Question

I wrote a simple program to solve the math problem:

A^2+B^2 = 12
A*B = 9
(A+B)^2 = y

My program is looping through each number starting at 0.000000001, and checking to see if the first equation is true (the second one is always true because I always assign double B = 9 / A).

Then it writes the results and pauses. This is taking forever to run since its doing a lot of math, I was wondering if I made any mistakes so I can fix them? Here is my code:

namespace AnnoyingMath1
{
    class Program
    {
        static void Main(string[] args)
        {
            for (double a = 0.00000001; a < 9; a += 0.00000001) {
                double b = 9 / a;
                if (Math.Pow(a, 2) + Math.Pow(b, 2) == 12)
                {
                    if (a * b == 9) // always true because b = 9 / a
                    {
                        Console.WriteLine("SUCCESS! a = " + a.ToString() + ", b = " + b.ToString() + ", y = (a+b)^2 = " + (Math.Pow(a + b, 2)).ToString() );
                        Console.ReadLine();
                    }
                }
                else {
                    Console.WriteLine("fail1 " + a.ToString() + " , " + b.ToString());
                }
            }
        }
    }
}

Answer 1

Yes. It does.

statements such as:

if (a * b == 9) // always true because b = 9 / a  WRONG, because you are dealing with floating point

are incorrect.

Never directly compare floating point values. Use a delta instead:

epsilon = 0.000001
if (Abs(a*b - 9) <= epsilon )

There are many similar questions on StackOverflow.

Please read What Every Computer Scientist Should Know About Floating-Point Arithmetic