Question

Hi
this a part of my code :
I have a list which its size is three and I consider that 2 last items are in one line (p and q) I need to get the angle between the first item of this list and these two points (p,q)

    private Point partition(List<Point> list, Point p, Point q) {



    double x1 = p.getX();
    double x2 = q.getX();
    double y1 = p.getY();
    double y2 = q.getY();
    double pQ = Math.sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
    for (int i = 0; i < list.size()-2; i++) {
        double pointX = list.get(i).getX();
        double pointY = list.get(i).getY();
        double pointQ = Math.sqrt((x2 - pointX) * (x2 - pointX) + (y2 - pointY) * (y2 - pointY));
        double pointP = Math.sqrt((pointX - x1) * (pointX - x1) + (pointY - y1) * (pointY - y1));
        double angle = Math.acos((pQ * pQ - pointP * pointP - pointQ * pointQ) /(- 2 * pointP * pointQ));
        System.out.println(angle);


    }

but instead of printing an angle for the first item it will print :(first item is not in the line of two last items).

1.6288442476732894

those points that print this result are :

[X :143.0  Y: 217.0, X :93.0  Y: 163.0, X :193.0  Y: 165.0]

please help me thanks.

EDITED : really it makes me confused .in such a way it will print this value ,sorry all !!!

Answer 1

Your arccos is bad, you need a parenthesis and a - :

 arccos((pQ^2 - pointP^2 - pointQ^2)/(-2 * pointP * pointQ))

see How to calculate an angle from three points?