AUDIT: r17421 - gnucash/trunk/lib/libqof/qof - Add a new function to the gnc_numeric library that converts denominators to exact powers of ten.

[Charles Day]

On Mon, Sep 1, 2008 at 1:57 PM, Christian Stimming <stimming at tuhh.de> wrote:

> Am Montag, 1. September 2008 22:44 schrieb Charles Day:
> > > > +  fraction = converted_val.denom;
> > > > +  if (fraction <= 0)
> > > > +    return FALSE;
> > >
> > > out of curiosity, what is the reason for disallowing negative
> > > denominators?
>
> My immediate reaction to this question: Denominators should be positive so
> that a comparison of equal numbers gives predictable results, and also to
> avoid any confusion about the actual meaning of a negative denominator.
>
> > > I am not sure whether there is a gnc_numeric API function
> > > to switch signs of nom and denom safely, but I am sure that doing it is
> > > possible :-) But maybe the current behavior is actually desired.
> >
> > Honestly, I'm not 100% sure what negative denominators mean. The function
> I
> > added is essentially a modified version of is_decimal_fraction() from
> > app-utils/gnc-ui-utils.c, and since negative denominators were not
> > considered decimal numbers there, I did the same.
> >
> > On the other hand, I believe that a negative denominator of -3 may
> actually
> > mean a denominator equal to 1/3. So if a gnc_numeric of 5/-3 is equal to
> > 5/(1/3), then that is also equal to 5*3=15.
>
> Uh oh? No, not at all, please. A negative denominator must mean at most
> that
> the sign of the rational number is changed from positive to negative, or,
> if
> the numerator is negative as well, the sign of the number is changed from
> negative to positive. No more, no less. As this doesn't make much sense, it
> is sensible to restrict the denominator to positive numbers only.
>

The gnc-numeric.c code seems to assume that only the numerator is needed to
determine sign. For example, gnc_numeric_positive_p() and
gnc_numeric_negative_p() only check the sign of the numerator.


> For sure negative numbers will and must never suddently turn into the
> inverse
> of a number. Negative != inverted. Really. Always.
>

There are several mentions of "reciprocal" in gnc-numeric.c comments,
referring to a negative denominator. See gnc_numeric_compare() and
gnc_numeric_equal(), for example. They seem to be doing what I suggested,
i.e. 5/-3 == 15/1.


> Or did I miss anything here?
>
> Regards,
>
> Christian
>

Cheers,
Charles