quadratic formula with scanner inputs

Question

Okay so I am a complete Java noob, and I'm trying to create a program for class that runs a quadratic equation using scanner inputs. So far what I've got is this:

import java.util.*;

public class QuadraticFormulaSCN {


public static void main(String[]args) {
  System.out.println("insert value for a:");
  Scanner scan1 = new Scanner(System.in);
   double a = scan1.nextDouble();
    System.out.println("insert value for b:");
  Scanner scan2 = new Scanner(System.in);
    double b = scan2.nextDouble();
    System.out.println("insert value for C:");
  Scanner scan3 = new Scanner(System.in);
   double c = scan3.nextDouble();

    double answer =((Math.sqrt(Math.pow(b,2)-(4*a*c))-b)/2);
      double final2 =(-b + Math.sqrt(Math.pow(b,2)-(4*a*c)))/2;
  System.out.println("The x values are:" + answer + final2);
}
}

But I get a weird output, specifically NaNaN... What do I do to fix this? What am I doing wrong?

Also, it will help you if you (at least for testing purposes) avoid one-line code and separate the calculus in several variables, for ease of debug printing. Anyway, are you sure that the equation that you have set has at least one solution (and, IMHO, you should try to print both solutions if they do exist). — SJuan76, Apr 16 '13 at 21:30

DeltaLima · Answer 1 · 2013-04-16T21:53:32.937

NaN is something you get when the calculation is invalid. Such as dividing by 0 or taking the squareroot of -1.

When I test your code with a = 1, b = 0 and c = -4 the answers is 2.02.0

The formatting is not right and the calculation of final2 is not negated.

Otherwise the code is right.

To improve you could check whether the discriminant is negative.

double d = b*b -4 * a * c;
if (d < 0){
   System.out.println("Discriminant < 0, no real solutions" );
   return;
}
double x1 = (-b  -sqrt(d))/(2*a);
double x2 = (-b +sqrt(d))/(2*a);
System.out.format("The roots of your quadratic formula are %5.3f and %5.3f\n",x1,x2);

Or, if you prefer support for solutions from the complex domain:

if (d < 0) {
    System.out.println("Discriminant < 0, only imaginary solutions");
    double r = -b / (2 * a);
    double i1 = -sqrt(-d) / (2 / a);
    double i2 = sqrt(-d) / (2 / a);
    System.out.format("The roots of your quadratic formula are (%5.3f + %5.3fi) and (%5.3f + %5.3fi)\n",r, i1, r, i2);
    return;
}

syb0rg · Answer 2 · 2013-04-16T21:45:17.767

I'm a little late to answer, but I corrected your problems (described in the other answers), fixed one of your calculations, and cleaned up your code.

import java.util.*;

public class Test {

    public static void main(String[] args)
    {
        Scanner s = new Scanner(System.in);
        System.out.println("Insert value for a: ");
        double a = Double.parseDouble(s.nextLine());
        System.out.println("Insert value for b: ");
        double b = Double.parseDouble(s.nextLine());
        System.out.println("Insert value for c: ");
        double c = Double.parseDouble(s.nextLine());
        s.close();

        double answer1 = (-b + Math.sqrt(Math.pow(b, 2) - (4 * a * c))) / (2 * a);
        double answer2 = (-b - Math.sqrt(Math.pow(b, 2) - (4 * a * c))) / (2 * a);

        if (Double.isNaN(answer1) || Double.isNaN(answer2))
        {
            System.out.println("Answer contains imaginary numbers");
        } else System.out.println("The values are: " + answer1 + ", " + answer2);
    }
}

rgettman · Answer 3 · 2013-04-16T21:35:35.357

You are getting NaN because you are attempting to take the square root of a negative number. In math that's not allowed unless you are allowing complex numbers, e.g. 1 +/- 2i.

This can happen in quadratic formulas when the discriminant (the thing in the square root) is negative, e.g. x^2 + 6*x + 100: b^2 - 4ac = 36 - 400 = -364. Taking the square root of a negative number in Java leads to NaN. (not a number)

To test for NaN, use Double.isNaN and handle the NaN appropriately.

In addition, your calculations are incorrect even if NaN isn't being encountered:

$ java QuadraticFormulaSCN
insert value for a:
1
insert value for b:
5
insert value for C:
6
The x values are:-2.0-2.0

This should have outputted 2.0 and 3.0

score 0 · Answer 4 · answered Apr 16 '13 at 21:38

0

You should only do the calculation when discriminant is equal or greater than zero

if(((Math.pow(b,2)-(4*a*c))>= 0){ /* Calculation here */ } 
else {/*error message or complex number calculus*/};

answered Apr 16 '13 at 21:38

Ramiro Motteta

104
4

score 0 · Answer 5 · answered Nov 29 '15 at 04:02

One thing I always try to do is put all my math in appropriate parenthesis to avoid an, all too easy, Order of Operations mistake. The NaN is saying "Not a number." You would also get that message if the user input numbers that could not produce a result, such as a trying to get the square root of a negative number. Also, just as a note, you can save sometime by only using on Scanner for a,b, and c.

public class QuadraticFormula{

   public static void main(String[] args){
      java.util.Scanner input = new java.util.Scanner(System.in);
      double a = input.nextDouble();
      double b = input.nextDouble();
      double c = input.nextDouble();

      double quadPos = (-b + Math.sqrt(Math.pow(b,2)-(4*a*c)))/(2*a);
      double quadNeg = (-b - Math.sqrt(Math.pow(b,2)-(4*a*c)))/(2*a);

      System.out.println("-b - = " + quadNeg + "\n-b + = " + quadPos);

      }
   }

quadratic formula with scanner inputs

5 Answers5