[boost] Re: Dimensional Analysis Interest?

From: Deane Yang (deane_yang_at_[hidden])
Date: 2003-10-16 10:50:53

David Richardson wrote:

> 
> The library has to make some choice about the internal way it represents 
> the exponents, and the users just have to follow whatever convention the 
> library has.
> 
> The concern is if a user stuffs dimensions into the n-tuple that do not 
> have the same semantics the library expects (e.g. multiplication of two 
> numbers does not result in the exponents in their n-tuples being 
> added).  If the library is hard coded to do conversion as if the n-tuple 
> contained physical dimensions, the library may incorrectly compute types 
> for arithmetic expressions.
> 

I'm sorry if I confused people. I never meant to say that the user 
should be able to specify the semantics of the exponents. I only meant 
that the user should be able to define the fundamental dimensions/units 
to be used (so, not necessarily mass, length, and time). As far as I 
know, you always want to add the exponents when you multiply two 
quantities. I even think that this is a mathematical fact and therefore 
should be hardwired (just as 2+2 = 4 is hardwired).

But I do think it worthwhile to allow rational exponents (but NOT
arbitrary precision). I'd like someone to show me a real example
where the exponent had a numerator or denominator other than 1, 2, 3, or 
4. (Even 4 is, for me, a stretch)