How to fix Code so that Fibonacci Search Algorithm in C# works properly

Question

Code doesn't always find the index.

I'm trying to implement the fibonacci search algorithm in C#. Sometimes the algorithm doesn't find the element in the array. I wrote Unit Tests to check Code coverage and saw that 10 % of my code isn't reached. I also checked other implementations. I think the problem is this part:

if (GetFibonacciNumberOnIndex(index - 1) == 1 && hayStack[offset + 1] == needle)
                return offset + 1;

But logically this should run when one last element is left.

    public static int FibSearch(int[] hayStack, int needle)
    {
        if (hayStack == null)
        {
            throw new ArgumentNullException(nameof(hayStack), "The array containing values to search in mustn't be null");
        }

        if (hayStack.Length == 0)
        {
            return -1;
        }

        int index = 0;

        while (GetFibonacciNumberOnIndex(index) < hayStack.Length)
            index++;

        int offset = -1;

        while (GetFibonacciNumberOnIndex(index) > 1)
        {
            int i = Math.Min(offset + GetFibonacciNumberOnIndex(index - 2), hayStack.Length - 1);

            if (needle < hayStack[i])
                index -= 2;
            else if (needle > hayStack[i])
            {
                index--;
                offset = i;
            }
            else
                return i;               
        }

        if (GetFibonacciNumberOnIndex(index - 1) == needle && hayStack[offset + 1] == needle)
            return offset + 1;

        return -404;
    }


    private static int GetFibonacciNumberOnIndex(int index)
    {
        if (index < 1)
            return 0;
        else if (index == 1)
            return 1;

        int first = 0;
        int second = 1;
        int tmp = 0;

        for (int i = 0; i < index; i++)
        {
            tmp = first;
            first = second;
            second = tmp + second;
        }

        return first;
    }
}

I have a file with 1000000 numbers. I'm reading the file and converting the content to an integer array. Finally I'm searching for a number, knowing the number is in the array, but the algorithm returns -404. I verified that the number is in the array using Linq Extension Methods on the integer array.

No, I meant sorted. To make the Fabonacci Search possible, you need to have a sorted array. — Aldert, Jan 04 '19 at 11:08
Well, this explains your problem. You can better use a binarysearch because it will be faster. Let me write you a small example.. — Aldert, Jan 04 '19 at 11:10
@Aldert: For binary search to work you still need to presort the numbers. Fibonacci Search is considered an improvement over binary in some scenarios. They are both O(log N) though. — TToni, Jan 04 '19 at 13:07
When your numbers are unsorted and you search only a single time, a simple for loop until you find it is the best you can do. As Aldert said, for Fibonacci Search to work your numbers need to be sorted. — TToni, Jan 04 '19 at 13:09
Why did my question get downvoted? (: i only want to know for future questions (: — Gino Pensuni, Jan 04 '19 at 16:05

score 0 · Answer 1 · answered Jan 04 '19 at 14:10

I create a small sample with binary search. No advantage using Fabonacci search. You can use a streamreader to add the integers one at a time, they will get added and sorted at teh same time.

public class NumIndex:IComparable<NumIndex>
        {
            public int Number { get; set; }
            public int Index { get; set; }

            public int CompareTo(NumIndex other)
            {
                return this.Number.CompareTo(other.Number);
            }

        }

        public static void Main(string[] args)
        {
            int[] nums = { 8, 4, 8, 1, 6, 3, 4, 32, 43, 5, 8, 95 };

            List<NumIndex> intArray = new List<NumIndex>();
            int i = 0;
            foreach (int num in nums)
            {
                NumIndex newNum = new NumIndex() { Number = num, Index = i++ };
                int index = intArray.BinarySearch(newNum);
                if (index > -1)
                {
                    //Item already found, you decide what to do with it, I added all numbers
                }
                else index = ~index;

                intArray.Insert(index, newNum);
            }

            NumIndex dummy = new NumIndex() { Number = 18, Index = 0 };

            int findItemIndex = intArray.BinarySearch(dummy);
            if (findItemIndex < -1) throw new Exception("Item not found");
            int itemIndex = intArray[].Index;
        }

Thank you very much! I guess its downvoted because its not an answer to the proper question (; — Gino Pensuni, Jan 04 '19 at 16:03
Well, does the answer help you, that is what it is all about :-) — Aldert, Jan 04 '19 at 16:05

How to fix Code so that Fibonacci Search Algorithm in C# works properly

1 Answers1