[ARROW-9913] [C++] Outputs of Decimal128::FromString depend on presence of one another

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 2.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/25945
• Assignee: Mingyu Zhong
• Reporter: Mingyu Zhong
• Created: 03/Sep/20 22:51
• Updated: 11/Jan/23 08:09
• Resolved: 07/Sep/20 14:48

Description

https://github.com/apache/arrow/blame/bfac60dd73bffa5f7bcefc890486268036182278/cpp/src/arrow/util/decimal.cc#L365-L373 in Decimal128::FromString makes *out depend on whether scale is null, and makes *scale depend on whether out is null. For example, given an input "1e2",

1. if out is not null and scale is not null, then *out is 1
2. if out is null and scale is not null, then *scale is -2
3. if neither out nor scale is null, then *out is 100 and *scale is 0

It is very counter-intuitive that when an additional output-only pointer is given for receiving extra info, it alters the value of another output.

Similarly, *precision is also affected by presence of out and scale.

The block of adjustment was added in https://github.com/apache/arrow/commit/bfac60dd73bffa5f7bcefc890486268036182278 for preventing negative scale output (https://issues.apache.org/jira/browse/ARROW-2177). While the motivation looks reasonable, the change is not sufficient (see case #2 above).

I think we should make the outputs independent of one-another. For the input "1e2", *out should be 100 if out is not null, and *scale should be 0 if scale is not null.

Issue Links

• links to: GitHub Pull Request #8109