curve and curve with error objects : sorting , friend

[Hicham Mouline]

Hello,

I have a Curve class that holds variable data
x f(x)
------ ---------
0.5 7.8
0.9 6.7
-5.4 6.6
0.0 6.6
1.5 7.8

and a CurveWithError object that holds data as
x f(x) error
------ --------- ----------
0.5 7.8 0.1
0.9 6.7 0.2
-5.4 6.6 0.05
0.0 6.6 0.1
1.5 7.8 0.05

I maintain the Curve object sorted at all times. In particular, the last
thing done by Curve::Curve(...) is to sort the data based on x.

I implemented CurveWithError with the help of Curve so that

class CurveWithError {
private:
Curve mCurve;
ContainerType mErrors;
};

I am trying to implement CurveWithError ctor in a way that I maintain the
invariant of sorted Curve.

in pseudo-code:
// xbegin is the iterator that points to the beginning of the sequence of
x's
// xend is the iterator that points to the end of the sequence of f(x)'s
// fbegin is the iterator that points to the beginning of the sequence of
f(x)'s
// ferrbegin is the iterator that points to the beginning of the sequence of
errors
CurveWithError::CurveWithError(... xbegin, .... xend, .... fbegin,
....ferrbegin )
: mCurve( xbegin, xend, fbegin), // the Curve ctor finished by sorting its
internal container based on the x's
mErrors(xend-xbegin)
{
// I should sort mErrors as well in the same order Curve sorted its
container
}

Ideas to implement this are welcome.

regards,

 

[Victor Bazarov]

I have a Curve class that holds variable data
x f(x)
------ ---------
0.5 7.8
0.9 6.7
-5.4 6.6
0.0 6.6
1.5 7.8

and a CurveWithError object that holds data as
x f(x) error
------ --------- ----------
0.5 7.8 0.1
0.9 6.7 0.2
-5.4 6.6 0.05
0.0 6.6 0.1
1.5 7.8 0.05

I maintain the Curve object sorted at all times. In particular, the last
thing done by Curve::Curve(...) is to sort the data based on x.

I implemented CurveWithError with the help of Curve so that

class CurveWithError {
private:
Curve mCurve;
ContainerType mErrors;
};

I am trying to implement CurveWithError ctor in a way that I maintain the
invariant of sorted Curve.

in pseudo-code:
// xbegin is the iterator that points to the beginning of the sequence of
x's
// xend is the iterator that points to the end of the sequence of f(x)'s
// fbegin is the iterator that points to the beginning of the sequence of
f(x)'s
// ferrbegin is the iterator that points to the beginning of the sequence of
errors
CurveWithError::CurveWithError(... xbegin, .... xend, .... fbegin,
...ferrbegin )
: mCurve( xbegin, xend, fbegin), // the Curve ctor finished by sorting its
internal container based on the x's
mErrors(xend-xbegin)
{
// I should sort mErrors as well in the same order Curve sorted its
container
}

Ideas to implement this are welcome.

What seemed like a good idea at the time (to use inheritance) perhaps
isn't so hot, at the second look. The 'Curve' type has a closed design,
it keeps and sorts its own data, and without it providing some interface
to expose the sorting order your attempts aren't going to be successful.

I would probably consider two approaches: one is that a Curve should
keep the sorting order in a separate array (basically a permutation
vector) which you could later use to rearrange whatever else you need.
Just ask the curve what the new arrangement is, then move your objects
(in your case the 'error' values) around according to it. Another is to
make your 'Curve' a template and tell to to store (and sort) whatever
you need stored and sorted. Here is a mock-up:

template<class T> class CurveT {
std::vector<double> internal_container_of_x;
public:
template<class ItX, class ItData> CurveT(It xbegin, It xend,
ItData dbegin, ItData dend);
}; // the c-tor does the sorting

class Curve : public CurveT<double> {
std::vector<double> eff_of_ex; // your f(x)
public:
Curve(...) : CurveT<double>(...,
eff_of_ex.begin(), eff_of_ex(end)) { ... }
};

class CurveWithError : public CurveT<std:air<double,double> >
std::deque<std:air<double,double> > eff_and_error;
public:
CurveWithError(...) : CurveT<std:air<double,double> >(...,
eff_and_error.begin(), eff_and_error(end)) { ... }
};

(here I use ... as omitted code, not the variable number of args).

Of course, in this implementation, 'Curve' and 'CurveWithError' are not
related. You can make them related if you extract the data storage
mechanism into a separate type (abstract in case of 'Curve', for
example, and concrete in case of 'CurveWithError'). That's essentially
a third way (a bit more convoluted) to design this. It would allow you
to both keep the classes related (and use run-time polymorphism if you
need to), and make it generic enough so you can later extend the whole
thing to store more than just pairs of doubles.

V

 

[Jerry Coffin]

Hello,

I have a Curve class that holds variable data

[ examples elided ]

I maintain the Curve object sorted at all times. In particular, the
last thing done by Curve::Curve(...) is to sort the data based on
x.

[ ... ]

I am trying to implement CurveWithError ctor in a way that I
maintain the invariant of sorted Curve.

It seems to me that you're combining a couple of rather different
concerns. I'd have one class for data about a point, and a completely
separate class for storing that data:

struct Point {
double x, f_x;

bool operator<(Point const &other) {
return x < other.x;
}
};

struct PointWithError : public Point {
double error;
};

Template <class Point>
class SortedVector {
std::vector<Point> points;
public:
SortedVector(/* ... */) {
// ...
std::sort(points.begin(), points.end());
}
};

typedef SortedVector<Point> Curve;
typedef SortedVector<PointWithError> CurveWithError;