Lookup() 和 Dictionary(Of list()) 的区别 [英] Difference between Lookup() and Dictionary(Of list())

问题描述

我正在努力思考哪些数据结构最有效，以及何时/何地使用哪些数据结构.

I'm trying to wrap my head around which data structures are the most efficient and when / where to use which ones.

现在，可能是我只是不太了解这些结构，但是 ILookup(of key, ...) 与 Dictionary(of键，列表(...))?

Now, it could be that I simply just don't understand the structures well enough, but how is an ILookup(of key, ...) different from a Dictionary(of key, list(of ...))?

还有我想在哪里使用 ILookup 以及在程序速度/内存/数据访问等方面更有效的地方?

Also where would I want to use an ILookup and where would it be more efficient in terms of program speed / memory / data accessing, etc?

推荐答案

两个显着差异:

• Lookup 是不可变的.是的 :) (至少，我相信具体的 Lookup 类是不可变的，并且 ILookup 接口不提供任何可变成员.可以> 当然是其他可变实现.)
• 当您查找在查找中不存在的键时，您会得到一个空序列而不是 KeyNotFoundException.(因此没有 TryGetValue，AFAICR.)

• Lookup is immutable. Yay :) (At least, I believe the concrete Lookup class is immutable, and the ILookup interface doesn't provide any mutating members. There could be other mutable implementations, of course.)
• When you lookup a key which isn't present in a lookup, you get an empty sequence back instead of a KeyNotFoundException. (Hence there's no TryGetValue, AFAICR.)

它们可能在效率上是相同的——例如，查找很可能在幕后使用 Dictionary>.根据您的要求在它们之间进行选择.就个人而言，我发现查找通常比 Dictionary> 更适合，主要是由于上面的前两点.

They're likely to be equivalent in efficiency - the lookup may well use a Dictionary<TKey, GroupingImplementation<TValue>> behind the scenes, for example. Choose between them based on your requirements. Personally I find that the lookup is usually a better fit than a Dictionary<TKey, List<TValue>>, mostly due to the first two points above.

注意，作为一个实现细节，用于值的IGrouping<,>的具体实现实现了IList，这意味着它是高效的与 Count()、ElementAt() 等一起使用

Note that as an implementation detail, the concrete implementation of IGrouping<,> which is used for the values implements IList<TValue>, which means that it's efficient to use with Count(), ElementAt() etc.

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