Coordinate iterator of point - c++

Question

How can I get a coordinate iterator of a Point p where Point is fulfilling the Point Concept?

Answer 1

It depends what's the Point type exactly. If you're using bg::model::point<> then this may be problematic since the run-time to compile-time translation of dimension index is needed, so e.g. some number of if conditions in a for-loop or recursive function (as you implemented).
However you could also implement your own Point type and define whatever members you need (e.g. operator[]) or use Point type already implemented in another library (assuming that max dimension is known at compile-time). Then, to let know Boost.Geometry how to handle your Point type you'd be forced to adapt it to Point concept by:
using macros provided by Boost.Geometry for typical 2D and 3D cases
specializing traits the same way how it's done e.g. for model::point_xy<> or model::point<>

Answer 2

I need a coordinate iterator to calculate the smallest enclosing ball (minimum bounding sphere/circle) of points of a (boost geometry) linestring using this library. The following solution contains a modified version of this example at the end:
#include <boost/geometry.hpp>
#include "Miniball.hpp"
namespace bg = boost::geometry;
template<std::size_t>
struct int2type {
};
template<class Point, std::size_t I>
typename bg::coordinate_type<Point>::type
get_imp(std::size_t index, const Point &point, int2type<I>) {
return (I == index)
? bg::get<I>(point)
: get_imp(index, point, int2type<I - 1>());
}
template<class Point>
typename bg::coordinate_type<Point>::type
get_imp(std::size_t index, const Point &point, int2type<0>) {
return bg::get<0>(point);
}
template<class Point>
typename bg::coordinate_type<Point>::type
get(std::size_t index, const Point &point) {
static std::size_t const size = bg::dimension<Point>::value;
return get_imp(index, point, int2type<size - 1>());
}
template<class Point, std::size_t I>
void set_imp(std::size_t index,
Point &point,
typename bg::coordinate_type<Point>::type value,
int2type<I>) {
return (I == index)
? bg::set<I>(point, value)
: set_imp(index, point, value, int2type<I - 1>());
}
template<class Point>
void set_imp(std::size_t index,
Point &point,
typename bg::coordinate_type<Point>::type value,
int2type<0>) {
return bg::set<0>(point, value);
}
template<class Point>
void set(std::size_t index, Point &point, typename bg::coordinate_type<Point>::type value) {
static std::size_t const size = bg::dimension<Point>::value;
return set_imp(index, point, value, int2type<size - 1>());
}
template<class Point>
class CoordinateIterator {
using self_t = CoordinateIterator<Point>;
public:
using iterator_category = std::forward_iterator_tag;
using value_type = typename bg::coordinate_type<Point>::type;
using difference_type = std::size_t;
using pointer = value_type *;
using reference = value_type &;
private:
Point _point;
difference_type _pos;
public:
CoordinateIterator()
: CoordinateIterator(Point()) {}
CoordinateIterator(Point point)
: CoordinateIterator(point, 0) {}
CoordinateIterator(Point point, difference_type pos)
: _point(point), _pos(pos) {}
inline value_type operator*() {
return get(_pos, _point);
}
inline const value_type operator*() const {
return get(_pos, _point);
}
inline self_t &operator++() {
++_pos;
return *this;
}
inline self_t operator++(int) {
self_t copy(*this);
++_pos;
return copy;
}
};
template<typename Linestring>
struct CoordinateAccessor {
using Pit = typename Linestring::const_iterator;
using Cit = CoordinateIterator<typename bg::point_type<Linestring>::type>;
inline Cit operator()(Pit it) const { return Cit(*it); }
};
int main(int argc, char *argv[]) {
using point = bg::model::point<double, 2, bg::cs::cartesian>;
using linestring = bg::model::linestring<point>;
using coordinate_type = bg::coordinate_type<linestring>::type;
using PointIterator = CoordinateAccessor<linestring>::Pit;
const int dimension = bg::dimension<linestring>::value;
const int numberOfPoints = 1000000;
// initialize random number generator
const double seed = (argc != 2) ? 0 : std::atoi(argv[1]);
std::srand(seed);
// generate random points and store them in a linestring
// ----------------------------------------------------------
linestring line;
for (int i = 0; i < numberOfPoints; ++i) {
point p;
for (int j = 0; j < dimension; ++j) {
set(j, p, rand());
}
bg::append(line, p);
}
// create an instance of Miniball
// ------------------------------
using MB = Miniball::Miniball<CoordinateAccessor<linestring>>;
MB mb(dimension, line.begin(), line.end());
// output results
// --------------
// center
std::cout << "Center:\n ";
const coordinate_type *center = mb.center();
for (int i = 0; i < dimension; ++i, ++center)
std::cout << *center << " ";
std::cout << std::endl;
// squared radius
std::cout << "Squared radius:\n ";
std::cout << mb.squared_radius() << std::endl;
// number of support points
std::cout << "Number of support points:\n ";
std::cout << mb.nr_support_points() << std::endl;
// support points on the boundary determine the smallest enclosing ball
std::cout << "Support point indices (numbers refer to the input order):\n ";
MB::SupportPointIterator it = mb.support_points_begin();
PointIterator first = line.begin();
for (; it != mb.support_points_end(); ++it) {
std::cout << std::distance(first, *it) << " "; // 0 = first point
}
std::cout << std::endl;
// relative error: by how much does the ball fail to contain all points?
// tiny positive numbers come from roundoff and are ok
std::cout << "Relative error:\n ";
coordinate_type suboptimality;
std::cout << mb.relative_error(suboptimality) << std::endl;
// suboptimality: by how much does the ball fail to be the smallest
// enclosing ball of its support points? should be 0
// in most cases, but tiny positive numbers are again ok
std::cout << "Suboptimality:\n ";
std::cout << suboptimality << std::endl;
// validity: the ball is considered valid if the relative error is tiny
// (<= 10 times the machine epsilon) and the suboptimality is zero
std::cout << "Validity:\n ";
std::cout << (mb.is_valid() ? "ok" : "possibly invalid") << std::endl;
// computation time
std::cout << "Computation time was " << mb.get_time() << " seconds\n";
return 0;
}