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I have a list of object I wish to sort in C#.Net 3.5, the object is as follows

id | name | parent_id

1 | Superman | NULL

2 | Batman | 3

3 | Watchman | 1

4 | Wolverine | 2

I know some of you might find this easy, but I need to sort the list based on the parent_id, which is pointing to its own index(id) in the same table.

So can someone give me a good algorithm to sort this list without looping over and over again? I cannot really phrase it that well, therefore I can't seem to google the right results I wanted.

A collection of IEnumerable or DataTable solution would be great.

Thanks in advance.

EDIT:----------------NEW Example

id | name | parent_id

1 | TOP CHILD | NULL

2 | Child C | 3

3 | Child B | 4

4 | Child A | 1

----> The Output I want is

id | name | parent_id

1 | TOP CHILD | NULL

4 | Child A | 1

3 | Child B | 4

2 | Child C | 3

----> If I use OrderBy or Sort, the result I get is

id | name | parent_id

1 | TOP CHILD | NULL

4 | Child A | 1

2 | Child C | 3

3 | Child B | 4

--> Non of the solutions is what I really wanted, Sorry again for not being clear

Hope this example is clearer

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5
  • I don't know any C# but I'm sure there is something called a "comparator function/object" in c#. C# should have some sort of library functions for sorting. Simply pass in your collection and your comparator function and it should be able to sort the collection for you. Commented May 15, 2009 at 6:06
  • 1
    I'm not 100% clear on the desired order - what does "based on the parent_id, which is pointing to its own index(id) in the same table" mean? Perhaps show the desired output? Commented May 15, 2009 at 6:06
  • 1
    Its even less clearer after your edit. In what order is the sort required. Acc to me 1, 3, 4 is the correct sort order for parent id as opposed to 1, 4, 3 Commented May 15, 2009 at 6:43
  • Not by sequential number, but by parent. Child A's parent_id is 1 (Parent A), Child B's parent id is 4 (Child A), Child C's parent id is 3 (Child B), Therefore it should display child A, B, C in that order Commented May 15, 2009 at 6:49
  • I think I understand what you're looking for. Check my revised comparer. Commented May 15, 2009 at 7:01

5 Answers 5

Reset to default
2

after you edit: I think I get you and the comparer looks like:

public Int32 CompareTo(SuperHero right)
{
    if (this.ID == right.ID)
        return 0;

    return this.ParentID.CompareTo(right.ID);
}

in response to your comment:

The class looks like:

public class SuperHero : IComparable<SuperHero>
{
    public Int32 ID { get; set; }
    public String Name { get; set; }
    public Int32 ParentID { get; set; }

    public SuperHero(Int32 id, String name, Int32 pid)
    {
        this.ID = id;
        this.Name = name;
        this.ParentID = pid;
    }

    public Int32 CompareTo(SuperHero right)
    {
        if (this.ID == right.ID)
            return 0;

        return this.ParentID.CompareTo(right.ID);
    }

    public override string ToString()
    {
        return this.Name;
    }
}

and to use it:

static void Main(string[] args)
{
    // create your list
    List<SuperHero> heroes = new List<SuperHero>();

    // populate it
    heroes.Add(new SuperHero(1, "Superman", 0));
    heroes.Add(new SuperHero(2, "Batman", 3));
    heroes.Add(new SuperHero(3, "Watchman", 1));
    heroes.Add(new SuperHero(4, "Wolverine", 2));

    foreach (SuperHero hero in heroes)
        Console.WriteLine(hero.ToString());
    Console.WriteLine();

    // sort it
    heroes.Sort();

    foreach (SuperHero hero in heroes)
        Console.WriteLine(hero.ToString());

    Console.ReadKey();
}

The .NET sort (QuickSort) will use your comparer to sort the list.

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2 Comments

How do I use CompareTo() to sort the list?
Thanks, though its not working 100% as I wanted, I guess you have pointed me in the right direction...thanks again
2

You can create a class for the above data with id, name, parent_id as class members. Override the CompareTo method of IComparable

    public class Person : IComparable<Person>
        {
            public int id, parentID;
            public string name;

            public Person(int id, string name, int parentID)
            {
                this.id = id;
                this.name = name;
                this.parentID = parentID;
            }

            #region IComparable Members

            public int CompareTo(Person obj)
        {
        return this.parentID.CompareTo(obj.parentID);

        //OR 
        //if (this.parentID > obj.parentID)
        //{
        //    return 1;
        //}
        //else if (this.parentID < obj.parentID)
        //{
        //    return -1;
        //}

        //return 0;
        }        
            #endregion
        }

In the main code:

List<Person> peopleArray = new List<Person>();
            peopleArray.Add(new Person(1, "Jerry", 1));
        peopleArray.Add(new Person(2, "George", 4));
        peopleArray.Add(new Person(3, "Elaine", 3));
        peopleArray.Add(new Person(4, "Kramer", 2));    
            peopleArray.Sort();

            foreach (Person p in peopleArray)
                Console.WriteLine(p.parentID);

This will sort the list by parent id O/P of parent ids: 1 2 3 4

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8 Comments

I am not sure how OP represents NULL value for parentId. Maybe you need to consider that.
When comparing on a field that already implements IComparable, its better to return the result of their Compare method than -1, 1, and 0. For example, comparing two integers returns their difference. This can optimize comparisons. like: return this.parentID.CompareTo(((Person)obj).parentID)
Though in the case of Int32, it's probably an assload faster to just return this.parentID - ((Person)obj).parentID.
shahKalpesh -- int is not Nullable in my case, and default value would be 0
@Josh - that and using the Generic IComparable(T) as well to avoid the type conversion.
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1

Easy way:

using System.Linq;
...
var sorted = unsorted.OrderBy(x=>x.parent_id);

More complicated/reusable way:

In .NET you can use existing sorting facilities (like Array.Sort, List.Sort, LINQ's OrderBy, etc) just by replacing one single method. You need to implement a comparer (either by creating an object that implements IComparer, implementing IComparable on the objects being compared, or using a Comparison delegate.)

All the comparer's job is is to determine whether a given object is greater than or less than another object. As long as you do that and follow some basic rules (ie. if X is greater than Y, then Y must be less than X) then the sorting will be quick and painless.

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1

If you implement IComparable in you object, you can use the .Sort method on an Array or ArrayList collection.

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1

I guess you want the NULL parentId to be the first, followed by its child.
If so, you can call OrderBy on the list using linq.

OrderBy(x => Convert.ToInt32(x.ParentId == null ? "0" : x.ParentId));

I am assuming ParentId is a string (it can not be null if it is an int).
And assuming that the list won't have Id = 0.

EDIT: This will print the items in the following order
Superman
Watchman
Wolverine
Batman

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