std::partial_sort_copy in C++

std::partial_sort is used for sorting the range within the entire container. So, if we want to keep the original container intact and just copy the sorted sub-part of the container into another one, then for that purpose, we can use std::partial_sort_copy.

Just like std::partial_sort, partial_sort_copy() can be used in two ways as shown below:

1. Comparing elements using <:

   Syntax:

   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.
   
   Return Value: 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;
   }

   Output:

   1 1 3
   

   Here, since, we declared v1 to be of size 3, so only three elements were stored in it.

2. By comparing using a pre-defined function:

   Syntax:

   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.
   
   Return Value: 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;
   }

   Output:

   1 1 3 3 3 7 7
   

Where to use it ?

• For copying the sorted range: So, whenever we want the original container to remain unchanged after sorting and the resultant of partial_sort to be stored inside another container, then we can use this.

  // 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;
  }

  Output:

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

  So, here v remained the same, and its sorted form is stored into v1.