Calculating the Angle between two vectors

Questions & Answers
1

I am making a 2D game where I have to aim a grapple onto a wall. I need help calculating the angle, because I want the game to look like this:

90 degrees

^

|

|--------->0 degrees

the vertical component is the y-axis, and the horizontal component is the x-axis

2

Mathf.Acos(Vector3.Dot(vector1.normalized, vector2.normalized));

should give you the angle between two vectors. This answers the title of your question, but the body of your question is very vague.

3

For angle to the mouse in a 2D, need to know the mouse is in screen Pixels and the bow is in world units. Can convert the bow to screen pixels, or can convert the mouse to the world position (in 3D the world position of the mouse is anywhere on the line it shoots, for 2D it’s just a point.) These are some fo the commands (untested):

Vector3 bowP = Camera.main.WorldToScreenPoint(bowWoldPos);
// x,y are in pixels. z can be ignored

Vector3 aimP = Input.MousePosition();
// Same units: x,y are in pixels
// acos (see other answer) for the angle

OR, to use world pos:

Vector3 aimWorldP = Camera.main.ScreenToWorldPoint(Input.MousePosition());
aimWorldP.z = bowPosition.z;
bow.LookAt(aimWorldP);
// bow.forward now aims at the mouse