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# Angle from two points

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I should know this... but anyway... What''s a fast way to calculate the angle between 2 points in the Cartesian coordinates system. -Derek

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You mean the angle between the two vectors from the origin to the two points? Take the arccos of the dot product of the two vectors, divided by the magnitude of each vector.

cos theta = a . b / (/a/ * /b/).

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I forgot that the board mangles pipes. Those last two /a/ and /b/ respectively refer to magnitude of a and magnitude of b.

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Here''s some code if you want...

#include #include #define PI 3.1415926535897932385class CVector2d{	private:		float x, y;	public:		CVector2d() : x(0.0f), y(0.0f) {}		CVector2d(float vx, float vy) : x(vx), y(vy) {}		float Magnitude();		float Dot(CVector2d &a);};float CVector2d::Magnitude(){	return (float)sqrt(pow(x,2) + pow(y,2));}float CVector2d::Dot(CVector2d &a) {	return (a.x * x) + (a.y * y);}void main(void){	CVector2d a(1,0), b(0,1);	float cosTheta, degrees, radians = 0.0f;	cosTheta = a.Dot(b) / (a.Magnitude() * b.Magnitude()); 	radians = (float)(acos(cosTheta));	degrees = (float)(radians * (180/PI));	cout << "radians: " << radians << endl;	cout << "degrees: " << degrees << endl;}

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That was my post above, but it looks like it removed the includes....it''s including math.h (for acos()) and iostream.h (for cout()).

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