Question

I have a std::vector that holds a Point struct (x,y,z and some other non-pointer types).

These points are control points for drawing a bspline curve. I’m not having trouble drawing the curve, but complications arise when I have to close the curve, which involves adding control points (alredy existing inside the container) in certain order.

For example, if I have 5 control points

A B C D E

I would have to get 5 sequences like this:

A B C D  //curve is drawn from B to C
B C D E  //curve is drawn from C to D
C D E A  //curve is drawn from D to E 
D E A B  //curve is drawn from E to A
E A B C  //curve is drawn from A to B

Initially, I went with std::rotate, but then realized it wasn’t what I was looking for.

I’m having trouble implementing this. Best I got is a non-working version in C++ (the reason of this failing is not the question, here’s a snippet).

static char letters = 'A';
typedef struct Point{
        float x,y,z;
        char name;

        Point(float x,float y,float z=0):name(letters++){}

}Point;
typedef std::vector<Point> lpoints;

void
rotate(lpoints& points)
{
    for (unsigned int i =0;i<5;i++){
                lpoints::iterator beg =   lista.begin() + (i%5);
                lpoints::iterator dernier=lista.begin()+(4+i)%6; 

                lpoints subseq(beg,dernier); //4 points in subseq

                //do stuff with subseq                                                       
    }
}

Is there a way to do this? I know I can implement it with many nested for loops, but i’m trying to avoid that, looking for something more elegant (if the word fits).

Thanks in advance.

Answer 1

What, exactly, is wrong with using std::rotate()? For example,

std::vector<int> v(5);

std::rotate(v.begin(), v.begin() + 1, v.end());
std::vector<int> firstFour(v.begin(), v.begin() + 4);

firstFour then contains the first four elements from the rotated vector. If you use this in a loop and run it v.size() times, you will get the five vectors you have in the question.