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

[Andreas Köhler]

Hi Charles,

On Mon, 2008-09-01 at 14:24 -0700, Charles Day wrote:
> 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.

Indeed.  There is also a documentation for the macro
GNC_DENOM_RECIPROCAL.  Rest assured that I profoundly dislike this
"design choice".  Fortunately, it does not seem to be in use (a lot).
If possible, let us kill it, please.

Ciao,
-- andi5


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