IComparer problem

[livez]

Hello! I'm trying to have a custom-sorter class to sort my objects. I want my objects to always be in a certain order(not alphabetically). In my Compare(object a, object b) method I check which objects they are and return 1 or -1 depending on which one I want to come first. How exactly does it work, do I return -1 if I want object a before object b in my collection? It doesn´t seem consistent to me, some values get sorted correctly and some don´t.

[CPallini]

You probably should post your code and your test data. :)

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[DaveyM69]

-1 if a less than b, 0 if a equal b, 1 if a greater than b.

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

[J4amieC]

I wrote you a little sameple which might help. I have 3 types of pet:

public class Dog { }
public class Cat { }
public class Rabbit { }

I dont want to order these alphabetically, I want to order them in order of my preference of them as pets. I like cats more than dogs, and dogs more than rabbits. I could write a comparer to always order a list of pets in my prefered order:

public class MyPetPreferenceComparer : IComparer
{
#region IComparer Members

public int Compare(object x, object y)
{
    // if they are same type return 0
    if (x.GetType() == y.GetType())
        return 0;

    // I like cats best so if lhs is a cat it comes before whatever is rhs
    if (x is Cat)
    {
        return -1;
    }
                
    if(x is Dog)
    {
        // lhs is a dog, if rhs is a rabbit dog comes before
        if (y is Rabbit)
            return -1;
        // otherwise dog comes after
        return 1;
    }
    // I like rabbits least so they always come after
    if (x is Rabbit)
        return 1;

    return 0;
}

#endregion

}

Now the test code:

ArrayList list = new ArrayList();
list.Add(new Rabbit());
list.Add(new Cat());
list.Add(new Dog());
list.Add(new Cat());
list.Add(new Rabbit());
list.Add(new Dog());

list.Sort(new MyPetPreferenceComparer());
foreach (object obj in list)
{
Console.WriteLine(obj.GetType().Name);
}

Outputs:

Cat
Cat
Dog
Dog
Rabbit
Rabbit

One thing Id add - nowdays we tend to prefer the generic versions of these sorts of interface so try to use IComparer<T> in System.Collection.Generics

[livez]

thanks alot!

[Luc Pattyn]

Hi, an int comparer could just return the difference, as in: public static int Compare(int a, int b) {return a-b;} BTW: you MUST return zero when both objects are identical. :)

Luc Pattyn [Forum Guidelines] [My Articles]

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modified on Sunday, June 12, 2011 9:02 AM