std::nth_element

默认 (1)	template <class RandomAccessIterator> void nth_element (RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last);
自定义 (2)	template <class RandomAccessIterator, class Compare> void nth_element (RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);

默认 (1)

template <class RandomAccessIterator>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last);

自定义 (2)

template <class RandomAccessIterator, class Compare>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last, Compare comp);

Sort element in range

Rearranges the elements in the range [first,last), in such a way that the element at the nth position is the element that would be in that position in a sorted sequence.

The other elements are left without any specific order, except that none of the elements preceding nth are greater than it, and none of the elements following it are less.

元素使用第一个版本的 operator< 进行比较，第二个版本使用 comp 进行比较。

参数

first, last: Random-access iterators to the initial and final positions of the sequence to be used. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
Notice that in this function, these are not consecutive parameters, but the first and the third.
nth: Random-access iterator pointing to the location within the range [first,last) that will contain the sorted element.
Notice that the value of the element pointed by nth before the call is not relevant.

Random-access iterator pointing to the location within the range [first,last] that will contain the sorted element.
If this points to last, the function call has no effect.
Notice that the value of the element pointed by nth before the call is not relevant.
comp: Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
该函数不得修改其任何参数。
This can either be a function pointer or a function object.

返回值

无

示例

// nth_element example
#include <iostream>     // std::cout
#include <algorithm>    // std::nth_element, std::random_shuffle
#include <vector>       // std::vector

bool myfunction (int i,int j) { return (i<j); }

int main () {
  std::vector<int> myvector;

  // set some values:
  for (int i=1; i<10; i++) myvector.push_back(i);   // 1 2 3 4 5 6 7 8 9

  std::random_shuffle (myvector.begin(), myvector.end());

  // using default comparison (operator <):
  std::nth_element (myvector.begin(), myvector.begin()+5, myvector.end());

  // using function as comp
  std::nth_element (myvector.begin(), myvector.begin()+5, myvector.end(),myfunction);

  // print out content:
  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

可能的输出

myvector contains: 3 1 4 2 5 6 9 7 8

复杂度

On average, linear in the distance between first and last: Compares elements, and possibly swaps (or moves) them, until the elements are properly rearranged.

数据竞争

范围[first,last)内的对象将被修改。

异常

如果任何元素比较、元素交换（或移动）或迭代器操作抛出异常，则会抛出异常。
请注意，无效参数会导致未定义行为。

另见

sort: 对范围中的元素进行排序（函数模板）

partial_sort: 对范围中的元素进行部分排序（函数模板）

partition: 将范围划分为两部分（函数模板）

find_if: 在范围中查找元素（函数模板）