
Boost : 
From: Matthias Troyer (troyer_at_[hidden])
Date: 20060619 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 finitedifference 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
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk