Why does (int)55 == 54 in C++?

Question

So I\'m learning C++. I\'ve got my \"C++ Programming Language\" and \"Effective C++\" out and I\'m running through Project Euler. Problem 1...dunzo. Problem 2...not so mu

Answer 1

Break the code for generatring, at times float number dont behave the way we think when used in a long expression...break the code and check it.

Answer 2

I agree 100% with shog9's answer - with the algorithm you used to calculate Fibonacci, you have to be really careful with floating point values. I found that the page cubbi.com: fibonacci numbers in c++ seems to show other ways of obtaining them.

I looked around for a good idea on how to make your implementation of GenerateNthFibonacciNumber handle cases where it returns the double 54.999999, but when you cast to an int or a long you get 54.

I came across what appears to be a reasonable solution at C++ Rounding, which I have adapted below in your code.

Also, It's not a huge deal, but you may want to precalculate PHI, then either pass it as a parameter or reference it as a global - now you are recalculating it every time you call the function.

double GenerateNthFibonacciNumber(const int n)
{
        //This generates the nth Fibonacci Number using Binet's Formula   
        const double PHI = (1.0 + sqrt(5.0)) / 2.0;
        double x = ((pow(PHI,n)-pow(-1.0/PHI,n)) / sqrt(5.0));
        // inspired by http://www.codingforums.com/archive/index.php/t-10827.html
        return ((x - floor(x)) >= 0.5) ? ceil(x) : floor(x);
}

Finally, here's how I rewrote your Solve() method so that GenerateNthFibonacciNumber(FibIndex) is only called in one place in the code. I also added the column with the current running total of the even Fibonacci terms to your output:

double Solve() {
    long FibIndex = 0;
    double result = 0.0;
    double oldresult = 0.0;
    int done = 0;
    const double PHI = (1.0 + sqrt(5.0)) / 2.0;

    while (!done)
    {
        double currentFib = GenerateNthFibonacciNumber(++FibIndex);
        if ((int)currentFib % 2 == 0)
        {
            oldresult = result;

            if (currentFib >= 4000000.0)
            {
                done = 1;
            }
            else
            {
                result += currentFib;
            }

        }
        cout << currentFib << " " << (int)currentFib << " " << (int)currentFib % 2 << " " << (int)result << "\n";       
    }
    return result;
}