对于每个词典循环按索引顺序 [英] For Each Dictionary loop in Index order

问题描述

我已尝试使用为循环与字典但不能真正实现我想要的手。

I have tried my hand using for loop with Dictionary but couldn't really achieve what I want to.

我有一个变量 SomeVariable ，而对于这个变量的值我想要我的 foreach 工作。 SomeVariable 可以是 1,2,3或4

I have a certain variable SomeVariable and on the value of this variable I want my foreach to work. SomeVariable can be 1,2,3 or 4

所以让我们说 SomeVariable 是 1 我想检索最后一个 item.value 从 SomeCollection 中的前3个索引（0,1,2）。

So lets say SomeVariable is 1 I want to retrieve the last item.value from among the first 3 indexes(0,1,2) inside the SomeCollection.

如果 SomeVariable 是 2 我想检索最后一个 item.value 在 SomeCollection 之间的接下来的3个索引（3,4,5）中。

And if SomeVariable is 2 I want to retrieve the last item.value from among the next 3 indexes(3,4,5) inside the SomeCollection.

等等on ...

For Each item As KeyValuePair(Of String, Integer) In SomeCollection
    If SomeVariable = 1 Then
            //....
    ElseIf SwitchCount = 2 Then
           //....
    End If
Next

推荐答案

字典没有定义的顺序，所以您感觉到的任何顺序都是短暂的。从 MSDN ：

A dictionary has no defined order, so any order you perceive is transient. From MSDN:

.KeyCollection中的键的顺序是未指定的，但它与Values属性返回的.ValueCollection中的关联值的顺序相同。

The order of the keys in the .KeyCollection is unspecified, but it is the same order as the associated values in the .ValueCollection returned by the Values property.

尝试使用Keys集合来确定顺序显示它是如何暂态的：

Trying to use the Keys collection to determine the order shows how it is transient:

Dim myDict As New Dictionary(Of Integer, String)

For n As Int32 = 0 To 8
    myDict.Add(n, "foo")
Next

For n As Int32 = 0 To myDict.Keys.Count - 1
    Console.WriteLine(myDict.Keys(n).ToString)
Next

输出打印0 - 8，按照您的预期。然后：

the output prints 0 - 8, in order, as you might expect. then:

myDict.Remove(5)
myDict.Add(9, "bar")

For n As Int32 = 0 To myDict.Keys.Count - 1
    Console.WriteLine(myDict.Keys(n).ToString)
Next

输出为：0，1，2，3，4，9（！），6，7，8

The output is: 0, 1, 2, 3, 4, 9 (!), 6, 7, 8

如您所见，它重用旧插槽。任何代码取决于某个位置的东西最终会破裂。您添加/删除的越多，它获得的无序越多。如果您需要订购词典，请使用 SortedDictionary 。

As you can see, it reuses old slots. Any code depending on things to be in a certain location will eventually break. The more you add/remove, the more unordered it gets. If you need an order to the Dictionary use SortedDictionary instead.

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