IComparer (or the IComparable methods it relies upon) did not retu

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I'm trying to figure out why I'm getting the following error message when
trying to radomly sort a list of custom objects. In particular I've noticed
the following strange behaviour I'm hoping someone could shed some light
on....

1. When the list contains only 3 objects the sort works fine, unless I
remove the 'if x.equals(y)' check in the compare function.

2. When there are more than 3 objects I get the error even when this check
is in place (albeit it usually takes a bit longer).



------------------------ERROR MESSAGE----------------------------
IComparer (or the IComparable methods it relies upon) did not return zero
when Array.Sort called x. CompareTo(x)

-------------------------CODE LISTING--------------------------------

Module Module1

Sub Main()
Dim People As New System.Collections.Generic.List(Of Person)
Dim Counter As Integer = 0
People.Add(New Person(1, "Brian"))
People.Add(New Person(2, "Darragh"))
People.Add(New Person(3, "Aoife"))
People.Add(New Person(4, "Aidan")) ' when only 3 people I don't seem
to have a problem unless I remove the if x.Equals(y) check below!
While True
Counter += 1
Console.WriteLine("Comparison {0}", Counter)
People.Sort(New Person.Random())
End While
End Sub
End Module

Public Class Person
Public ID As Integer
Public Name As String

Public Sub New(ByVal ID As Integer, ByVal Name As String)
Me.ID = ID
Me.Name = Name
End Sub

Public Overrides Function ToString() As String
Return String.Format("ID={0}, Name={1}", Me.ID, Me.Name)
End Function

Class Random
Implements System.Collections.Generic.IComparer(Of Person)

Private Shared r As New System.Random

Public Function Compare(ByVal x As Person, ByVal y As Person) As
Integer Implements System.Collections.Generic.IComparer(Of Person).Compare
Console.Write("Comparing: x=[{0}], y=[{1}]", x, y)
If x.Equals(y) Then Compare = 0 Else Compare = r.Next(-1, 2)
'removing this gives error when only 3 items
'Compare = r.Next(-1, 2)
Console.WriteLine("{0}Returning: {1}", vbTab, Compare)
End Function

End Class

End Class
 
I tried your code... in C#. (I'm working with IComparers this week, so I
thought Id give your randomizer a run thru)


Here is mine:

int compareValue = 0;
if (x.Equals(y))

{

compareValue = 0;

}

else

{

compareValue = m_random.Next(-1, 2);

}

//'removing this gives error when only 3 items

compareValue = m_random.Next(-1, 2);

return compareValue;



and I have a member variable:
private System.Random m_random = new System.Random();



First, you may want to put

Option Strict On
Option Explicit On

at the top of your vb code.

Private Shared r As New System.Random

needs to be rewritten as:

Private Shared r System.Random = New System.Random()



So I think its either that. or the static/shared thing you are doing.





I did not experience the issue with 2 or 3 items in my collection.




Darragh Jones said:
I'm trying to figure out why I'm getting the following error message when
trying to radomly sort a list of custom objects. In particular I've noticed
the following strange behaviour I'm hoping someone could shed some light
on....

1. When the list contains only 3 objects the sort works fine, unless I
remove the 'if x.equals(y)' check in the compare function.

2. When there are more than 3 objects I get the error even when this check
is in place (albeit it usually takes a bit longer).



------------------------ERROR MESSAGE----------------------------
IComparer (or the IComparable methods it relies upon) did not return zero
when Array.Sort called x. CompareTo(x)

-------------------------CODE LISTING--------------------------------

Module Module1

Sub Main()
Dim People As New System.Collections.Generic.List(Of Person)
Dim Counter As Integer = 0
People.Add(New Person(1, "Brian"))
People.Add(New Person(2, "Darragh"))
People.Add(New Person(3, "Aoife"))
People.Add(New Person(4, "Aidan")) ' when only 3 people I don't seem
to have a problem unless I remove the if x.Equals(y) check below!
While True
Counter += 1
Console.WriteLine("Comparison {0}", Counter)
People.Sort(New Person.Random())
End While
End Sub
End Module

