迭代在`std :: multiset`的唯一元素 [英] Iterating over unique elements of `std::multiset`

问题描述

我需要的只是知道是否有东西存在以及存在的次数。

All I need is to know if something exists and how many times it exist. I will iterate over the existent things and query how much of that exists.

我的实现目前使用 multiset 我做如下：

My implementation so far uses multiset, I do as follow:

std::multiset<thing> a;
auto previous = a.end();
for( auto each = a.begin(); each != a.end(); ++each ) {
    if( previous == a.end() || *previous != *each ) {
        a.count(*each);
    }
    previous = each;
}

澄清

我有一个向量 thing s。但是他们有时重复的价值，我想迭代独特的东 s和每个独特的做某事。这个东西需要知道这个东西出现在向量上的时间。

Clarifications

I have a vector of things. But they repeat the value sometimes, I want to iterate over unique things and for each unique do something. This "something" needs to know the amount of time this thing appears on the vector.

上面是我现在解决我的问题，它似乎并不是最优雅的方式来做我想要的。

The code I posted above is how I am resolving my problem right now, it does not seems to be the most elegant way to do what I want.

我只是遵循Stackoverflow准则：我告诉我的问题是什么，我告诉我的（尝试的）解决方案。

I am just following the Stackoverflow guidelines: I tell what is my problem, and I tell my (tried) solution.

如果一个带有问号的句子真的需要，你去：通过 multiset ？

If a sentence with a question mark is really needed, there you go: Is there a way to iterate over unique elements over a multiset?

推荐答案

迭代独特元素三种可能的方法：

Three possible approaches:

• 使用 std :: unique 创建唯一值的临时集合。


• 使用 std :: multiset :: upper_bound 提高迭代器的效率。而不是增量： for（auto each = a.begin（）; each！= a.end（）; each = a.upper_bound（* each））您不需要 if 检查您的循环内容，并且保证大小对数。很酷（不知道，在我看起来之前）。对于以下建议，所有的信用到 @MarkRansom ：使用 std :: upper_bound 从< algorithm> code>，可以指定一个范围来查找上限。在你的情况下，你已经有一个很好的候选人的范围的开始，所以这种方法可能会更有效率，这取决于在标准库中的实现。


• 如果这是一个真正的性能问题为你和以前的解决方案仍然不够好，考虑切换到 map< thing，unsingned> 或甚至 unordered_map< thing ，unsigned> 其中 unsigned 只是跟踪等效有。这意味着重写您的插入/删除代码虽然。



• Use std::unique to create a temporary collection of unique values. This might make the code a little more readable, but less efficient.
• Advance your iterator by using std::multiset::upper_bound rather than increments: for( auto each = a.begin(); each != a.end(); each=a.upper_bound(*each)) - that way you don't need the if check insider your loop, plus it is guaranteed to be logarithmic in size. Pretty cool (didn't know that before I looked it up). For the following suggestion, all credit goes to @MarkRansom: Using std::upper_bound from <algorithm>, you can specify a range in which to look for the upper bound. In your case, you already have a good candidate for the start of that range, so this method is likely to be more efficient, depending on the implementation in your standard library.
• If this is a real performance problem for you and the previous solution still isn't good enough, consider switching to map<thing, unsingned> or even unordered_map<thing,unsigned> where the unsigned just keeps track of the number of equivalent things you have. That implies rewriting your insertion/deletion code though.

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