Issue #7037 has been updated by headius (Charles Nutter).


If there's nothing further to do here and we all agree that the details of rounding logic are implementation-dependent, this can be closed.
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Bug #7037: float formatting inconsistently rounds half to even
https://bugs.ruby-lang.org/issues/7037#change-30664

Author: headius (Charles Nutter)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: 
ruby -v: 2.0.0dev


MRI does not appear to consistently round half to even. I'm not sure what rounding strategy this is, but it rounds xx05 and xx15 to odd for xxx, and other values to even:

irb(main):001:0> "%1.1f" % 1.05
=> "1.1"
irb(main):002:0> "%1.1f" % 1.15
=> "1.1"
irb(main):003:0> "%1.1f" % 1.25
=> "1.2"
irb(main):004:0> "%1.1f" % 1.35
=> "1.4"
irb(main):005:0> "%1.1f" % 1.45
=> "1.4"
irb(main):006:0> "%1.1f" % 1.55
=> "1.6"

None of the tie-breaking strategies I could find (http://en.wikipedia.org/wiki/Rounding#Tie-breaking) seem to support MRI's model.

If MRI is indeed using "half even", xx05 should round to xx0 and xx15 should round to xx2. An example with Java's BigDecimal appears to support this:

irb(main):029:0> java.math.BigDecimal.new('1.05').round(java.math.MathContext.new(2, java.math.RoundingMode::HALF_EVEN)).to_s
=> "1.0"
irb(main):030:0> java.math.BigDecimal.new('1.15').round(java.math.MathContext.new(2, java.math.RoundingMode::HALF_EVEN)).to_s
=> "1.2"
irb(main):031:0> java.math.BigDecimal.new('1.25').round(java.math.MathContext.new(2, java.math.RoundingMode::HALF_EVEN)).to_s
=> "1.2"
irb(main):032:0> java.math.BigDecimal.new('1.35').round(java.math.MathContext.new(2, java.math.RoundingMode::HALF_EVEN)).to_s
=> "1.4"

We would like clarification about the proper rounding tie-breaker strategy to use so we can fix this JRuby issue properly: http://jira.codehaus.org/browse/JRUBY-6889


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