Public Class Person
Public ID As Integer
Public Name As String

Public Sub New(ByVal ID As Integer, ByVal Name As String)
Me.ID = ID
Me.Name = Name
End Sub

Public Overrides Function ToString() As String
Return String.Format("ID={0}, Name={1}", Me.ID, Me.Name)
End Function

Class Random
Implements System.Collections.Generic.IComparer(Of Person)

Private Shared r As New System.Random

Public Function Compare(ByVal x As Person, ByVal y As Person) As
Integer Implements System.Collections.Generic.IComparer(Of Person).Compare
Console.Write("Comparing: x=[{0}], y=[{1}]", x, y)
If x.Equals(y) Then Compare = 0 Else Compare = r.Next(-1, 2)
'removing this gives error when only 3 items
'Compare = r.Next(-1, 2)
Console.WriteLine("{0}Returning: {1}", vbTab, Compare)
End Function

End Class

End Class
Click to expand...
 
Actually, now I see what you're talking about.

I had to run it 3 times before I got the error. But I did get it.

One thing is that one of my objects (x or y) is null/Nothing for some
unknown reason.

Hmmm.. I'll still looking at this sucker.
 
http://msmvps.com/blogs/jon.skeet/archive/2005/12/02/77520.aspx


I think it has something to do with thread safety.


I've even tried a more anal RandomNumber getter:

I don't have an answer. As soon as you take out the Random.Next... it
doesn't blow up.

Please post a solution here if you figure it out.



class RandomNumberGetter

{

private System.Random m_random = null;//new System.Random();

//private static readonly object padlock = new object();

//private readonly object padlock = new object();

private object padlock = new object();

public RandomNumberGetter()

{

lock (padlock)

{

this.m_random = new Random();

}

}



public int GetRandomNumber(int x, int y)

{

lock (padlock)

{

return this.m_random.Next(x, y);

}

}



}



Darragh Jones said:
I'm trying to figure out why I'm getting the following error message when
trying to radomly sort a list of custom objects. In particular I've noticed
the following strange behaviour I'm hoping someone could shed some light
on....

1. When the list contains only 3 objects the sort works fine, unless I
remove the 'if x.equals(y)' check in the compare function.

2. When there are more than 3 objects I get the error even when this check
is in place (albeit it usually takes a bit longer).



------------------------ERROR MESSAGE----------------------------
IComparer (or the IComparable methods it relies upon) did not return zero
when Array.Sort called x. CompareTo(x)

-------------------------CODE LISTING--------------------------------

Module Module1

Sub Main()
Dim People As New System.Collections.Generic.List(Of Person)
Dim Counter As Integer = 0
People.Add(New Person(1, "Brian"))
People.Add(New Person(2, "Darragh"))
People.Add(New Person(3, "Aoife"))
People.Add(New Person(4, "Aidan")) ' when only 3 people I don't seem
to have a problem unless I remove the if x.Equals(y) check below!
While True
Counter += 1
Console.WriteLine("Comparison {0}", Counter)
People.Sort(New Person.Random())
End While
End Sub
End Module

Public Class Person
Public ID As Integer
Public Name As String

Public Sub New(ByVal ID As Integer, ByVal Name As String)
Me.ID = ID
Me.Name = Name
End Sub

Public Overrides Function ToString() As String
Return String.Format("ID={0}, Name={1}", Me.ID, Me.Name)
End Function

Class Random
Implements System.Collections.Generic.IComparer(Of Person)

Private Shared r As New System.Random

Public Function Compare(ByVal x As Person, ByVal y As Person) As
Integer Implements System.Collections.Generic.IComparer(Of Person).Compare
Console.Write("Comparing: x=[{0}], y=[{1}]", x, y)
If x.Equals(y) Then Compare = 0 Else Compare = r.Next(-1, 2)
'removing this gives error when only 3 items
'Compare = r.Next(-1, 2)
Console.WriteLine("{0}Returning: {1}", vbTab, Compare)
End Function

End Class

End Class
Click to expand...
 
