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C++ 算法中的 nth_element函数

C++算法 nth_element()函数用于按升序对第一个元素与第n个元素之间的元素进行排序，而对第n个元素与最后一个元素之间的元素不进行排序。但是，第n个元素和最后一个元素之间的元素都不比第一个元素和第n个元素之间的元素小。

nth_element - 语法

default (1)      template <class RandomAccessIterator>
                         void nth_element (RandomAccessIterator first, RandomAccessIterator nth,
                            RandomAccessIterator last);

custom (2)       template <class RandomAccessIterator, class Compare>
                         void nth_element (RandomAccessIterator first, RandomAccessIterator nth,
                           RandomAccessIterator last, Compare comp);

nth_element - 参数

first:一个随机访问迭代器，指向要使用的参数内的第一个元素。

last:一个随机访问迭代器，指向要使用的参数中的最后一个最后一个元素。

comp :用户定义的二进制谓词函数，该函数接受两个参数，如果两个参数顺序正确，则返回true，否则返回false。

nth :一个随机访问迭代器，该迭代器将访问包含已排序元素的参数[first，last)中的位置。

nth_element - 返回值

没有

nth_element - 例子1

让我们看一个简单的示例来演示nth_element()的用法:

在线运行复制代码

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

void print(vector<int> ar)
{
  for(auto x : ar) cout << x << " "; 
  cout << endl;
}
int main()
{	
  vector<int> ar = {1, 3, 6, 1, 2, 4, 7, 0};
  cout<<"Before: ";
 //will print 1 3 6 1 2 4 7 0
  print(ar); 

 //mid = 5th element (ar.begin() + 4)
  auto mid = ar.begin() + distance(ar.begin(), ar.end())/2;

 //lets nth_element ar to mid
  nth_element(ar.begin(), mid, ar.end());
  cout<<"\nAfter: ";
 //will print 2 0 1 1 3 4 7 6
 //mid points to element 3
  print(ar);

  return 0;
}

输出:

Before: 1 3 6 1 2 4 7 0 

After: 2 0 1 1 3 4 7 6

nth_element - 例子2

让我们看另一个简单的例子:

在线运行复制代码

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>

using namespace std;
 
int main()
{
    vector<int> v{5, 6, 4, 3, 2, 6, 7, 9, 3};
    cout<<"Elements are: ";
    for (vector<int>::iterator it=v.begin(); it!=v.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';
 
    nth_element(v.begin(), v.begin() + v.size()/2, v.end());
    cout << "The median is " << v[v.size()/2] << '\n';
 
    nth_element(v.begin(), v.begin()+1, v.end(), greater<int>());
    cout << "The second largest element is " << v[1] << '\n';
    
    return 0;
}

输出:

Elements are:  5 6 4 3 2 6 7 9 3
The median is 5
The second largest element is 7

nth_element - 例子3

让我们看另一个简单的例子:

复制代码

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

using namespace std;

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

int main () {
  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

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

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

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

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

  return 0;
}

输出:

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

nth_element - 例子4

让我们看另一个简单的例子:

在线运行复制代码

#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( ) );  
   cout << "Position 3 partitioned vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
  //To sort in descending order, specify binary predicate  
   nth_element( v1.begin( ), v1.begin( ) + 4, v1.end( ),  
          greater<int>( ) );  
   cout << "Position 4 partitioned (greater) vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   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;  
   
   return 0;
}

输出:

Original vector:
 v1 = ( 0 3 6 9 12 15 1 4 7 10 13 16 2 5 8 11 14 17 )
Position 3 partitioned vector:
 v1 = ( 1 0 2 3 8 5 9 4 7 6 10 16 13 15 12 11 14 17 )
Position 4 partitioned (greater) vector:
 v1 = ( 16 17 14 15 13 12 11 9 7 8 10 6 1 4 5 3 2 0 )
Shuffled vector:
 v1 = ( 13 10 6 3 5 2 0 17 11 8 15 9 7 14 16 1 12 4 )
Position 5 partitioned (UDgreater) vector:
 v1 = ( 14 17 15 16 13 12 10 11 9 8 0 2 7 5 3 1 6 4 )

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