C# fibonacci function returning errors

Question

I am practising a C# console application, and I am trying to get the function to verify if the number appears in a fibonacci series or not but I'm getting errors.

What I did was:

class Program
{
    static void Main(string[] args)
    {
        System.Console.WriteLine(isFibonacci(20));
    }
    static int isFibonacci(int n)
    {
        int[] fib = new int[100];
        fib[0] = 1;
        fib[1] = 1;
        for (int i = 2; i <= 100; i++)
        {
            fib[i] = fib[i - 1] + fib[i - 2];

            if (n == fib[i])
            {
                return 1;
            }



        }
        return 0;
    }
}

Can anybody tell me what am I doing wrong here?

Answer 1

Here's a fun solution using an infinite iterator block:

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;
   }
}

bool isFibonacci(int n)
{
    foreach (int f in Fibonacci())
    {
       if (f > n) return false;
       if (f == n) return true;
    }
}

I actually really like this kind of Fibonacci implementation vs the tradition recursive solution, because it keeps the work used to complete a term available to complete the next. The traditional recursive solution duplicates some work, because it needs two recursive calls each term.

Answer 2

The problem lies in <= the following statement:

for (int i = 2; i <= 100; i++)

more to the point the =. There is no fib[100] (C# zero counts) so when you check on i=100 you get an exception.

the proper statement should be

for (int i = 2; i < 100; i++)

or even better

for (int i = 2; i < fib.Length; i++)