[GitHub] [arrow] pitrou commented on a change in pull request #10100: ARROW-11660: [C++] Move RecordBatch::SelectColumns method from R to C++ library

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

pitrou commented on a change in pull request #10100:
URL: https://github.com/apache/arrow/pull/10100#discussion_r617619087



##########
File path: cpp/src/arrow/record_batch.cc
##########
@@ -253,6 +253,27 @@ bool RecordBatch::ApproxEquals(const RecordBatch& other) const {
   return true;
 }
 
+Result<std::shared_ptr<RecordBatch>> RecordBatch::SelectColumns(
+    const std::vector<int>& indices) const {
+  int n = static_cast<int>(indices.size());
+
+  std::vector<std::shared_ptr<Field>> fields(n);
+  std::vector<std::shared_ptr<Array>> columns(n);

Review comment:
       Nit, but you can use the `FieldVector` and `ArrayVector` type aliases, respectively.

##########
File path: cpp/src/arrow/record_batch.cc
##########
@@ -253,6 +253,27 @@ bool RecordBatch::ApproxEquals(const RecordBatch& other) const {
   return true;
 }
 
+Result<std::shared_ptr<RecordBatch>> RecordBatch::SelectColumns(
+    const std::vector<int>& indices) const {
+  int n = static_cast<int>(indices.size());
+
+  std::vector<std::shared_ptr<Field>> fields(n);
+  std::vector<std::shared_ptr<Array>> columns(n);
+
+  for (int i = 0; i < n; i++) {
+    int pos = indices[i];
+    if (pos < 0 || pos > num_columns() - 1) {
+      return Status::Invalid("Invalid column index ", pos, " to select columns.");
+    }
+    fields[i] = schema()->field(pos);
+    columns[i] = column(pos);
+  }
+
+  auto new_schema =
+      std::make_shared<arrow::Schema>(std::move(fields), schema()->metadata());
+  return RecordBatch::Make(new_schema, num_rows(), columns);

Review comment:
       `std::move(columns)` will avoid a vector copy here.




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