Re: [boost] [PQS] rank of a physical quantity?

From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-06-19 04:04:21

On Jun 18, 2006, at 1:23 AM, Janek Kozicki wrote:

> Gerhard Wesp said:     (by the date of Thu, 15 Jun 2006 11:39:15  
> +0200)
>
>> On Thu, Jun 15, 2006 at 02:17:38PM +0700, Oleg Abrosimov wrote:
>>> the problem here is that PQS deals with only dimensions, but  
>>> physical
>>> quantities have not only that, they have rank also.
>>
>> What is the rank of a physical quantity?
>
> good question indeed. I can grasp the idea of that. In fact above  
> quote
> written by Oleg was a revelation for me. But how to include the  
> concept
> or "rank" into the library design? Would it boil down to
> abstract_quantity_id, but with different name?
>
> I'm really not sure if one can hold energy inside a vector (say:  
> vector
> field of energy), perhaps some physicist here can answer this  
> question.
> But I'm sure that momentum can be hold as a vector (ie. momentum  
> vector).
>
> Could it possibly mean, that some quantities can be represented as
> vectors, while representing others as vector doesn't make sense -  
> would
> it be "rank" then?

As a physicist I am completely baffled and confused. What do you mean  
by rank of a quantity? Do you mean the size of a vector/matrix? If  
so, then this is completely orthogonal to a unit library. You can  
hold any physical quantity inside a vector, or inside a multi_array  
of arbitrary dimensions. Just consider a finite-difference or finite  
element representation of a field theory, and you have multi- 
dimensional arrays of quantities of essentially any unit you can  
think of.

In my opinion thus the "rank" (if I understand what is meant here) is  
orthogonal to the unit system. The unit is a property of the value  
type of the container, and the size (or rank) is a property of the  
container.

Matthias