From: merch-redmine@...
Date: 2020-11-24T18:25:48+00:00
Subject: [ruby-core:101051] [Ruby master Bug#17264] BigDecimal exponentiation cannot be used with #** method

Issue #17264 has been updated by jeremyevans0 (Jeremy Evans).

Assignee set to mrkn (Kenta Murata)
Status changed from Open to Assigned

karatedog (F�ldes L�szl�) wrote:
> My suggestion is the #** method and #power method should work the same way or the #** method retired.

The methods actually do operate the same way.  `#**` is basically:

```ruby
def **(other)
  power(other)
end
```

So the issue is basically the precision for `power` when given one argument is not high enough, at least for fractional powers.  I've submitted a pull request to increase the precision, but the downside is it makes the power calculations much slower: https://github.com/ruby/bigdecimal/pull/171. It is up to @mrkn whether the benefits of increasing the precision are worth the performance decrease.

----------------------------------------
Bug #17264: BigDecimal exponentiation cannot be used with #** method
https://bugs.ruby-lang.org/issues/17264#change-88723

* Author: karatedog (F�ldes L�szl�)
* Status: Assigned
* Priority: Normal
* Assignee: mrkn (Kenta Murata)
* ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
As stated in Bug #17214 (https://bugs.ruby-lang.org/issues/17214) when exponentiating a BigDdecimal number even when using small numbers, a precision argument must be passed to the operation or the operation will return wrong result, 2222 and 3.5 not being 'huge' numbers:
``` ruby
(BigDecimal("2222",10000) ** BigDecimal("3.5",10000)).to_i
# => 517135311000
```

However the #** method cannot be passed a precision argument and as seen above it will return wrong values for small numbers even if the BigDecimal numbers themselves have a large precision argument.
Therefore this operation can only be valid if used with the #power method and provided with a larger precision argument:
``` ruby
BigDecimal(2222).power(3.5, 15).to_i #=> 517135308457
```
My suggestion is the #** method and #power method should work the same way or the #** method retired.



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>