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From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-08-20 08:28:59

On Aug 20, 2006, at 10:42 AM, Andy Little wrote:

> "Janek Kozicki" <janek_listy_at_[hidden]> wrote in message
> news:20060819212430.739b0920_at_absurd...
>> Andy Little said: (by the date of Sat, 19 Aug 2006 13:03:42
>> +0100)
>>> In fact there is a 2D and 3d vector in quan so you can do:
>>> quan::some_vect< quan::length:: m > position_a, position_b;
>>> quan::length:: m distance_a_to_b = magnitude(position_b -
>>> position_a);
>>> Because of that useage It might get confusing.
>> I still hold my position that linear algerbra library should not be a
>> part of _two_ separate libraries: Dimensions and Units.
> FWIW I brought this up only in relation to magnitude. That said, I
> have found
> that being able to write code like the above is a lot of fun, much
> more so than
> if I was using doubles to represent lengths. However suddenly you are
> restricted in what you can do. For example:
> double var1= 10; // var1 meters
> double var2 = 10 ; // var2 represents meters.
> var1 *= var2 ; // var1 now represents an area in square meters.
> Of course you cant do this with a fixed_quantity:
> quan::length::m var1(10);
> quan::length::m var2(10);
> var1 *= var2 ; // Error
> I would guess that is an unacceptable restriction for many authors
> of algebra
> libraries.

Why should this be an unacceptable restriction? All it says is that a
quantity with units is not a field, but that is obvious from the
start. I can multiply a vector (3m, 2m, 1m) by a factor of 5 but not
by a factor of 5m. I see no problem here at all, except if you assume
that the element type of a vector is the same type as the scalar
factor in your expression.

> As far as I am concerned though I am more interested to see what
> happens if I do
> play by the rules that are imposed by having strongly typed
> quantities, but it
> is kind of difficult to ask everyone to play by these rules, so I
> am happy to
> create vector types myself, that work with Quan.

I don't see any reason why a correctly designed linera algebra
library should not work with quantities with units
> Maybe, if we can get Quan into Boost then we will be in a stronger
> position and
> there may then be interest in creating a linear generic algebra
> library for
> physical quantities, but I suspect that due to the above kind of
> issues, there
> will always be a great divide between a 'raw' linear algebra
> library and one
> that is designed to work with physical quantities.

  I don't agree. For me there is no such thing as 'raw' linear
algebra. All linear algebra concepts work perfectly with quantities
with units.


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