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    copied!<p>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 <a href="http://cubbi.com/fibonacci/c++.html" rel="nofollow noreferrer" title="cubbi.com: fibonacci numbers in c++">cubbi.com: fibonacci numbers in c++</a> seems to show other ways of obtaining them.</p> <p>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.</p> <p>I came across what appears to be a reasonable solution at <a href="http://www.codingforums.com/archive/index.php/t-10827.html" rel="nofollow noreferrer" title="C++ Rounding">C++ Rounding</a>, which I have adapted below in your code.</p> <p>Also, It's not a <strong>huge</strong> 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.</p> <pre><code>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)) &gt;= 0.5) ? ceil(x) : floor(x); } </code></pre> <p>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:</p> <pre><code>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 &gt;= 4000000.0) { done = 1; } else { result += currentFib; } } cout &lt;&lt; currentFib &lt;&lt; " " &lt;&lt; (int)currentFib &lt;&lt; " " &lt;&lt; (int)currentFib % 2 &lt;&lt; " " &lt;&lt; (int)result &lt;&lt; "\n"; } return result; } </code></pre>
 

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