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From: John Max Skaller (skaller_at_[hidden])
Date: 20010622 17:38:52
Msk_at_[hidden] wrote:
>
> Would there be any interest in a small domain library for
> boost that handles dimensions (e.g. length, mass, power)
> and units (e.g. meters, feet, furlongs) of physical
> quantities?
> I would appreciate feedback on three main questions:
>
> 1) Is this appropriate for boost?
IMHO, yes.
> 2) Has this already been done?
Yes.
> 3) Does anyone see any fatal flaws in the general approach,
It looks good to me.
Some operations seems to be missing.
Suppose I write:
pow(x,y)
and x,y have units. What is the result?
A more pragmatic example is Pythagoras:
sqrt(x * x + y * y)
Here, x is in metres, so the argument to sqrt is
square metres, and the result should be in metres.
More generally, root(n,x) yields division
of the dimension by n (and an error if it isn't
integral  unless you want to support fractal dimensions :).
pow(x,n) yields multiplication of the unit by n.
So log(n,x) also yields division. (Guess?)
When it comes to trig functions ...
I guess I'm saying: dimensional correctness is a problematic
thing in physics, because all sorts of approximations
and kludges and special functions are used. At some stage,
you need to 'cast away' dimensions, do a pure computation,
then add the dimensions back.
sin ( undim(x) ) metres()
It would be kind of nice to wrap this up in a single
'cast away/cast back' operation (but I can't see how).
[A way to wrap existing 'pure' functions with dimensions?]
Is there a cast away function?
 John (Max) Skaller, mailto:skaller_at_[hidden] 10/1 Toxteth Rd Glebe NSW 2037 Australia voice: 61296600850 checkout Vyper http://Vyper.sourceforge.net download Interscript http://Interscript.sourceforge.net
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