C++ std::partial_sort_copy用法及代码示例

std::partial_sort用于对整个容器内的范围进行排序。因此，如果我们要保持原始容器不变，而只是将已排序的容器的sub-part复制到另一个容器中，则可以使用std::partial_sort_copy。

就像std::partial_sort一样，partial_sort_copy()可以通过两种方式使用，如下所示：

1. 使用<：比较元素
   

   用法:

   Template 
   RandomAccessIterator partial_sort_copy (InputIterator first, InputIterator last,
                                           RandomAccessIterator result_first,
                                           RandomAccessIterator result_last);
   
   first: Input iterator to the first element in the container.
   last: Input iterator to the last element in the container.
   result_first: Random-Access iterator pointing to the initial
   position in the destination container.
   result_last: Random-Access iterator pointing to the final
   position in the destination container.
   
   返回值：It returns an iterator pointing to the element that 
   follows the last element written in the result sequence.
   

   // C++ program to demonstrate the use of 
   // std::partial_sort_copy 
   #include <iostream> 
   #include <vector> 
   #include <algorithm> 
   using namespace std; 
   int main() 
   { 
       vector<int> v = { 1, 1, 3, 10, 3, 3, 7, 7, 8 }, v1(3); 
     
       vector<int>::iterator ip; 
     
       // Using std::partial_sort_copy 
       std::partial_sort_copy(v.begin(), v.end(), v1.begin(), v1.end()); 
     
       // Displaying the vector after applying 
       // std::partial_sort_copy 
       for (ip = v1.begin(); ip != v1.end(); ++ip) { 
           cout << *ip << " "; 
       } 
     
       return 0; 
   }

   输出：

   1 1 3
   

   在这里，由于我们声明v1的大小为3，因此仅存储了三个元素。

2. 通过使用预定义函数进行比较：

   用法:

   Template
    RandomAccessIterator partial_sort_copy (InputIterator first, InputIterator last,
                                            RandomAccessIterator result_first,
                                            RandomAccessIterator result_last,
                                            Compare comp);
   
   Here, first, last, result_first and result_last are 
   the same as previous case.
   
   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.
   The function shall not modify any of its arguments.
   This can either be a function pointer or a function object.
   
   返回值：It returns an iterator pointing to the element that 
   follows the last element written in the result sequence.
   

   // C++ program to demonstrate the use of 
   // std::partial_sort_copy 
   #include <iostream> 
   #include <algorithm> 
   #include <vector> 
   using namespace std; 
     
   // Defining the BinaryFunction 
   bool comp(int a, int b) 
   { 
       return (a < b); 
   } 
     
   int main() 
   { 
       vector<int> v = { 1, 1, 3, 10, 3, 3, 7, 7, 8 }, v1(7); 
     
       vector<int>::iterator ip; 
     
       // Using std::partial_sort_copy 
       std::partial_sort_copy(v.begin(), v.end(), v1.begin(), 
                              v1.end(), comp); 
     
       // Displaying the vector after applying 
       // std::partial_sort_copy 
       for (ip = v1.begin(); ip != v1.end(); ++ip) { 
           cout << *ip << " "; 
       } 
     
       return 0; 
   }

   输出：

   1 1 3 3 3 7 7
   

Where to use it ?

• 复制排序范围：因此，只要我们希望原始容器在排序后保持不变并将partial_sort的结果存储在另一个容器中，则可以使用它。

  // C++ program to demonstrate the use of 
  // std::partial_sort_copy 
  #include <iostream> 
  #include <vector> 
  #include <algorithm> 
  using namespace std; 
  int main() 
  { 
      vector<int> v = { 100, 45, 78, 23, 220 }, v1(5); 
    
      vector<int>::iterator ip; 
    
      // Using std::partial_sort_copy 
      std::partial_sort_copy(v.begin(), v.end(), v1.begin(), 
                             v1.end()); 
    
      // Displaying the vectors after applying 
      // std::partial_sort_copy 
      cout << "v = "; 
    
      for (ip = v.begin(); ip != v.end(); ++ip) { 
          cout << *ip << " "; 
      } 
    
      cout << "\nv1 = "; 
      for (ip = v1.begin(); ip != v1.end(); ++ip) { 
          cout << *ip << " "; 
      } 
      return 0; 
  }

  输出：

  v = 100 45 78 23 220
  v1 = 23 45 78 100 220
  

  因此，此处v保持不变，并且其排序形式存储在v1中。