Angle of a vector pointing from A to B

Question

I'm not the best in Maths, but for what I am doing now I need to calculate the angle of the vector which is shown as arrow in the picture below:

I have a point A and a point B in a 2D plane. I need to calculate the following:

• The angle in which the arrow must be rotated in order to point to B

Answer 1

arctan((A.y - B.y) / (A.x - B.x)) and note the special case where A.x = B.x

Answer 2

atan2(yB-yA, xB-xA), assuming your library has atan2. Otherwise you need to use atan, which will return the correct answer if B is to the right of A, and will be 180 degrees off otherwise. Also note that the return value is in radians, you can convert radians to degrees by multiplying by 180/pi if necessary.

Wikipedia has a detailed explanation of the geometry.