对对象不为空的列表进行排序

发布于 2024-10-19 20:20:06 字数 218 浏览 1 评论 0原文

我正在尝试对类列表进行排序，并且我需要具有子类的类，该子类在列表中不是第一个。我以为以下内容会起作用，但事实并非如此。

ListOfClasses.Sort(Function(x, y) If(x.SubClass IsNot Nothing, 1, 0))

我知道这充其量只是一个黑客（并不是说它有效），但我认为它会将类向上移动，使子类不等于任何东西？

原文

I am trying to sort a list of classes, and I need the classes with a subclass that isnot nothing first in the list. I thought the following would work but it doesnt.

ListOfClasses.Sort(Function(x, y) If(x.SubClass IsNot Nothing, 1, 0))

I know this is a hack at best (not that it works) but I thought it would move the classes up the order where the subclass was not equal to nothing?

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许一世地老天荒 2024-10-26 20:20:06

如果 x 小于、等于或大于 y，您的比较器函数需要返回 -1、0 或 1。在您的情况下（因为您希望后面有 Nothing 值）：

Dim xNull = x Is Nothing
Dim yNull = y Is Nothing

If xNull = yNull Then Return 0 ' either both are Nothing, or neither is.
If xNull Then Return 1
Return -1

但请注意，在这里使用 Sort 效率很低。您需要的操作称为 partition 并以 O(n) 运行。

Your comparer function needs to return -1, 0 or 1 if x is less than, equal to or greater than y. In your case (since you want to have Nothing values at the back):

Dim xNull = x Is Nothing
Dim yNull = y Is Nothing

If xNull = yNull Then Return 0 ' either both are Nothing, or neither is.
If xNull Then Return 1
Return -1

But be advised that using Sort here is unnecessarily inefficient. The operation you need is called partition and runs in O(n).

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谁把谁当真 2024-10-26 20:20:06

这里的问题是您的比较器委托没有遵循此处列出的正确比较规则。特别是，它需要确保如果您说“x 大于 y”，那么您也说“y 小于 x”。在这里，您只说过“x 大于 y”，但实际上从未说过相反的话。

这是一个比较器函数，它将正确对这些元素进行排序。

Function Compare(ByVal x as TheType, ByVal y as TheType) As Integer
  If x.SubClass Is Nothing AndAlso y.SubClass Is Nothing Then
    Return 0
  Else If x.SubClass IsNot Nothing AndAlso y.SubClass IsNot Nothing Then
    Return 0
  Else If x.SubClass IsNot Nothing Tehn
    Return -1
  Else
    Return 1
  End If 
End Function

这也可以表示为语句 lambda，但由于它们仅在 Visual Studio 2010 中受支持，所以我选择编写完整的函数。

The problem here is that your comparer delegate doesn't follow the proper comparison rules as listed here. In particular it needs to ensure that if you say 'x is greater than y' that you also say 'y is less than x'. Here you're only ever saying 'x is greater than y' but you never actually say the opposite.

Here is a comparer function that will properly sort these elements

Function Compare(ByVal x as TheType, ByVal y as TheType) As Integer
  If x.SubClass Is Nothing AndAlso y.SubClass Is Nothing Then
    Return 0
  Else If x.SubClass IsNot Nothing AndAlso y.SubClass IsNot Nothing Then
    Return 0
  Else If x.SubClass IsNot Nothing Tehn
    Return -1
  Else
    Return 1
  End If 
End Function

This could also be expressed as a statement lambda but as they're only supported in Visual Studio 2010 I chose to write a full function.

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七婞 2024-10-26 20:20:06

如果您想要的只是最后的所有“Nothing”值（并且在这种情况下性能不是一个大问题），您可以在通用列表上使用默认的 .Sort() 。这将为您提供前面的“Nothing”值。然后你在列表上调用.Reverse。

Dim ListOfClasses As New List(Of Object)
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 1")
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 2")
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 3")

ListOfClasses.Sort()
ListOfClasses.Reverse()

If all you want is all of the 'Nothing' values at the end (and performance isn't a huge concern in this situation) you can use the default .Sort() on a generic list. This will give you the 'Nothing' values at the front. Then you call .Reverse on the list.

Dim ListOfClasses As New List(Of Object)
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 1")
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 2")
ListOfClasses.Add(Nothing)
ListOfClasses.Add("Something 3")

ListOfClasses.Sort()
ListOfClasses.Reverse()

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