C++中nth_element函数的用法

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本文详细介绍了C++标准库中的std::nth_element函数，该函数能够将指定位置的元素调整到其在完全排序序列中应有的位置，同时确保该元素之前的所有元素都不大于它，之后的所有元素都不小于它。文章通过示例代码展示了如何使用此函数。

std::nth_element

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

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 resulting nth position is the element that would be in that position in a sorted sequence, with none of the elements preceding it being greater and none of the elements following it smaller than it. Neither the elements preceding it nor the elements following it are guaranteed to be ordered.

The elements are compared using operator< for the first version, and comp for the second.

Parameters

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 third ones.

nth

Random-Access iterator pointing to the location within the range [first,last) that will have the sorted element.

comp

Comparison function object that, taking two values of the same type than those contained in the range, returns true if the first argument goes before the second argument in the specific strict weak ordering it defines, and false otherwise.

// alg_nth_elem.cpp
// compile with: /EHsc
#include <vector>
#include <algorithm>
#include <functional> // For greater<int>( )
#include <iostream>

// Return whether first element is greater than the second
bool UDgreater ( int elem1, int elem2 ) {
return elem1 > elem2;
}

int main() {
using namespace std;
vector <int> v1;
vector <int>::iterator Iter1;

int i;
for ( i = 0 ; i <= 5 ; i++ )
v1.push_back( 3 * i );

int ii;
for ( ii = 0 ; ii <= 5 ; ii++ )
v1.push_back( 3 * ii + 1 );

int iii;
for ( iii = 0 ; iii <= 5 ; iii++ )
v1.push_back( 3 * iii +2 );

cout << "Original vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;

nth_element(v1.begin( ), v1.begin( ) + 3, v1.end( ) );
//
//nth_element(v1.begin( ), v1.begin( ) + 18, v1.end( ) );
//cout << "Position 18 partitioned vector:\n v1 = ( " ;
cout << "Position 3 partitioned vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
  cout << *Iter1 << " ";
cout << ")" << endl;
cout<<v1[3]<<endl;       //输出第3大的元素
// To sort in descending order, specify binary predicate
nth_element( v1.begin( ), v1.begin( ) + 4, v1.end( ),
  greater<int>( ) );  //greater<int>由大到小排序
cout << "Position 4 partitioned (greater) vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
  cout << *Iter1 << " ";
cout << ")" << endl;
cout<<v1[4]<<endl;   //输出第4大的元素（从0开始排的）

random_shuffle( v1.begin( ), v1.end( ) );
cout << "Shuffled vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;

// A user-defined (UD) binary predicate can also be used
nth_element( v1.begin( ), v1.begin( ) + 5, v1.end( ), UDgreater );
cout << "Position 5 partitioned (UDgreater) vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;
cout<<v1[5]<<endl; //输出第5大的元素，第0大，第1大....
}

测试结果：