[UDUNITS #ONU-754617]: Comments on udunits-2

Hi Harvey,

Sorry for the delayed reply.  I've been busy with other things and I 
wanted to give your email a considered response.

> I am pleased to see you have produced a new udunits. I gather udunits-2
> is still in beta release so I hope some changes are still possible.
> 
> I had also been thinking about things like logarithmic units, so I
> thought I would share some of these ideas.  In fact this is the first
> real task in my new (from today) role as a CSIRO honorary fellow after
> retiring last Friday. I considered posting to the netcdf and/or udunits
> groups, but decided to start with a more private communication with you
> and Russ.

Honorary fellow, eh?  Sounds nice.  Does that mean you have authority
but no responsibility?  :-)

> Function ut_raise should be modified so the power argument is double
> rather than int.  This simple change would allow operations such as
> square-root, which occur when doing things like calculating the side of
> a square from its area.

You're right.  I forgot to create an inverse to raising a unit to an
integral power.

I'd rather not generalize ut_raise() to accept floating-point exponent,
however, because that would lead to problems in determining the rational
number the user had in mind (e.g., 2/3, 3/4, 7/9).  It could be done by
limiting the allowed set of rational numbers to a select few -- but 
the elements of that set would be arbitrary.

I think I'd rather add two more functions: ut_root() and ut_power().
ut_root(ut_unit unit, int i) would take the "i"th root of "unit" and
ut_power(ut_unit unit, int numer, int denom) would take "unit" to
the power "numer/denom".  Therefore, ut_raise(unit, i) and
ut_root(unit, i) would be equal to ut_power(unit, i, 1) and 
ut_power(unit, 1, i), respectively.

> There is a need for functions named say ut_exp and ut_log to convert
> from and to logarithmic units.  This would enable one to do things like
> multiplying by adding logarithms, then taking the anti-log (exp).  One
> might want to do this to decrease the chance of overflow in calculating
> a geometric mean. I suggest storing an int representing the number of
> applications of function exp(), with negative values representing the
> number of applications of the inverse function log().  Thus unit "pH"
> would have the value -1.  Value -2 would mean log(log()).

The function ut_log(ut_unit unit, double base, ut_unit reference) already
exists to convert a non-logarithmic unit to a logarithmic one.

I can create a new function, ut_exp(ut_unit unit), to return a
logarithmic unit to a non-logarithmic one.

I'm afraid I don't see the need, however, for units that are associated
with quantities that are, for example, the logarithm of the logarithm of a
physical quantity or the exponent of the exponent of a physical quantity.
Do you have an example of a formula or mathematical expression in which
this is the case?

> Cheers to all at Unidata,

You too.

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: ONU-754617
Department: Support UDUNITS
Priority: Normal
Status: Closed