPrint, we need to change child null bitmap to ensure the correct output.




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