get vector orientation from coordinates of two ends

I a starting point of a vector at (x1,y1) and it ends at (x2, y2).

Is there anyway I can get the vector orientation theta in rad? when theta = 0 is completely arbitrary. However, maybe it is easier when the vector is pointing to right, since sin(theta) = 0 at that point.

If use the slope approach, cases where vector is pointing straight up or down cannot be solved because the slope is inf or -inf.

Any suggestions? Thanks very much!

Answer 1

[dpb]

doc atan2

theta=atan2(x2-x1,y2-y1);

If you'll write x,y as vectors as

x=[x(1) x(2)]; y=[y(1) y(2)];

then you can write

theta=atan2(diff(x),diff(y));

[Ohad Givaty]

you should do atan2(diff(y),diff(x));

see documentation: https://www.mathworks.com/help/matlab/ref/atan2.html