[Pharo-dev] float & fraction equality bug
tudor at tudorgirba.com
Thu Nov 9 09:48:59 EST 2017
Thanks for the answer. The example I provided was for convenience.
I still do not understand why it is wrong to expect 0.1 = (1/10) to be true.
> On Nov 9, 2017, at 3:36 PM, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com> wrote:
> Nope, not a bug.
> If you use Float, then you have to know that (x -y) isZero and (x = y) are two different things.
> Example; Float infinity
> In your case you want to protect against (x-y) isZero, so just do that.
> 2017-11-09 15:15 GMT+01:00 Tudor Girba <tudor at tudorgirba.com>:
> I just stumbled across this bug related to the equality between fraction and float:
> In essence, the problem can be seen that by doing this, you get a ZeroDivide:
> x := 0.1.
> y := (1/10).
> x = y ifFalse: [ 1 / (x - y) ]
> The issue seems to come from the Float being turned to a Fraction, rather than the Fraction being turned into a Float:
> Fraction(Number)>>adaptToFloat: rcvr andCompare: selector
> "If I am involved in comparison with a Float, convert rcvr to a
> Fraction. This way, no bit is lost and comparison is exact."
> rcvr isFinite
> ifFalse: [
> selector == #= ifTrue: [^false].
> selector == #~= ifTrue: [^true].
> rcvr isNaN ifTrue: [^ false].
> (selector = #< or: [selector = #'<='])
> ifTrue: [^ rcvr positive not].
> (selector = #> or: [selector = #'>='])
> ifTrue: [^ rcvr positive].
> ^self error: 'unknow comparison selector'].
> ^ rcvr asTrueFraction perform: selector with: self
> Even if the comment says that the comparison is exact, to me this is a bug because it seems to fail doing that. What do you think?
> "Problem solving should be focused on describing
> the problem in a way that makes the solution obvious."
"We are all great at making mistakes."
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