Bug #19321: BigDecimal multiplication giving different result since Ruby 3.1

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.1.3
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

[alexandruanca (Alexandru Anca)]

I have this math operation 120.to_d * 0.07 and I noticed that since upgrading to Ruby 3.1 I get different results. I would expect to get the same result as in previous versions.

rbenv local 3.0.3
➜  irb
require 'bigdecimal/util.rb'
(120.to_d * 0.07).to_s('F')
=> "8.4000000000000012"

rbenv local 3.1.3
✗ irb
require 'bigdecimal/util.rb'
(120.to_d * 0.07).to_s('F')
=> "8.4"

It looks like the change has been introduced in Ruby 3.1. I tested Ruby 2.7.0 and I get the same result as in 3.0 ("8.4000000000000012")
I saw in the release notes that there were changes to bigdecimal so I am not sure if this is expected or it's a bug

[nobu (Nobuyoshi Nakada)]

The newer result seems arithmetically correct.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Closed

The old result was incorrect. The new, correct result is due to a bug fix. I'm going to close this. If you can find a result that is incorrect in Ruby 3.1 or 3.2, please post it.

[nobu (Nobuyoshi Nakada)]

JFI: it is https://github.com/ruby/bigdecimal/pull/180, which fixed https://github.com/ruby/bigdecimal/issues/70 and #13331.

[alexandruanca (Alexandru Anca)]

The old result was incorrect. The new, correct result is due to a bug fix. I'm going to close this. If you can find a result that is incorrect in Ruby 3.1 or 3.2, please post it.

I see, makes sence. Thanks for the explanation