RulerOfNothing

Not really. You just need:

class Base
{
   virtual int getSpecialNum();
};

class A : public Base
{
   int getSpecialNum(){return 0;}
};

class B: public Base
{
   int getSpecialNum(){return 1;}
}

although it has been a while since I've had to do this.

If you redraw the entire screen (or at least all parts of the screen that change) every frame then removing clear(screen) won't make any difference otherwise part of the old frame will still be on the screen.

I have a question: what exactly does clear(my_pic); do? If it clears the loaded image you probably want clear(screen); or whatever clears the screen instead.

I think the problem here is that your delete function gets rid of the associated node data, so this happens:

1. Program decides to delete s.
2. Program deletes s and its associated node n.
3. Program tries to get the next member of n, but can't because that node has been deleted and can no longer be assumed to hold valid data.

How I would solve this is by moving the line n=n->next; to immediately after the program extracts the element from n, so that n will always be valid when you extract the next member from it.

I'm not exactly sure what you are trying to say, so correct me if I misinterpret you. You are saying that you are analysing dot products between the x unit vector (1,0) and a collection of vectors representing points in 2D space. In this case the dot product is simply the x co-ordinate of the vector, so if you had vectors (3,0), (3,10) and (3,-10) then they would all have the same dot product. If you normalise the collection of vectors you still have problems if some of the vectors are parallel with each other (for example, the vectors (3,4) and (6,8) would both normalise to (0.6,0.8)). Hopefully this helps.