Re: [boost] [Review][PQS] Review deadline

From: Andy Little (andy_at_[hidden])
Date: 2006-06-12 16:57:37

"Michael Fawcett" wrote
> On 6/9/06, Andy Little  wrote:
>> In cases of vectors, I have only used vectors where all elements are one type
>> of
>> quantity. The vectors are used to represent position, direction and so on in
>> 3
>> dimensions. A container that holds different quantities I would consider to
>> be a
>> tuple. But I stress I am not an expert.
>
> I am not an expert either, but in practice I have used (incorrectly?
> should I have used a tuple or a custom data type?) vectors for
> representing latitude/longitude/altitude with mixed units. Something
> like vec3<double, unsigned short, double> for decimal degrees, meters,
> decimal degrees.

Well IMO this sounds more like a navigation coordinate(or whatever is the
standard name for it) , and this sounds specific enough to make it its own
class, maybe you could convert it to a vector relative to the center of the
earth:

struct nav_coord{
    boost::pqs::angle::s latitude,longtitude;
    boost::pqs::length::m altitude;
    operator boost::pqs::three_d::vect<boost::pqs::length::m>();
};

AFAIK a vector classically represents a magnitude and direction without more
information though, so the vector would be a position vector

> I often need to move back and forth from those units
> to nautical miles and feet, and I bet a library like PQS would help me
> out more than meaningful variable names.

You could  return the distance (or distance vector) from a distance function
(between two navigation coordinates presumably) in (say) meters. In PQS
conversions to non-SI units are automatic so the result can be assigned to ( a
vector in) nautical miles or feet. (In PQS vectors are designed to work this way
as well as scalars) :
/*
    show that conversion works for vectors as well as scalars
    NB not tested in pqs_3_1_1 release version
*/
// note:ideally its ..
//#include <boost/pqs/three_d/out/vect.hpp>
// but in pqs_3_1_1...
#include <boost/pqs/three_d/vect_out.hpp>
#include <boost/pqs/t1_quantity/types/out/length.hpp>

namespace pqs = boost::pqs;

int main()
{
    typedef pqs::length::naut_mile naut_mile;
    typedef pqs::length::ft foot;

    pqs::three_d::vect<naut_mile> naut_mile_vect(
        naut_mile(1),naut_mile(1),naut_mile(1)
    );

    pqs::three_d::vect<foot> ft_vect = naut_mile_vect;

    std::cout << ft_vect <<'\n';

    // convert absolute length of vect to meters FWIW
    pqs::length::m length_of_vect = magnitude(ft_vect);

    std::cout << length_of_vect <<'\n';
}
//output:
//[6076.12 ft, 6076.12 ft, 6076.12 ft]
//3207.76 m

> I am ambivalent about whether the vector should allow mixed data types
> or not.  I could settle for using a tuple instead, as long as vector
> functions like normalize(my_vec3) or length(my_vec) worked on tuple
> types as well.

My guess is the nav coordinates would work best as a class? This would enable
for example conversion to from position vectors etc as above, which isnt
possible with a tuple.

regards
Andy Little