Bubble sort issues

Question

I keep getting a NullReferenceException was unhandled error when trying to run this code and dont know why. I am trying to sort by a persons name I have only been working with c# for about 4 Mths.

    public static student[] players = new student[30];
    public struct student
    {
        public string lastname;
        public string firstname;
        public string likes;
    }

    public static void read()
    {
        StreamReader sr = new StreamReader(@"names.txt");
        int i = 0;
        while (!sr.EndOfStream)
        {
            players[i].lastname = sr.ReadLine();
            players[i].firstname = sr.ReadLine();
            players[i].likes = sr.ReadLine();
            i++;
        }
        sr.Close();
    }

        public static void sort()
        {
            //alphabetically lists players
            student temp;

            for (int i = 0; i < players.Length - 1; i++)
            {
                for (int pos = 0; pos < players.Length - 1; pos++)
                {

                    if (players[pos + 1].lastname.CompareTo(players[pos].lastname) < +0)
                    {
                        temp = players[pos + 1];
                        players[pos + 1] = players[pos];
                        players[pos] = temp;
                    }
                }
            }

        }

How are you populating your players array? Are all items in the array valid objects? Please post your code where you populate 'players'. — Baldrick, Nov 13 '13 at 01:51
I'm imagining that 'players' is a static array of 'student' declared in the class definition. — Baldrick, Nov 13 '13 at 01:52
NullReference is usually referring to an object is not created — jhyap, Nov 13 '13 at 02:02
There is space for 30 students preallocated in `students` array. `names.txt` probably contains less than 90 lines. For a quick fix store last value of `i` and use that value to limit first `for loop` instead of `players.Length` which is always 30. — Nikola Markovinović, Nov 13 '13 at 02:32

jhyap · Answer 1 · 2013-11-13T02:17:49.687

1

I might have caught the NullReference

while (!sr.EndOfStream)
{
    player[i] = new student();
    players[i].lastname = sr.ReadLine();
    players[i].firstname = sr.ReadLine();
    players[i].likes = sr.ReadLine();
    i++;
}

edited Nov 13 '13 at 02:17

answered Nov 13 '13 at 02:05

jhyap

3,779
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cannot apply indexing with [] to an expression – Kiwilad Nov 13 '13 at 02:15
this is error message you received when you tried player[i][pos]? Because initially when I see your for loops, I presumed it was a jagged array. lol.. So, just use player[pos] will do.. – jhyap Nov 13 '13 at 02:17

score 1 · Accepted Answer · answered Nov 13 '13 at 02:57

You need to check the array item is not null before you compare the lastname.

private static void Sort(Player[] players)
{
    //alphabetically lists players

    for (int i = 0; i < players.Length; i++)
    for (int pos = 0; pos < players.Length - 1; pos++)
    {
        var currentPlayer = players[pos] != null ? players[pos].LastName : null;
        var nextPlayer = players[pos + 1] != null ? players[pos + 1].LastName : null;

        if (Compare(currentPlayer, nextPlayer) > 0)
        {
            var temp = players[pos + 1];
            players[pos + 1] = players[pos];
            players[pos] = temp;
        }
    }
}

private static int Compare<T>(T current, T next)
    where T: IComparable
{
    if (current == null) return -1;
    if (next == null) return 1;
    return current.CompareTo(next);
}

score 0 · Answer 3 · answered Nov 13 '13 at 02:24

0

Sorting is already built into the .NET framework via Enumerable.OrderBy and EnumerableOrderByDescending. In your case you can greatly reduce the function:

public static IOrderedEnumerable<student> sort()
{
   IOrderedEnumerable<student> orderedPlayers = players.OrderBy(p => p.lastname);
   return orderedPlayers;
}

This will order the results without throwing an exception even if lastName is null.

answered Nov 13 '13 at 02:24

P.Brian.Mackey

43,228
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238
348

1

Thank you that sorted it, pun intended lol – Kiwilad Nov 13 '13 at 02:58
Haha! If I answered your question please mark choose the answer. Thanks! – P.Brian.Mackey Nov 13 '13 at 03:03

Bubble sort issues

3 Answers3