Re: Announce: Grace project site operational

[dennison@telepath.com (Ted Dennison)]

"Martin Dowie" <martin.dowie@nospam.baesystems.com> wrote in message news:<3ce1424a$1@pull.gecm.com>...
> "Ted Dennison" <dennison@telepath.com> wrote in message
> news:4519e058.0205140718.3ddb1b2a@posting.google.com...
> > To do this kind of thing *right*, you'd really need a language that
> > relies on structural equivalence for types, and automaticly generates
> > the proper new type given the operator and the types of its operands.
> > In Ada, you just can't do that.
...
> In the same vein as the standard Ada maths library doesn't try to
> cover _all_ possible maths packages, an 'elementary SI' package should
> be possible (I feel) to cover 99% of project needs (e.g. SI units and
> support for powers to the +/- 3 plus a base 'dimensionless' unit).
...
> It might be big, it might not be pretty, but once its there it's not
> exactly going to be changing! :-)

I'm certianly not an engineering math expert, but I've watched enough
of them try to go this route and fail that I'm skeptical. The big
problem was always the geometric explosion of operators required for
every new unit added. Its no biggie when you just have positions and
velocities, but at ((n-1)n * p)/2 custom operators requred (where n =
# of units in your system and p = the average number of operators per
unit combo), you could potentially have to add 45p operators when the
10th unit is added, 105p operators when the 15th unit is added, and
190p operators when the 20th unit is added. If p is 2 ("*" and "/"),
then that's 90, 210, and 380 operator subprograms respectively.

According to http://www.unc.edu/~rowlett/units/index.html, there are
actually 7 "fundamental" units, 22 more derived units with approved
names, and a theoreticly limitless amount of derived units without
approved names (eg: volume, luminance). There are 21 prefixes (powers
of 10 scalings) any one of these units may have applied to them. Of
course many of those would make no sense and could probably be left
out (eg: yottaseconds, zeptoamperes). But if you try to do the math
from here, you get into a ludicrous number of operator routines you
are going to have to create.

Now its quite possible that p is not constant in this equation, in
other words that there are only a limited number of other units that
any one unit will ever have to interact with (not related at all to
n). That might make things manageable. That's one of the things I
wouldn't know, not being an engineering math user myself.

Now I'm not saying I think an SI package is a Bad Idea. Every project
I've worked on had its own auxililary math ops and types package, so
that would be a silly claim. Even without a separate type for every
SI, it would be useful. I just want to make sure you really know what
you are getting into. If you do go down this path, at least figure out
up front how you are going to cull things down to keep it humanly
doable.

-- 
T.E.D. 
Home     -  mailto:dennison@telepath.com (Yahoo: Ted_Dennison)
Homepage -  http://www.telepath.com/dennison/Ted/TED.html (down)