js8544 commented on code in PR #36289:
URL: https://github.com/apache/arrow/pull/36289#discussion_r1259131419
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docs/source/cpp/compute.rst:
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@@ -563,30 +563,34 @@ representation based on the rounding criterion.
+-------------------+------------+-------------+-------------------------+----------------------------------+--------+
| floor | Unary | Numeric | Float32/Float64/Decimal |
| |
+-------------------+------------+-------------+-------------------------+----------------------------------+--------+
-| round | Unary | Numeric | Float32/Float64/Decimal |
:struct:`RoundOptions` | (1)(2) |
+| round | Unary | Numeric | Input Type |
:struct:`RoundOptions` | (1)(2) |
+-------------------+------------+-------------+-------------------------+----------------------------------+--------+
-| round_to_multiple | Unary | Numeric | Float32/Float64/Decimal |
:struct:`RoundToMultipleOptions` | (1)(3) |
+| round_to_multiple | Unary | Numeric | Input Type |
:struct:`RoundToMultipleOptions` | (1)(3) |
+-------------------+------------+-------------+-------------------------+----------------------------------+--------+
| trunc | Unary | Numeric | Float32/Float64/Decimal |
| |
+-------------------+------------+-------------+-------------------------+----------------------------------+--------+
-* \(1) Output value is a 64-bit floating-point for integral inputs and the
- retains the same type for floating-point and decimal inputs. By default
- rounding functions displace a value to the nearest integer using
- HALF_TO_EVEN to resolve ties. Options are available to control the rounding
- criterion. Both ``round`` and ``round_to_multiple`` have the ``round_mode``
- option to set the rounding mode.
+* \(1) By default rounding functions displace a value to the nearest
+ integer using HALF_TO_EVEN to resolve ties. Options are available to
control
+ the rounding criterion. Both ``round`` and ``round_to_multiple`` have the
+ ``round_mode`` option to set the rounding mode.
* \(2) Round to a number of digits where the ``ndigits`` option of
:struct:`RoundOptions` specifies the rounding precision in terms of number
of digits. A negative value corresponds to digits in the non-fractional
part. For example, -2 corresponds to rounding to the nearest multiple of
100 (zeroing the ones and tens digits). Default value of ``ndigits`` is 0
- which rounds to the nearest integer.
+ which rounds to the nearest integer. For integer inputs a non-negative
+ ``ndigits`` value is ignored and the input is returned unchanged. For integer
+ inputs, if ``-ndigits`` is larger than the maximum number of digits the
+ input type can hold, it is truncated to the maximum digit. For example,
+ ``round([123], ndigits=-4, round_mode=DOWN)`` returns [100] for ``int8``
type.
Review Comment:
I agree that we should align with RoundToMultiple. I changed the behavior to
rejecting the operation when -ndigits is too large.
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