Question

Currently I am learning how to build my own linked list in C#. I have created a function called AddTrees that adds a tree to the end of the list. User create their trees through textboxes input:tree_name, tree_height, tree_price and tree_instock. I am requesting help with my InsertTree which is not inserting a tree between current and current.next_tree? But instead of instead of inserting it between, it adds it to the top of the list.

   namespace Tree_farm
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }

            public class TheTrees
            {
                private string tree_type = " ";
                private int tree_height = 0;
                public double tree_price = 0;
                private int tree_instock = 0;


                public TheTrees next_tree;

                public TheTrees(string newtree, int newheight, int newinstock, double newprice)
                {
                    tree_type = newtree;
                    tree_height = newheight;
                    tree_price = newprice;
                    tree_instock = newinstock;


                    next_tree = null;
                }

                public override string ToString()
                {
                    return tree_type + " " + tree_height + " " + tree_price + " " + tree_instock;
                }

            }


            public class ListForTrees
            {
                public TheTrees first_tree;
                public TheTrees last_tree;

                public int count = 0;

                public ListForTrees(TheTrees new_tree)
                {
                    first_tree = new_tree;
                    last_tree = new_tree;
                    count = 1;
                }

                public ListForTrees()
                {

                }

                public void InsertTree(TheTrees new_tree)
                {
                    TheTrees current = first_tree;

                    if (count == 0)
                    {
                        first_tree = new_tree;
                        last_tree = new_tree;
                        count = 1;
                    }

                    else if (count != 0)
                    {
                        if (new_tree.tree_price <= first_tree.tree_price)
                        {
                            new_tree.next_tree = first_tree;
                            first_tree = new_tree;
                        }

                        else if (new_tree.tree_price >= last_tree.tree_price)
                        {
                            last_tree.next_tree = new_tree;
                            last_tree = new_tree;
                        }

                        else
                        {
                            while (new_tree.tree_price > current.next_tree.tree_price)
                            {
                                 current.next_tree = current;
                            }

                            new_tree.next_tree = current.next_tree;
                            current.next_tree = new_tree;
                        }

                        count++;
                    }
                }

 public void AddTree(TheTrees new_tree)
            {
                TheTrees current = first_tree;

                if (count == 0)
                {
                    first_tree = new_tree;
                    last_tree = new_tree;
                    count = 1;
                }

                else if (count != 0)
                {
                    if (new_tree.tree_price <= first_tree.tree_price)
                    {
                        new_tree.next_tree = first_tree;
                        first_tree = new_tree;
                    }

                    else if (new_tree.tree_price >= last_tree.tree_price)
                    {
                        last_tree.next_tree = new_tree;
                        last_tree = new_tree;
                    }

                    else
                    {
                        while (new_tree.tree_price > current.next_tree.tree_price)
                        {
                            current = current.next_tree;
                        }

                        new_tree.next_tree = current.next_tree;
                        current.next_tree = new_tree;
                    }

                    count++;
                }
            }

                public void ClearTrees()
                {
                    first_tree = null;
                    count = 0;
                }
            }

            ListForTrees mainlist = new ListForTrees();

            private void Form1_Load(object sender, EventArgs e)
            {

            }


            private void BtnInsertTree_Click(object sender, EventArgs e)
            {
                //Insert Code
                try
                {
                    int height = Convert.ToInt32(TxtTreeHeight.Text);
                    int stock = Convert.ToInt32(TxtTreeStock.Text);
                    double price = Convert.ToDouble(TxtTreePrice.Text);

                    TheTrees treeinsert = new TheTrees(TxtTreeName.Text, height, stock, price);

                    mainlist.InsertTree(treeinsert);
                }
                catch
                {
                    MessageBox.Show("Please check intput fields");
                }
            }


        private void BtnAddTree_Click(object sender, EventArgs e)
        {
            try
            {
                int height = Convert.ToInt32(TxtTreeHeight.Text);
                int stock = Convert.ToInt32(TxtTreeStock.Text);
                double price = Convert.ToDouble(TxtTreePrice.Text);

                TheTrees treeadd = new TheTrees(TxtTreeName.Text, height, stock, price);

                mainlist.AddTree(treeadd);
            }
            catch
            {
                MessageBox.Show("Please check intput fields");
            }
        }
        }
    }

Answer 1

First, I think your expected result is wrong. It appears you want the linked list sorted in order of price (lowest to highest). If so, Cypress should actually go at the end of your list, not in the middle (80.00 > 50.00 > 2.00).

Secondly, I believe the issue rests in your while loop.

 while (new_tree.tree_price > current.next_tree.tree_price)
 {
      current.next_tree = current;
 }

I believe what you’re trying to do is walk down your linked list. What you’re actually doing is mucking up your linked list by changing current.next_tree to point to itself. Changing it to the following is a positive first step towards correcting your algorithm:

 while (new_tree.tree_price > current.next_tree.tree_price)
 {
      current = current.next_tree;
 }

EDIT: It should be noted that I do not get the same error that you get in your original post. When I insert items in the same order that you claim to, I get the list in correct sorted order. This is due to the fact that Cypress goes to the end, as expected. How did you get the result that you claim in the question?