[R] Improve efficiency of decimal casting

[asfimport]

arrow/r/src/array_to_vector.cpp

Lines 975 to 990 in 04641a3

	Status Ingest_some_nulls(SEXP data, const std::shared_ptr<arrow::Array>& array,
	R_xlen_t start, R_xlen_t n, size_t chunk_index) const {
	auto p_data = REAL(data) + start;
	const auto& decimals_arr = checked_cast<const arrow::Decimal128Array&>(*array);
	auto ingest_one = [&](R_xlen_t i) {
	p_data[i] = std::stod(decimals_arr.FormatValue(i).c_str());
	return Status::OK();
	};
	auto null_one = [&](R_xlen_t i) {
	p_data[i] = NA_REAL;
	return Status::OK();
	};
	return IngestSome(array, n, ingest_one, null_one);
	}

"this looks quite inefficient (it roundtrips through a string representation instead of creating a double directly)."

from: #11898 (comment)

Reporter: Jonathan Keane / @jonkeane

Related issues:

• [R] Switch to cpp11 (is fixed by)

PRs and other links:

• Fixed by

_{Note: This issue was originally created as ARROW-15167. Please see the migration documentation for further details.}

[asfimport]

Dragoș Moldovan-Grünfeld / @dragosmg:
I think row 981 is the issue as std::stod converts a string to double

arrow/r/src/array_to_vector.cpp

Line 981 in 04641a3

p_data[i] = std::stod(decimals_arr.FormatValue(i).c_str());