problem with angles

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 !!!

Why `i < list.size()-2;`? I would expect `i < list.size()-1;`. — Matt Ball, Nov 22 '10 at 04:17
@Matt: there are no functions in the code above that take radian arguments. acos takes a number between -1 and 1 and returns a value in radians. — R. Martinho Fernandes, Nov 22 '10 at 04:18
Why do people continue to help somebody with a 33% accept rating??? — camickr, Nov 22 '10 at 05:16

score 2 · Accepted Answer · edited May 23 '17 at 11:56

2

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?

edited May 23 '17 at 11:56

Community

1
1

answered Nov 22 '10 at 04:27

I have changed it but still doesn't work well.it will print value more than one – user472221 Nov 22 '10 at 04:48

score 0 · Answer 2 · answered Nov 22 '10 at 04:24

0

NaN is a special double value meaning "not a number". It is generated as a result of some "bad" calculations:

dividing 0 by 0
dividing infinity by infinity (any combination of positive and negative)
multiplying 0 by infinity, either positive and negative, and vice-versa
adding negative and positive infinity
subtracting negative from positive infinity, and vice-versa
the square root of a negative number
the logarithm of a negative number
the inverse sine or cosine of a number not between -1 and 1
any calculation involving one or more NaN values.

Check the result of the argument to the inverse cosine function (acos). I bet it is off the valid range.

answered Nov 22 '10 at 04:24

R. Martinho Fernandes

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You mean `1.5707963267948966` is the value of `(pQ * pQ - pointP * pointP - pointQ * pointQ) / 2 * pointP * pointQ` ? – R. Martinho Fernandes Nov 22 '10 at 04:27
See RC's answer. He knows why you're trying to calculate the inverse cosine of a number out of the [-1,1] range. – R. Martinho Fernandes Nov 22 '10 at 04:31

problem with angles

2 Answers2