C# Sort object array by specific parameter

Question

I am relatively new to programming. I have an array of objects which isn't necessarily full (may include null rows). And I want to sort it by one of the class parameters "int moveScore". This is my array (currently holds only 32 entries)

Score[] firstPlyScore = new Score[1000];

I tried 2 things for sorting

1 In the "Score" class, i inherited "IComparable" and used the "CompareTo" method as follows

public int CompareTo(object obj)
        {
            Score x = (Score)obj;

            if (this.moveScore < x.moveScore)
                return -1;

            if (this.moveScore > x.moveScore)
                return 1;

            return 0;
        }

I called it using;

Array.Sort(firstPlyScore);

The problem is that it does sort correctly but at the end of the array. Meaning rows 0-966 are "null" and 967-999 are sorted correctly (967 with highest "int", 999 with lowest). Is there any way to fix this.

2 I also tried this

Array.Sort(firstPlyScore, delegate
    (Score x, Score y) { return x.moveScore.CompareTo(y.moveScore); });

Here the problem was that it crashed when it reached a "null" row.

Help most appreciated!

Have you considered using a list instead of an array? That way you can easily avoid those null entries. — Jon Skeet
– Jon Skeet, Commented Dec 11, 2013 at 19:25
to Jon Skeet - I don't actually now how to use lists yet (still a student) :-) — GuyZ
– GuyZ, Commented Dec 11, 2013 at 19:29
@user3092553 List<Score> scoreList = new List<Score>();, that will create an 'open-ended' collection of Score objects, in which you can Add() to. See List<T> — xenolightning
– xenolightning, Commented Dec 11, 2013 at 19:30

Mike Strobel · Accepted Answer · 2013-12-11 19:59:38Z

5

The default comparison behavior is for null values to be ordered before non-null values. If you want to override this behavior, a custom Comparison<Score> like in your second example would be the way to go.

delegate (Score x, Score y) {
    if (x == null)
        return y == null ? 0 : 1;
    if (y == null)
        return -1;
    return x.moveScore.CompareTo(y.moveScore);
}

This will keep the null items at the end of the array.

To sort in descending order, just swap the x and y references in the last line.

edited Dec 11, 2013 at 19:59

answered Dec 11, 2013 at 19:31

Mike Strobel

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3 Comments

user2864740 Over a year ago

+1 because, as much as a think arrays here are poo, this ought to work work (and it won't "lose" all the null entries).

GuyZ Over a year ago

Thanks! This actually works only it sorts them in a ascending order. How do i get it to sort descending?

Samuel Edwin Ward Over a year ago

@user3092553, return y.moveScore.CompareTo(x.moveScore); should do it

L.B · Accepted Answer · 2013-12-11 19:29:35Z

3

firstPlyScore = firstPlyScore
                    .Where(x => x != null)
                    .OrderByDescending(x => x.moveScore)
                    .ToArray();

answered Dec 11, 2013 at 19:29

L.B

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7 Comments

GuyZ Over a year ago

thanks! it works but... i need the array to be 1000 for future iterations. Is it possible to keep it at 1000?

L.B Over a year ago

@user3092553 just add .Concat(firstPlyScore.Where(x => x == null)) after OrderByDescending. Final linq would be:

firstPlyScore = firstPlyScore                         .Where(x => x != null)                         .OrderByDescending(x => x.moveScore)                         .Concat(firstPlyScore.Where(x => x == null))                         .ToArray();

xenolightning Over a year ago

Even though this does work, the code is quite verbose and would be hard to follow. The custom comparison option, above, is a better approach.

L.B Over a year ago

@Xenolightning hard to follow? maybe for you. not for all.

L.B Over a year ago

@Xenolightning maybe, But it shows another way of doing it with Linq. So unless it is wrong , it can be thought as an answer.

|

BRAHIM Kamel · Accepted Answer · 2013-12-11 20:18:53Z

2

You can use Linq to Entities to sort and then convert back to an array it will re-size your array to the correct length needed without null issue

 var list = firstPlyScore.OrderByDescending(x => x.MoveScore).ToList();
       //here  how you can get always  1000 array  length  as you asked  
            for (int i = list.Count-1; i < 1000; i++)
            {
                list.Add(null);  
            }
            firstPlyScore = list.ToArray();  

        }

edited Dec 11, 2013 at 20:18

answered Dec 11, 2013 at 19:27

BRAHIM Kamel

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4 Comments

Mauricio Gracia Gutierrez Over a year ago

since there are some entries which are null x.MoveScore will fail

GuyZ Over a year ago

Thanks for the reply but I still get a "NullReferenceException".

L.B Over a year ago

Adding where and changing Order to OrderByDescending makes it my answer :)

GuyZ Over a year ago

thanks! it works but... 1) i need the array sorted in an descending order. 2) I do need the array to be 1000 for future iterations

Konstantin · Accepted Answer · 2013-12-11 19:28:41Z

1

In the beginning of your compare method

if(obj == null) return 0;

answered Dec 11, 2013 at 19:28

Konstantin

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Comments

Fabian Bigler · Accepted Answer · 2013-12-11 19:30:24Z

0

The problem is that it does sort correctly but at the end of the array. Meaning rows 0-966 are "null" and 967-999 are sorted correctly (967 with highest "int", 999 with lowest). Is there any way to fix this.

If you need something, whose size can change, you are looking for a List<int>, instead of using arrays. Arrays should be used for Lists that do not change (e.g. a chess board).

The List also provides you a method called Sort.

As mentioned by others, you can also use LINQ to achieve what you seek for.

answered Dec 11, 2013 at 19:30

Fabian Bigler

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Comments

Pic Mickael · Accepted Answer · 2013-12-11 19:32:16Z

0

Do you really need to keep 1000 items in the list even if most of them are null ? I would suggest to check the logic behind it to see if you can prevent that because that's taking a lot of space for nothing.

Otherwise, 2 possibilities:

Check for Obj == null in your compare function. But it might still fail if the comparing item is null
Create a custom class for your score and make Icomparable (Check this link for several example about how to sort arrays

answered Dec 11, 2013 at 19:32

Pic Mickael

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