using System;

​

namespace ConsoleApp1

{

    class Program

    {

        static void Main(string[] args)

        {

            string SearchFor = "kAriM";

            string[] data = new[]{"ali", "Behnaz", "Davood", "Elnaz", "Fatemeh", "Gholam", "Hanie", "Iman", "Javad", "Karim", "Leilla", "Nazgol", "Nazila"};

            Console.WriteLine($"Search  For [{SearchFor}] in :\n\n");

            Console.WriteLine($"[{string.Join(",", data)}]\n\n");

            int Index = Find(data, SearchFor);

            Console.WriteLine($"Find Index is [{Index}]");

            Console.ReadLine();

        }

​

        static int Find(string[] data, string SearachItem)

        {

            //It is mandatory to turn off case-sensitive by convert values to lowercase or uppercase

            ConvertArrayToLower(data);

            string Lower = SearachItem.ToLower();

            //initiate

            char XChar = Lower[0]; // we will compare first char of Search Item by first char of array values

            int FirstIndex = 0;

            int LastIndex = data.Length - 1;

            while (FirstIndex <= LastIndex)

            {

                int MiddelIndex = (FirstIndex + LastIndex) / 2;

                char middel = data[MiddelIndex][0];

                if (XChar > middel)

                    FirstIndex = MiddelIndex + 1;

                else if (XChar < middel)

                    LastIndex = MiddelIndex - 1;

                else if (data[MiddelIndex] != Lower) // maybe the first char of two or more values will be the same so we should go to the next step

                {

                    FirstIndex = MiddelIndex + 1;

                }

                else

                    return MiddelIndex; // if found

            }

​

            //if not found

            return -1;

        }

​

        private static void ConvertArrayToLower(string[] data)

        {

            for (int i = 0; i < data.Length; i++)

            {

                data[i] = data[i].ToLower();

            }

        }

    }

}