Bug #13331: Inconsistent conversion of Float into Decimal

• Status: Closed
• Assignee: mrkn (Kenta Murata)
• Target version: -
• ruby -v: ruby 2.4.0
• Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
• Tags: ext

[msatkiewicz (Maciej Satkiewicz)]

One would reasonably expect the equality 1.to_d * f == f.t_d to hold for every Float value f, i.e. multiplying by DigDecimal's unit can be viewed as an alternative way of casting Float into Decimal; or simply one could expect that BigDecimal#* should always invoke Float#to_d on it's argument. Yet for some Floats (e.g. 64.4) the results differ, as showed in the provided script. This can cause a bug in some application contexts.

Originally I asked about that on Stackoverflow:
http://stackoverflow.com/questions/40472933/inconsistent-conversion-of-float-into-decimal-in-ruby/40473007

See the comprehensive answer by Stefan who points out that it is due to different precisions being used in Float#to_d and BigDecimal#*.

[msatkiewicz (Maciej Satkiewicz)]

• Description updated (diff)

[mrkn (Kenta Murata)]

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

Continue this in https://github.com/ruby/bigdecimal/issues/70

• Status changed from Assigned to Closed

Applied in changeset git|e1424c35012ccf1f3822c9046081b225aef4142e.

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[bigdecimal] Fix the default precision of Float#to_d

Fix https://github.com/ruby/bigdecimal/issues/70
[Bug #13331]

https://github.com/ruby/bigdecimal/commit/aa536cd4b5