Notebook

// Notebook program

using System;

class Submission
{
    // -------------------------
    // Subprograms
    // -------------------------
    // Return valid note index (0-9)
    static int get_note_number()
    {
        int note_number = 0;
        bool valid_input = false;
        while (!valid_input)
        {
            note_number = Convert.ToInt32(Console.ReadLine());
            if (note_number >= 0 && note_number < 10)
            {
                valid_input = true;
            }
            else
            {
                Console.WriteLine("Invalid input. Enter 0-9.");
            }
        }
        return note_number;
    }


    // Write a new note
    static void add_note(string[] notebook)
    {
        int index = get_note_number();
        string note = Console.ReadLine();
        notebook[index] = note;
    }


    // Clear the whole notebook (after confirmation)
    static void clear_notes(string[] notebook)
    {
        string choice = "";
        while (choice != "y" && choice != "n")
        {
            choice = Console.ReadLine();
        }
        if (choice == "y")
        {
            for (int index = 0; index < notebook.Length; index++)
            {
                notebook[index] = "";
            }
        }
    }


    // Sort notes alphabetically
    static void order_notes(string[] notebook)
    {
        Array.Sort(notebook);
    }


    // Show the notebook
    static void show_notebook(string[] notebook)
    {
        for (int index = 0; index < notebook.Length; index++)
        {
            Console.WriteLine($"{index} : {notebook[index]}");
        }
    }


    // Menu (returns false when the user quits)
    static bool menu(string[] notebook)
    {
        Console.WriteLine();
        Console.WriteLine("a. Add note");
        Console.WriteLine("c. Clear notebook");
        Console.WriteLine("o. Order notes");
        Console.WriteLine("q. Quit");
        string choice = Console.ReadLine();
        if (choice == "q")
        {
            return false;
        }
        switch (choice)
        {
            case "a":
                add_note(notebook);
                break;
            case "c":
                clear_notes(notebook);
                break;
            case "o":
                order_notes(notebook);
                break;
        }
        return true;
    }


    // -------------------------
    // Main program
    // -------------------------
    public static void Main(string[] args)
    {
        string[] notebook = new string[10];
        for (int i = 0; i < notebook.Length; i++)
        {
            notebook[i] = "";
        }
        bool running = true;
        while (running)
        {
            show_notebook(notebook);
            running = menu(notebook);
        }
    }

}
You can submit as many times as you like. Only your latest submission will be taken into account.