[boost] Re: Re: Physical Quantity- boostified

From: Andy Little (andy_at_[hidden])
Date: 2003-11-13 11:30:25

"Daryle Walker" <darylew_at_[hidden]> wrote in message
news:BBD8B7C7.27CC%darylew_at_hotmail.com...
> On 11/6/03 8:33 PM, "Andy Little" <andy_at_[hidden]> wrote:
>
> > "Deane Yang" <deane_yang_at_[hidden]> wrote in message
> > news:boekps$3rl$1_at_sea.gmane.org...
> >> I want to know why it is useful to be able to add two quantities with
> >> different units and have the operator+ do the unit conversion inside of
it.
> >> I'm sure there's an obvious answer that will make me feel very dumb.
> >
> > There is a very obvious answer:
> > from , random book close at hand:
> > Ross Garrett  "The symmettry of sailing ,1987 Appendix B"
> > Length           m    km      inch     foot     nautical mile
> > 1 metre     =    1    10-3    39.37    3.281    5.4x10-4
> > 1 kilometre =    ......
> > etc.
> >
> > The = sign says it all.
>
> No it doesn't.

Oh ...yes ... it ... Does.   :-)

>
> > 1 metre is equal to 1000mm
> > is equal to etc...
> > They are all different ways of saying the same thing. Its an everyday
thing, a
> > commonly understood concept. If you have =, you soon need +, * ... etc
>
> Yikes!  The "=" in the table has nothing to do with the one for C++ ("is
> equivalent to" vs. "assignment").

hmm... Did I say "The Symmetry of Sailing " is written in C++ ?

> In fact, the table "=" is a writing
> convention and has no algorithmic meaning (unlike C++'s "=" or "==").

????...

An example algorithm using my physical_quantities class:

#include <iostream>
#include "physical_quantities.hpp"
#include "physical_quantities_demo_util.hpp"

using namespace physical_quantities;

int main()
{
    pq_velocity<float>::m_per_s     v1(1);              // metres per second
    pq_velocity<float>::miles_per_s      v2   =  v1;    //v1 in miles per
second

    std::cout << v1 << " is equivalent to " << v2
    << " is " << std::boolalpha << ( v2 == v1 ) <<'\n';

    return 0;
}
///////////////
output :
1 m.s-1 is equivalent to 0.000621371 miles.s-1 is true
//////////

If you dont like the sailing example see
NIST special publication 811,
guide for the Use of the International System of Units,
P8, table 6 etc

or any cookery book :-)

>
> > However
> > The different units have evolved because ,when dealing with big lengths
it
> > is easier to use a bigger unit.
>
> That sounds kind-of arbitrary.

The arbitrariness or otherwise of evolution is slightly off topic....

>
> > There are just less zeros to stick on there. Its easier to deal with.
>
> No, it's not.  The zeros just get tacked on the other end.

????  Oh you mean if you  change units :-)

I GHz   =  1000000 kHz
 =   1000000000 Hz
 =    1000000000000000000000000000000000 yHz.
=  0.000000000000001 YHz
(note ... I could have missed a couple of 0's here and there... apologies)

>
> > That goes for Floating Point too. If you only need big lengths you want
them
> > at the scale of the unit.
> [TRUNCATE]
>
> That last sentence doesn't make sense.  (Which "unit" do you mean?)

see here for an overview... :

http://www.servocomm.freeserve.co.uk/Cpp/physical_quantity/abstract.html

general note: I hope to put an implementation on the boost files
section sooner or later. I feel that I should
provide some level of robustness, adequate explanation justification etc,
which all takes time.

>
> The real problem is that computer numbers don't have infinite precision
> _nor_ infinite digit space.

hmm...I usually muddle through somehow, without infinite precision. :-)
Physics is based on measurement.
Measurement is inaccurate.
Floating point numbers generally do more than a good enough job.
I conjecture that most problems are of the "division by zero" type. ie lazy
programming (yes guilty :- )  )

regards
Andy Little