[GitHub] [arrow] fsaintjacques commented on a change in pull request #7435: ARROW-8779: [R] Implement conversion to List

[GitBox <gi...@apache.org>]

fsaintjacques commented on a change in pull request #7435:
URL: https://github.com/apache/arrow/pull/7435#discussion_r440369138



##########
File path: r/src/array_from_vector.cpp
##########
@@ -201,6 +201,39 @@ struct VectorToArrayConverter {
     return Status::OK();
   }
 
+  template <typename T>
+  arrow::enable_if_t<arrow::is_struct_type<T>::value, Status> Visit(const T& type) {
+    using BuilderType = typename TypeTraits<T>::BuilderType;
+    ARROW_RETURN_IF(!Rf_inherits(x, "data.frame"),
+                    Status::RError("Expecting a data frame"));
+
+    auto* struct_builder = checked_cast<BuilderType*>(builder);
+
+    int64_t n = 0;
+    if (XLENGTH(x) > 0) {
+      // TODO: need a more generic way to get the vec_size(<data.frame>)
+      //       perhaps using vctrs::::short_vec_size
+      n = XLENGTH(VECTOR_ELT(x, 0));
+    }
+    RETURN_NOT_OK(struct_builder->Reserve(n));
+
+    // TODO: double check but seems fine that no value is NULL

Review comment:
       You can remove the TODO, this is correct.

##########
File path: r/tests/testthat/test-Array.R
##########
@@ -445,6 +445,9 @@ test_that("Array$create() handles vector -> list arrays (ARROW-7662)", {
   expect_array_roundtrip(list(character(0)), list_of(utf8()))
   expect_array_roundtrip(list("itsy", c("bitsy", "spider"), c("is")), list_of(utf8()))
   expect_array_roundtrip(list("itsy", character(0), c("bitsy", "spider", NA_character_), c("is")), list_of(utf8()))
+
+  # struct
+  expect_array_roundtrip(list(tibble::tibble(a = integer(0))), list_of(struct(a = int32())))

Review comment:
       Add a bit more columns: float, string, bool. Also add a failing case where columns are not of the same size.




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