[GitHub] [arrow] mapleFU commented on a diff in pull request #34961: GH-34960: [C++] test util Fixing arrow Random Generator for lost nullable info

["mapleFU (via GitHub)" <gi...@apache.org>]

mapleFU commented on code in PR #34961:
URL: https://github.com/apache/arrow/pull/34961#discussion_r1161544365


##########
cpp/src/arrow/testing/random.cc:
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@@ -793,7 +793,7 @@ std::shared_ptr<Array> RandomArrayGenerator::ArrayOf(const Field& field, int64_t
                 values_length, alignment, memory_pool);                              \
     const auto offsets = OffsetsFromLengthsArray(lengths.get(), force_empty_nulls,   \
                                                  alignment, memory_pool);            \
-    return *ARRAY_TYPE::FromArrays(*offsets, *values);                               \
+    return *ARRAY_TYPE::FromArrays(field.type(), *offsets, *values);                 \

Review Comment:
   No, the problem is a bit tricky. Let me explain it:
   1. Arrow has `DataType`, `DataType` doesn't including some nullable info itself. `Field` has nullable info.
   2. For `DataType` , it child would be array of `Field`, so DataType has children's nullable info
   
   Here if doesn't pass `field.type()` in ctor, it will mark child as nullable. And Map's constructor already marks it.



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