My situation

• create a list of objects using just classes

This how code work

• with the class ListRatings i can create a list of Rating (without using list method that c# offer with the library System.Collections.Generic)

(you can see what attribute and method have class ListRatings and class Rating in code bellow)

Issue

when i try to print all rating i added to my list, my program print just first and last!

My code

class ListRating:

public class ListRatings
{
    private Rating first;
    private Rating last;

    public ListRatings()
    {
        first = null;
        last = null;
    }
    public void InsertNewRat(Rating rating)
    {
        if (first == null)
        {
            first = rating;
            last = rating;
        }
        else
        {
            first.setNext(rating);
            last.setNext(rating);
        }
    }
    public void PrintRats()
    {
        Rating e = first;

        Console.WriteLine(e.getMatter());
        Console.WriteLine(e.getDate());
        Console.WriteLine(e.getRate());

        e = e.getNext();

        Console.WriteLine(e.getMatter());
        Console.WriteLine(e.getDate());
        Console.WriteLine(e.getRate());

        e = e.getNext();

        Console.WriteLine(e.getMatter());
        Console.WriteLine(e.getDate());
        Console.WriteLine(e.getRate());

    }
}

class Rating:

public class Rating
{
    private int rate;
    private string matter;
    private DateTime date;
    private Rating next;

    public Rating(int rate, string matter, DateTime date)
    {
        this.rate = rate;
        this.matter = matter;
        this.date = date;
        next = null;
    }

    public int getRate()
    {
        return rate;
    }

    public string getMatter()
    {
        return matter;
    }

    public DateTime getDate()
    {
        return date;
    }

    public Rating getNext()
    {
        return next;
    }

    public void setNext(Rating valutazione)
    {
        next = valutazione;
    }
}

Main:

    static void Main(string[] args)
    {
        ListRatings lr = new ListRatings();
        Rating r1 = new Rating(9, "Math", new DateTime(2021, 10, 5));
        Rating r2 = new Rating(10, "sport", new DateTime(2021, 11, 3));
        Rating r3 = new Rating(6, "English", new DateTime(2021, 11, 7));

        lr.InsertNewRat(r1);
        lr.InsertNewRat(r2);
        lr.InsertNewRat(r3);

        lr.PrintRats();
        Console.ReadKey();

    }

OUTPUT

Math
05/10/2021 00:00:00
9
English
07/11/2021 00:00:00
6

and the program stop and say error: System.NullReferenceException [is in the second e.getNext(); i used in the class ListRating]

with this output you can see that is printing the first one and jump to the last one without printing the second one.

The output i need is

Math
05/10/2021 00:00:00
9
Sport
10
03/11/07 00:00:00
English
07/11/2021 00:00:00
6

sorry for my bad english

CodePudding user response：

You aren't correctly updating your linked list. This code:

else
{
    first.setNext(rating);
    last.setNext(rating);
}

always sets both the first and last node's next node to the one you're trying to insert.

Instead, you want

else
{
    last.setNext(rating);
    last = rating;
}

This sets the node that's currently last to the new node, then updates your pointer to the last node to rating, which should now be last in the linked list.

As for the NullReferenceException, @TheVillageIdiot is correct

CodePudding user response：

You can adjust this solution:

class Program
{
    static void Main(string[] args)
    {
        var list = new List<int>();
        list.Append(1);
        list.Append(2);
        list.Append(3);
        list.Append(4);
        list.Append(5);

        list.Print();

        var list2 = new List<int>(11, 22, 33);
        list.Concat(list2);
        list.Print();
    }
}

public class List<T>
{
    Node<T> first;
    Node<T> last;

    public List()
    {

    }

    public List(params T[] data)
    {
        Append(data);
    }

    public void Append(params T [] data)
    {
        foreach (T d in data)
        {
            var n = new Node<T> { data = d };
            if (last == null)
            {
                first = n;
                last = n;
            }
            else
            {
                last.next = n;
                last = n;
            }
        }
    }

    public void Concat(List<T> list)
    {
        if (last == null)
        {
            first = list.first;
            last = list.last;
        }
        else
        {
            last.next = list.first;
            last = list.last;
        }
    }

    public void Print() 
    {
        var n = first;
        while (n != null)
        {
            Debug.Write(n.data);
            if (n.next != null)
                Debug.Write(" -> ");

            n = n.next;
        }
        
        Debug.WriteLine("");
    }
}

public class Node<T>
{
    public T data { get; set; }
    public Node<T> next { get; set; }
}

CodePudding user response：

You are trying to create a linked list implementation. Not sure if you need last in your RatingsList class. Personally, I would rename first to rating as well. The issue is how you are iterating over the ratings in PrintRats function. Change it to this and then try:

public void PrintRats()
{
    Rating e = first;
    
    while(e!=null)
    {
        Console.WriteLine(e.getMatter());
        Console.WriteLine(e.getDate());
        Console.WriteLine(e.getRate());

        e = e.getNext();
    }
}

Also, you can move all those Console.WriteLine in the above method into Rating class. Lastly, naming methods PrintRats instead of PrintRatings does not make it concise.

PS: If I'm sounding cranky, this is still before morning tea.