using System;

public class Program

{

    public static void Main()

    {

        int[][] inputs = new[]

                         {

                             new[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10},

                             new[] {1, 2, 3, 3, 3, 3, 3, 5},

                             new[] {1, 2},

                             new[] {1},

​

                             //null

                         };

​

        foreach (int[] input in inputs)

        {

            foreach (var val in new int[] { 1, -1, 5, 3, 2, 10, 100 })

            {

                Console.WriteLine(String.Join(",", input));

                Console.WriteLine("first index of {0} is {1}", val, FirstIndexOf(input, val));

                Console.WriteLine();

            }

        }

    }

    public static int FirstIndexOf(int[] arr, int val)

    {

        if (arr == null)

        {

            return -1;

        }

​

        int startIndex = 0;

        int endIndex = arr.Length - 1;

        while (startIndex < endIndex)

        {

            int midPointIndex = (int)Math.Floor((decimal) ((endIndex + startIndex + 1) / 2));

​

            if (arr[midPointIndex] > val)

            {

                endIndex = midPointIndex - 1;

            }

            else if (arr[midPointIndex] < val)

            {

                startIndex = midPointIndex; // check if one off

            }

            else if (arr[midPointIndex - 1] >= val)

            {

                endIndex = midPointIndex;

            }

            else

            {

                return midPointIndex;

            }

        }

​

        if (arr[startIndex] == val)

        {

            if (startIndex == 0 || arr[startIndex - 1] == val)

            {

                return startIndex;

            }

        }

​

        return -1;

    }

​

}