<=?Utf-8?B?RGFycmFnaCBKb25lcw==?= <Darragh
I'm trying to figure out why I'm getting the following error message when
trying to radomly sort a list of custom objects.
Click to expand...

I suspect the problem is in the exception message. Basically, by
implementing IComparer randomly, you're breaking the contract by not
making it reflexive. You're saying that at the samt time, x > y and
y > x can both be true. That makes it impossible to sort.

What's your actual aim here? If you want to shuffle the list, there are
much better ways of doing it.
 
<=?Utf-8?B?RGFycmFnaCBKb25lcw==?= <Darragh
I'm trying to figure out why I'm getting the following error message when
trying to radomly sort a list of custom objects. In particular I've noticed
the following strange behaviour I'm hoping someone could shed some light
on....
Click to expand...

<snip>

Here's a short but complete program which demonstrates the problem with
no randomness:

using System;
using System.Collections.Generic;

class Test
{
static void Main()
{
List<string> list = new List<string>();
list.Add("a");
list.Add("b");
list.Add("c");
list.Add("d");

list.Sort(new DodgyComparer<string>());
}
}

class DodgyComparer<T> : IComparer<T>
{
int[] results = {-1, 0, 1, -1, 0, -1, -1, -1, 100};
int index=0;

public int Compare(T x, T y)
{
Console.WriteLine (x+" "+y+" "+results[index]);
return results[index++];
}
}

(The 100 is there to show that it's not going off the end of the array.
As a side issue, the same exception is thrown if the comparer throws
IndexOutOfRangeException, which made this awkward to diagnose.)

I've reported it to MS:
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?
FeedbackID=145253
 
Thanks Jon for your help. I see now what I was doing wrong. Using Sort to
randomize the list was probably not the cleverest thing I've ever done.

I've decided to take a more straight forward approach...

Private Function Randomize(ByVal list As Collections.Generic.List(Of
Person)) As Collections.Generic.List(Of Person)
Randomize = New Collections.Generic.List(Of Person)
Dim index As Integer
Dim r As New Random()
While list.Count > 0
index = r.Next(list.Count)
Randomize.Add(list.Item(index))
list.RemoveAt(index)
End While
End Function
 
Darragh Jones said:
Thanks Jon for your help. I see now what I was doing wrong. Using Sort to
randomize the list was probably not the cleverest thing I've ever done.

I've decided to take a more straight forward approach...

Private Function Randomize(ByVal list As Collections.Generic.List(Of
Person)) As Collections.Generic.List(Of Person)
Randomize = New Collections.Generic.List(Of Person)
Dim index As Integer
Dim r As New Random()
While list.Count > 0
index = r.Next(list.Count)
Randomize.Add(list.Item(index))
list.RemoveAt(index)
End While
End Function
Click to expand...

While that will do it, it's not a terribly nice way of doing it, as it
involves creating a new list unnecessarily. There's a really neat
technique which shuffles a list. Imagine that the list has a dividing
line between unshuffled values and shuffled values. Everything starts
unshuffled. You randomly pick one of the unshuffled values and swap it
with the first unshuffled value, then move the imaginary line to past
that element. Keep going until everything is shuffled.

It's O(n) and very simple to implement. Unfortunately I don't have time
to do it for you in VB.NET right now, but here's an implementation in
C#:

static readonly Random rng = new Random();
public static void Shuffle (IList list)
{
// Note: last element doesn't need shuffling! (Couldn't
// pick a different one to itself anyway.)
for (int i=0; i < list.Count-1; i++)
{
object x = list;

int index = rng.Next(list.Count-i)+i;
list=list[index];
list[index]=x;
}
}
 
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