c# - 偶数的斐波那契数列和

Question

我在 stackoverflow 上遇到了这个问题:

"I'm having some trouble with this problem in Project Euler. Here's what the question asks: Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... Find the sum of all the even-valued terms in the sequence which do not exceed four million."

最佳答案是这个(在 VS2010 中不能为我编译...为什么？):

    IEnumerable<int> Fibonacci()
    {
        int n1 = 0;
        int n2 = 1;

        yield return 1;
        while (true)
        {
          int n = n1 + n2;
          n1 = n2;
          n2 = n;
          yield return n;
        }
    }

    long result=0;

    foreach (int i in Fibonacci().TakeWhile(i => i<4000000).Where(i % 2 == 0))
    {
        result+=i;
    }
    Console.WriteLine(result);

在寻找答案之前，我决定自己尝试一下，并想出了这个(请告诉我为什么这是解决这个问题的好方法或坏方法):

我在类里面写它是因为我可以在未来为类添加更多内容，而不仅仅是解决一个斐波那契问题。

class Fibonacci
{
    private int prevNum1 = 1;
    private int prevNum2 = 2;
    private int sum = 0;

    public int GetSum(int min, int max)
    {
        prevNum1 = min;
        prevNum2 = prevNum1 + prevNum1;
        if (prevNum1 % 2 == 0)
        {
            sum += prevNum1;
        }
        if (prevNum2 % 2 == 0)
        {
            sum += prevNum2;
        }
        int fNum = 0;
        while (prevNum2 <= max)
        {
            fNum = prevNum1 + prevNum2;
            if (fNum % 2 == 0)
            {
                //is an even number...add to total
                sum += fNum;
            }
            prevNum1 = prevNum2;
            prevNum2 = fNum;

        }

        return sum;
    }

}

        Fibonacci Fib = new Fibonacci();
        int sum = Fib.GetSum(1, 4000000);

        Console.WriteLine("Sum of all even Fibonacci numbers 1-4,000,000 = {0}", sum);

同样，我正在寻找关于为什么这是解决此问题的好方法或坏方法的答案。还有为什么第一个解决方案无法编译。我是一名初级程序员，正在努力学习。谢谢!

Answer 1

有了这个它必须编译:

foreach (int i in Fibonacci().TakeWhile(i => i < 4000000).Where(i => i % 2 == 0))
{
    result += i;
}

代码无法编译的问题是错误的 lambda 表达式，它是:

.Where(i % 2 == 0)

但必须是

.Where(i => i % 2 == 0)