C# - 2 Dimensional List cannot be printed

Question

I created 2 dimensional list in C#, when I want to print the items, it didn't show anything, but the list is not empty.

public static List<List<string>> GetSymetricOrder(List<List<string>> main_list){
            List<List<string>> new_main_list = new List<List<string>>();
            List<string> list = new List<string>();
            for(int i=0;i<main_list.Count;i++){
                for(int j=0;j<main_list[i].Count;j+=2){
                    list.Add(main_list[i][j]);
                }
                if(main_list[i].Count % 2 == 0){
                    for(int k = main_list[i].Count-1;k>=0;k-=2){
                        list.Add(main_list[i][k]);
                    }
                }else{
                    for(int l = main_list[i].Count-2;l>=0;l-=2){
                        list.Add(main_list[i][l]);
                    }
                }
                new_main_list.Add(list);
                list.Clear();
            }
            return new_main_list;
        }

        public static void Display(List<List<string>> new_main_list){
            int i = 1;
            foreach(var list in new_main_list){
                Console.WriteLine("SET"+i);
                foreach(var s in list){
                    Console.WriteLine(s);
                }
                i++;
            }
        }

    public static void Main(string[] args)
    {
                int num;
                List<List<string>> main_list = new List<List<string>>();
                List<string> list = new List<string>();
                while((num = int.Parse(Console.ReadLine()))!=0){
                    for(int i=0;i<num;i++){
                        string input = Console.ReadLine();
                        list.Add(input);
                    }
                    main_list.Add(list);
                    list.Clear();
                }

                List<List<string>> SymetricOrder = GetSymetricOrder(main_list);
                if(SymetricOrder != null){
                        Display(SymetricOrder);
                }

The code above receive input string in list list in main method and add the list to main_list. This will be processed in GetSymetricOrder and displayed using Display method. The list is not actually empty because the line:

if(SymetricOrder != null){
    Display(SymetricOrder);
}

actually print Console.WriteLine("SET"+i); inside Display method, but without list item. Here's the example input:

4   
AAA 
AA  
AAA 
AA  
3   
BBB 
BB  
BBB 
2   
RERE
RE  
0   

and here is the printed result:

SET1

SET2

SET3

No list items were printed, don't know why. Is there anything I missed?, thank you.

Answer 1

The problem is that the "list" variable. You add that to the "main_list" and then clear it => the list inside the "main_list" is cleared as well.

Basically you cannot reuse the "list" variable, since then the "main_list" only contains one instance several times. An easy way to fix it is to just assign a new list to the "list" variable instead of clearing it.