Question

When running the following code I end up getting negative values. I am only using positive numbers so I am really confused. I have tried to see if this negative number was a return code but there dose not seem to be any codes that return this as a number.

  public static int fib(int n)
    {
        int a = 0;
        int b = 1;
        for (int i = 0; i < n; i++)
        {
            int temp = a;
            a = b;
            b = temp + b;
        }

        return b;
    }

    static void Main(string[] args)
    {
        int n = 0;
        bool Run = true;

        while (Run == true)
        {
            n = fib(n + 1);
            Console.WriteLine(n);
        }
    }

Here are the results when the code is run:

1
2
3
5
13
610
-121099088
1

Answer 1

You’re just seeing integer overflow. You’re trying to compute fib(610) which is way more than an Int32 can hold – and when you add two large integers together, you’ll get a negative number.

If you build it in “checked” mode, an exception will be thrown instead – or you can do it just for the one expression you need:

for (int i = 0; i < n; i++)
{
    int temp = a;
    a = b;
    b = checked(temp + b);
}

See MSDN for more on checked arithmetic.