Re: [boost] [review] pqs

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2006-06-09 04:02:59

Carlo Wood wrote:
> On Thu, Jun 08, 2006 at 05:36:40PM -0500, David Greene wrote:
> > the library.  Andy presented this example to justify kg's 0 power:
> > 
> >     mass::kg m(1); // 1 kg == 1000 g
> >     acceleration::m_div_s2 a(1); // 1 meter per second squared
> >     force::N f =  m * a;
> > 
> >     The value of f should be 1000 Newtons right?
> > 
> > Of course, the value is 1 Newton.  But this has nothing at all to
> > do with kg having a prefix power of 0 or 3.  The calculation can
> 
> The point is,
> 
>   1 N = 1 kg * 1 m / 1 s / 1 s
> 
> The 1 comes from 10^0, hence 0 is needed.
> 
>   10^0 N = 10^0 kg * 10^0 m / 10^0 s / 10^0 s
> 
>   0 = 0 + 0 - 0 - 0
> 
> The calculation of the left-hand 0 is done with generic
> templates using '+' to add up exponents. How is it logical
> to make an exception here for kg?
> 
> If you'd force 'kg' to be 'g' internally, then you will
> have to apply the same trick to N that is now applied to kg,
> with the exception that then BOTH are internally not in S.I.
> units (mN and g, instead of N and kg). That is, you'd end up
> with:
> 
>   3 = 3 + 0 - 0 - 0
> 
> and thus mN. But you still want to print N.
> I don't think it will get any better by using a prefix power of 3 for kg.

If you know that kg ==> 10^3g and that N = kg.m.s-2 then you
have the information right there! Given that N is expressed in terms
of kg, the library will know that the powers of each unit are:

   N = [ 3, 0, 0, 0 ]

and that mN is:

   N = [ 0, -3, -3, -3 ]

You don't need to treat kg as a special case here where the math is
concerned, as per my unit system toy example.

- Reece
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