multimap相对于矢量图的优势是什么？ [英] What's the advantage of multimap over map of vectors?

问题描述

我不明白为什么multimap存在，如果我们可以创建矢量或集合地图。
对我来说，区别只是：

I don't understand why multimap exists if we can create map of vectors or map of sets. For me only differences are:

• 在multimap中使用 equal_range 获取键的元素和向量的映射，我们只需使用 [] 运算符并具有元素的向量。


• 使用 multimap.insert（make_pair（key，value））在多图中添加元素和 map_of_vectors [key] .push_back（value）

• using equal_range in multimap for getting elements of a key and in map of vectors we simply use [] operator and have vector of elements.
• using multimap.insert(make_pair(key,value)) in multimap for adding elements and map_of_vectors[key].push_back(value) in map of vectors.

那么为什么要使用multimap？对我来说，最好有一个向量而不是两个迭代器来获取所有的键值。

So why use multimap? For me it's better to have a vector than two iterators to get all values of a key.

这个问题也适用于unordered_map的向量和unordered_multimap。 b $ b

This question applies also to unordered_map of vectors and unordered_multimap.

推荐答案

我会说这取决于具有相同键值的所有值是否具有您要解决的关系。

I would say it depends on whether all the values with same key have a relationship that you want to address.

例如，你经常使用键X来遍历所有元素，或者将它们传递给一个函数，等等？

So for example, do you often go through all elements with key X, or pass them to a function, and so on? Then it is more convenient to already have them in their separate container, that you can address directly.

然而，如果你只有一个项目的集合，可以共享同一个键值，或为什么在两者之间使用向量？使用迭代器运行multimap比为地图，向量case使用嵌套的for循环更方便。

However, if you just have a collection of items, that may share same key value, or not, why use vectors in between? It is more convenient to run through the multimap with iterators than have a nested for loop for the map, vector case.

另一种看法是：如果多个条目键是很常见的，你的结构在地图中更有效率，向量case。如果他们很少发生，则是相反的。

Another way of looking at this: If multiple entries per key is very common, your structure is more efficient in the map, vector case. If they seldomly happen, it is the opposite.

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