paleolimbot commented on a change in pull request #12533:
URL: https://github.com/apache/arrow/pull/12533#discussion_r818222944
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File path: r/src/recordbatchreader.cpp
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@@ -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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