using System;

    using System.Linq;

    using System.Collections.Generic;

    public class Program

    {

        public static void Main()

        {

            var list = new List<Double?> { null, null, 2, 3, 4, null, null, 3, 3, null, 7 , 8, null};

            var groupLists = new List<GroupList>();

            GroupList currentGroupList = null;

            for (var i = 0; i < list.Count; i++)

            {

                if (list.ElementAt(i).HasValue)

                {

                    if (currentGroupList == null)

                    {

                        currentGroupList = new GroupList();

                        currentGroupList.Start = i;

                        groupLists.Add(currentGroupList);

                    }

                    currentGroupList.Items.Add(list.ElementAt(i).Value);

                }

                else

                {

                    if (currentGroupList != null)

                    {

                        currentGroupList.End = i - 1;

                    }

                    currentGroupList = null;

                }

            }

            if (currentGroupList != null)

            {

                currentGroupList.End = list.Count -1;

            }

            foreach(var groupList in groupLists)

            {

                Console.WriteLine("{0} {1} [{2}]", groupList.Start, groupList.End, string.Join(",",groupList.Items.ToArray()));

            }

        }

        public class GroupList

        {

            public GroupList()

            {

                Items = new List<Double>();

            }

            public int Start { get; set; }

            public int End { get; set; }

            public List<Double> Items { get; set; }

        }

    }