What type of triangle?

Question

I have made a program where I insert the length of 2 sides of a triangle and the angle between them. The program shall use this information to tell me if the triangle is equilateral, isosceles or opposite of equilateral.

If you know length a, b & x(angle between them) you can calculate the last side using the following formula: c = sqrt(aa + bb -2*abcos(x))

I have no idea why it won't work. I have done it all correctly mathematically. I believe I am using the language Java incorrectly and therefor I can't get it to work.

I believe the fault lies with the first "if" thingy. So I tried disabling the other two just for now, to be able to try if the program can recognize if the triangle is at least equilateral to begin with, it can't however.

import javax.swing.*;

public class Upp46 {
    public static void main(String[] arg){

        String a, b, x;
        a = JOptionPane.showInputDialog("Length of a");
        b = JOptionPane.showInputDialog("Length of b");
        x = JOptionPane.showInputDialog("Angle between a & b");
        /* "x must be converted to radians"
            before it can be applied */

        double ai, bi, xi, c;
        ai = Double.parseDouble(a); //Length of a
        bi = Double.parseDouble(b); //Length of b
        xi = Double.parseDouble(x); //Angle x in degrees
        xi = xi * (Math.PI/180); //Converts from degree to Radians
        c = Math.sqrt(ai*ai + bi*bi -2*ai*bi*Math.cos(xi)); //Length of c

        if (ai == bi && c == ai){
            JOptionPane.showMessageDialog(null, "The triangle is equilateral");
        }
        /*else if (ai == bi && c != bi ||
                 ai == c  && bi !=c  ||
                 bi == c  && c  !=bi){
            JOptionPane.showMessageDialog(null, "Triangeln är likbent");
        }
        else {
            JOptionPane.showMessageDialog(null, "Triangeln är oliksidig");
        }
        */
    }
}

Your real problem is that you do not understand that programming is not mathematics. There is **no** way to represent PI "correctly" on a computer. Beyond that, float and double are inherently **limited** to a certain precision. You want to read https://stackoverflow.com/questions/1661273/floating-point-arithmetic-not-producing-exact-results for example , esp. that link in the first answer. — GhostCat, Sep 07 '19 at 09:39

score 0 · Answer 1 · answered Sep 07 '19 at 08:05

0

I think java is not using exact numbers for PI and therefor this is not working. In my book it recommends I instead use abs value and < for a small value.

Egz: e = 1e-10 if (Math.abs(a-b) < e

answered Sep 07 '19 at 08:05

Aaravos

1
3

There are no exact numbers for PI. But your solution makes sense. – Erwin Bolwidt Sep 07 '19 at 08:10
So you agree with my original solution but the issue here is just java, right? – Aaravos Sep 07 '19 at 08:13
I mean does the calculator use the abs value system as well ? – Aaravos Sep 07 '19 at 08:14
Your solution will fail for large values (because of limited precision). Use relative error, not absolute. – Sep 07 '19 at 09:05

What type of triangle?

1 Answers1