Windows program gives me -1.#IND as a number result. What the heck is that?

[floofy]

Hello all!
First I would like to thank all of those who helped me regarding my first assignment...I'm much better with my semicolons now.

A couple assignments later...I'm having problems again. I'm supposed to create a very simple C++ program calculating the answers to a quadratic equation in form a(x^2) + b(x) + c. I thought i had no problems typing the code, but when I test 1, 2 and 3 for a, b, and c variables respectively, I get "-1.#IND" as my answer when it should be -1. What's going on??

#include <iostream>
#include <cmath>
 
using namespace std; 
 
int main()
{
  double a = 0;
  double b = 0;
  double c = 0;
 
  cout << "The current value of a is "<<a << endl;
  cout << "The current value of b is "<<b << endl;
  cout << "The current value of c is "<<c << endl;
 
  cout << "Enter the coefficients a, b, and c for the quadratic equation in the form"
       << endl;
  cout << "a(x^2) + b(x) + c" << endl;
  cout << "For example, if the equation is 2(x^2) + 3(x) + 4" << endl;
  cout << "Enter as 2.0 3.0 4.0" << endl;
 
  cin >> a >> b >> c; 
 
  cout << "Your expression is "<<a<<"(x^2) + "<<b<<"x + "<<c<< endl;
 
  double factor = a * c;
 
  double x1 = 1;
  double x2 = ((-1*b) - sqrt(pow(b,2.0) - 4*a*c))/(2*a);
 
  cout << x1 << endl;
  cout << x2 << endl;
 
 
  system("PAUSE");
  return 0;
}

I'm focussing on the x2 equation here...x1 is just to test to see if the value pops up okay and it does. In the x2 equation, it seems that the 4*a*c is causing the -1.#IND I think...because when I take out the c in 4*a*c, a value does pop up instead of the -1.#IND thing. Whatever value I test, I get -1.#IND.

Help!!

[horace1]

so long as you are dealing with real roots you only had one problem and that was you set the result of x1 to 1, e.g.

 double x1 = 1;
  double x2 = ((-1*b) - sqrt(pow(b,2.0) - 4*a*c))/(2*a);
 

I have fixed that

 
#include <iostream>
#include <cmath>
 
using namespace std; 
 
int main()
{
  double a = 0;
  double b = 0;
  double c = 0;
 
  cout << "The current value of a is "<<a << endl;
  cout << "The current value of b is "<<b << endl;
  cout << "The current value of c is "<<c << endl;
 
  cout << "Enter the coefficients a, b, and c for the quadratic equation in the form"
       << endl;
  cout << "a(x^2) + b(x) + c" << endl;
  cout << "For example, if the equation is 2(x^2) + 3(x) + 4" << endl;
  cout << "Enter as 2.0 3.0 4.0" << endl;
 
  cin >> a >> b >> c; 
 
  cout << "Your expression is "<<a<<"(x^2) + "<<b<<"x + "<<c<< endl;
 
  double factor = a * c;
 
  double x1 = ((-1*b) + sqrt(pow(b,2.0) - 4*a*c))/(2*a); //** fixed
  double x2 = ((-1*b) - sqrt(pow(b,2.0) - 4*a*c))/(2*a);
 
  cout << x1 << endl;
  cout << x2 << endl;
 
 
  system("PAUSE");
  return 0;
}
 

if you try these values your program works

        a = 2.0     b = -6.0     c = 4.0    real roots = 2.0 and 1.0
        a = 61.0    b = 159.0    c = 87.0   real roots = -0.781449 and -1.825108
 
 

however the values you used yeild complex roots

        a = 1.0     b = 2.0     c = 3.0  complex roots -1.0 +-j1.414
 

when you have complex roots the expression (pow(b,2.0) - 4*a*c) is negative so when you take the square root you get an undefined value which gives you your "-1.#IND" output. If you are suposed to solve for complex roots you need to test for this situation and evaluate complex values

[floofy]

Thank you so much for your help!! I didn't realize that I maybe shouldn't have used 1, 2, and 3.

[Banfa]

It is easy to test if the result will be complex, compare

pow(b,2.0) AND 4*a*c

if pow(b,2.0) > 4*a*c
   2 real roots

if pow(b,2.0) == 4*a*c
   1 real root

if pow(b,2.0) < 4*a*c
   2 complex roots


Also before performing the calculation it would be good to test that a != 0 otherwise you will get a divide by 0 error.


In most programming the code to handle the success case (when the program operates successfully as specified) is normally <60% of the code, sometimes <40%, these rest is all error handling conditions. However it is this error handling that most often gets leftout by inexperienced programmers.

This is also why programmes are twice as complex to write as they initially appear, because a lot of people initially forget to think about the error handling cases.