[rules-users] [rules user] different behavior on double type

[Zeke]

Whether the comparison is precarious or not should depend on how you handle
the literal. If you handle 99.9 as BigDecimal("99.9") which precision can
be controlled, you can trust the comparison.

Anyway, since it has different behavior for these two cases, it should be a
bug, right? I just want to know it is Drools' bug or MVEL's... If it is
MVEL's, I may need ask for MVEL guys' help...

On 26 March 2012 19:50, Wolfgang Laun <wolfgang.laun at gmail.com> wrote:

> 4.0.7 is rather old. 5.1.1 and later do not exhibit this behaviour.
>
> However, it should be noted that comparing double values using == is a
> precarious approach. For instance, 99.9 (the mathematical number)
> cannot be represented as a double machine number. Hence, you do not
> truly test for
> equality with that number, just with its approximation by some machine
> number.
>
> -W
>
> On 26/03/2012, Zeke <xanadu860122 at gmail.com> wrote:
> > Hi, guys:
> >   I am using Drools 4.0.7. I find a strange behavior. When my LHS is
> > "Message( varDouble == 99.9 )" and "varDouble" equals "new Double(99.9)",
> > the condition is true. But when my LHS is "Message( types.doubleType ==
> > 99.9 )" and the "doubleType" field is also equal to "new Double(99.9)",
> the
> > condition is false. I debug the code, and find that:
> >
> >   When the LHS is "Message( varDouble == 99.9 )", the logic will handle
> the
> > literal 99.9 as *Double.valueOf( “99.9” )*, and the comparison is between
> > *(new
> > Double(99.9)).doubleValue(*) and *Double.valueOf( “99.9”
> ).doubleValue()*,
> >   When the LHS is "Message( types.doubleType == 99.9 )", the logic is
> > different from above. The comparison is between *new BigDecimal((Double)
> > 99.9, MathContext.DECIMAL64)* (which double value is 99.90000000000001)
> and
> > *new BigDecimal(“99.9”, MathContext.DECIMAL64)* (which double value is
> > 99.9).
> >
> >   I think we need make things consistent. "varDouble" and
> > "types.doubleType" are of the same type and value. It doesn't make sense
> to
> > have different behavior just because of one field is nesting while the
> > other is not.  I know Drools is using MVEL to evaluate the condition.
> But I
> > am not sure it is a MVEL bug or Drools uses MVEL wrong. It will be much
> > appreciated if you can give me any comments!
> >
>
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