获取C ++ multiset中的前N个元素 [英] Get first N elements in a C++ multiset

问题描述

如何获取多重集结构中的前N个元素，而不必始终获取第一个（.begin（））元素，然后将其擦除？

How can I get the first N elements from a multiset structure, without constantly getting the first (.begin()) element and then erasing it?

推荐答案

我只想总结前N个元素，而不影响multiset。

I just want to sum the first N elements without affecting the multiset.

#include <numeric>
#include <iterator>

// ...

int sum = std::accumulate(my_set.begin(), std::next(my_set.begin(), N));

std :: next +11库添加。这里是一个旧的编译器的解决方案：

std::next is a C++11 library addition. Here is a solution for older compilers:

std::multiset<int>::iterator it = my_set.begin();
std::advance(it, N);
int sum = std::accumulate(my_set.begin(), it);

两个解决方案在多重集上迭代两次。如果你想防止这种情况，使用手动循环：

Both solutions iterate over the multiset twice. If you want to prevent that, use a manual loop:

int sum = 0;
std::multiset<int>::iterator it = my_set.begin();
for (int i = 0; i < N; ++i)
{
    sum += *it++;
}

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