[GitHub] [arrow] paleolimbot commented on a change in pull request #12533: ARROW-15817: [R] Use TableSourceNode instead of InMemoryDataset

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paleolimbot commented on a change in pull request #12533:
URL: https://github.com/apache/arrow/pull/12533#discussion_r818222944



##########
File path: r/src/recordbatchreader.cpp
##########
@@ -51,6 +51,20 @@ std::shared_ptr<arrow::Table> Table__from_RecordBatchReader(
   return table;
 }
 
+// [[arrow::export]]
+std::shared_ptr<arrow::Table> RecordBatchReader__Head(
+    const std::shared_ptr<arrow::RecordBatchReader>& reader, int64_t num_rows) {
+  std::vector<std::shared_ptr<arrow::RecordBatch>> batches;
+  std::shared_ptr<arrow::RecordBatch> this_batch;
+  while (num_rows > 0) {
+    this_batch = ValueOrStop(reader->Next());
+    if (this_batch == nullptr) break;
+    batches.push_back(this_batch->Slice(0, num_rows));
+    num_rows -= this_batch->num_rows();
+  }
+  return ValueOrStop(arrow::Table::FromRecordBatches(reader->schema(), batches));
+}

Review comment:
       My reading of this is that it may give more rows than `num_rows`...is that the intention? I get that it's awkward because the record batch reader is mutable (as Jon noted below).




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