how to solve cos 90 problem in java? [duplicate]

Question

I have some problems with calculating cosinus 90 in Java using Math.cos function :

public class calc{
      private double x;
      private double y;

      public calc(double x,double y){
                   this.x=x;
                   this.y=y;
      }

      public void print(double theta){
           x = x*Math.cos(theta);
           y = y*Math.sin(theta);
           System.out.println("cos 90 : "+x);
           System.out.println("sin 90 : "+y);
      }

      public static void main(String[]args){
           calc p = new calc(3,4);
           p.print(Math.toRadians(90));

      }

}

When I calculate cos90 or cos270, it gives me absurb values. It should be 0. I tested with 91 or 271, gives a near 0 which is correct.

what should I do to make the output of cos 90 = 0? so, it makes the output x = 0 and y = 4.

Thankful for advice

Answer 1

What you're getting is most likely very, very small numbers, which are being displayed in exponential notation. The reason you're getting them is because pi/2 is not exactly representable in IEEE 754 notation, so there's no way to get the exact cosine of 90/270 degrees.

Answer 2

Just run your source and it returns:

cos 90 : 1.8369701987210297E-16
sin 90 : 4.0

That's absolutely correct. The first value is nearly 0. The second is 4 as expected.

3 * cos(90°) = 3 * 0 = 0

Here you have to read the Math.toRadians() documentation which says:

Converts an angle measured in degrees to an approximately equivalent angle measured in radians. The conversion from degrees to radians is generally inexact.

Update: You can use for example the MathUtils.round() method from the Apache Commons repository and round the output to say 8 decimals, like this:

System.out.println("cos 90 : " + MathUtils.round(x, 8));

That will give you:

cos 90 : 0.0
sin 90 : 4.0