Bug #17264: BigDecimal exponentiation cannot be used with #** method

• Status: Closed
• Assignee: mrkn (Kenta Murata)
• Target version: -
• ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[karatedog (Földes László)]

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:

(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:

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.

[jeremyevans0 (Jeremy Evans)]

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

karatedog (Földes László) 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:

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 (Kenta Murata) whether the benefits of increasing the precision are worth the performance decrease.

• Status changed from Assigned to Closed

Applied in changeset git|a86c147579745859ea064ec22b2901a7ac7e4abf.

---

Import bigdecimal 2.0.2 (#3905)

• remove duplicated include

• Make BigDecimal#round with argument < 1 return Integer

Fixes [Bug #12780]

• Use a higher default precision for BigDecimal#power and #**

When a fractional power is given, increase the precision if the
precision isn't specified via power's second argument:

Float: increase by 15 (rough number of decimal precision in float)
BigDecimal: increase by adding similar precision modifier as done to
calculate the base precision.
Rational: double the precision, since a BigDecimal is created, but
the created BigDecimal uses the same precision.

Increasing the precision for these power calculations has the obvious
tradeoff of making the calculations slower.

Fixes Ruby Bug #17264

• Use DBLE_FIG for a Float value

• Version 2.0.1

Co-authored-by: pavel [e-mail]
Co-authored-by: Jeremy Evans [e-mail]