2017-11-09 15:34 GMT+01:00 Martin McClure <[email protected]>: > On 11/09/2017 06:15 AM, Tudor Girba wrote: > > Hi, > > I just stumbled across this bug related to the equality between fraction > and float: > https://pharo.fogbugz.com/f/cases/20488/x-y-iff-x-y-0-is- > not-preserved-in-Pharo > > 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? > > > I think exact comparison is the best thing to do here (even though ANSI > says otherwise). And an exact comparison will answer false, because there > is no float that is exactly equal to 1/10 -- it is an infinitely repeating > decimal in binary. > > For consistency, though, it might be better to compute x - y by converting > the Float to a Fraction rather than the other way around (though this would > also contravene ANSI) since Fraction is the more general format (every > Float can be represented exactly as a Fraction, but the reverse is not > true). > > The POV is that Float are potentially inexact (some quantity rounded to nearest representable Float). While Fraction are exact.
If you mix exact quantity with inexact, then the result is inexact (thus Float). It does not hurt that this choice is CPU friendly. Regards, > -Martin >
