Coding a Math Equation

Question

So I'm trying to code this equation using java:

I'm taking a, b, and c from the user. This is the code I have so far:

import java.util.Scanner;
class QaudraticFunction{
    public static void main(String []args){
        Scanner input = new Scanner(System.in);
        System.out.println("Enter a: ");
        double a = input.nextDouble();

        System.out.println("Enter b: ");
        double b = input.nextDouble();

        System.out.println("Enter c: ");
        double c = input.nextDouble();

        double val1 = (Math.pow(b,2.0)) - (4.0*a*c);
        double discriminant = Math.sqrt(val1);
        double val2 = (-b)-(discriminant);
        double r2 = val2/(2.0*a);

        System.out.println("r2 = " + r2);
    }
}

I think my issue is a logical error because the program compiles and runs correctly. When I enter the values for a, b and c. I get r2 = NaN

`When I enter the values for a, b and c` What values do you enter? — copeg, Sep 21 '16 at 22:38
Your code looks fine. You're probably providing coefficients for a parabola which does not intersect the x-axis. I forget the exact math terminology, but try some other numbers — Kon, Sep 21 '16 at 22:38
you'll get NaN when you do something not allowed like dividing by 0 or taking the square root of a negative number which are both possibilities based on your formula. It all comes down to your numbers being used — RAZ_Muh_Taz, Sep 21 '16 at 22:39
Code seems ok, but `(Math.pow(b,2.0))` is better written as `b*b` *(better performance and simpler)*, and `(4.0*a*c)` can be simply `4*a*c`. You don't need all those parenthesis, since Java knows standard [operator precedence](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html) rules. — Andreas, Sep 21 '16 at 22:41
@Kon I mostly entered small values such as 1, 2, 3 or 5, 10, 15 — Vishal, Sep 21 '16 at 22:43
With input 1,2,3 `b*b-4*a*c` is `2*2-4*1*3 = 4 -12 = -8`. Trying to take square root of a negative number gives the error (see all other comments). You have the same problem with 5,10,15, `10*10-4*15*15 = 100 - 900 = -800`. Try with 1, 3, 2, you should get -1 and -2. (You can also do the computations by hand on a piece of paper to get a better understanding.) — Sci Prog, Sep 22 '16 at 01:44

score 3 · Accepted Answer · answered Sep 21 '16 at 22:44

3

Two possible reasons you are seeing a NaN.

The denominator is 0. This is only possible if a is set to 0 or 0.0. I'm going to assume that's not the case.

The other possibility is that you are performing a square root of a negative number, which (in java) is a NaN. See Math.sqrt javadoc for more details.

If the argument is NaN or less than zero, then the result is NaN.

answered Sep 21 '16 at 22:44

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Oh I understand now. So what you're saying is that the value of 'b' should be higher than 'a' and 'c' otherwise the square root will be negative. – Vishal Sep 21 '16 at 22:49
1

Sort of. I'm speculating here since you haven't given the value of a,b and c but that will be my best guess. If you need recommendations on how to deal with it, one cheap way will be to just check if val1 is NaN or b^2 > 4*a*c; if not throw an error. Other one is to use [Apache Common's Complex](https://commons.apache.org/proper/commons-math/userguide/complex.html) data type. – blr Sep 21 '16 at 23:17

Coding a Math Equation

1 Answers1