jorisvandenbossche commented on code in PR #19706:
URL: https://github.com/apache/arrow/pull/19706#discussion_r1068333553
##########
r/R/expression.R:
##########
@@ -89,6 +92,56 @@ Expression$create <- function(function_name,
expr
}
+
+#' @export
+`[[.Expression` <- function(x, i, ...) {
+ # TODO: integer (positional) field refs are supported in C++
+ assert_that(is.string(i))
+ get_nested_field(x, i)
+}
+
+#' @export
+`$.Expression` <- function(x, name, ...) {
+ assert_that(is.string(name))
+ if (name %in% ls(x)) {
+ get(name, x)
+ } else {
+ get_nested_field(x, name)
+ }
+}
+
+get_nested_field <- function(expr, name) {
+ if (expr$is_field_ref()) {
+ # Make a nested field ref
+ out <- compute___expr__nested_field_ref(expr, name)
+ } else {
+ # Use the struct_field kernel, but that only works if:
+ # * expr has a knowable type (has a schema set)
+ # * that type is struct
+ # * `name` exists in the struct (bc we have to map to an integer position)
Review Comment:
FYI, nowadays it shouldn't be need to map it to an integer position, the
"struct_field" kernel now also accepts a string name field ref
##########
r/src/expression.cpp:
##########
@@ -46,13 +46,26 @@ std::shared_ptr<compute::Expression>
compute___expr__call(std::string func_name,
compute::call(std::move(func_name), std::move(arguments),
std::move(options_ptr)));
}
+// [[arrow::export]]
+bool compute___expr__is_field_ref(const std::shared_ptr<compute::Expression>&
x) {
+ return x->field_ref() != nullptr;
+}
+
// [[arrow::export]]
std::vector<std::string> field_names_in_expression(
const std::shared_ptr<compute::Expression>& x) {
std::vector<std::string> out;
+ std::vector<arrow::FieldRef> nested;
+
auto field_refs = FieldsInExpression(*x);
for (auto f : field_refs) {
- out.push_back(*f.name());
+ if (f.IsNested()) {
+ // We keep the top-level field name.
Review Comment:
This might not be used in practice (in a `mutate` call where you select the
field, you also directly specify the resulting column name), but otherwise it
might also make sense to keep the innermost field name?
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