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C++ algorithm partial_sort_copy() function

C++ algorithm partial_sort_copy() function with c++ tutorial for beginners and professionals with examples on adjacent_find(),any_of(), copy(), copy_if(), count(), count_if(), equal(), find(), find_end(), find_first_of(), find_if(), find_if_not(), for_each() etc.

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C++ Algorithm partial_sort_copy()

C++ Algorithm partial_sort_copy() function is similar to partial_sort() function which is used to rearrange the elements in the range[first, last), in such a way that the elements between the first and middle will be sorted and the elements between middle and last will be in an unspecified order. But partial_sort_copy() function puts the result in a new range[result_first, result_last).

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

Syntax

default (1)      template <class InputIterator, class RandomAccessIterator>
                         RandomAccessIterator
                           partial_sort_copy (InputIterator first,InputIterator last,
                            RandomAccessIterator result_first,
                             RandomAccessIterator result_last);

custom (2)      template <class InputIterator, class RandomAccessIterator, class Compare>
  		RandomAccessIterator
   		 partial_sort_copy (InputIterator first,InputIterator last,
                            RandomAccessIterator result_first,
                              RandomAccessIterator result_last, Compare comp);

Parameter

first: An input iterator pointing to the first element in the source range to be partially sorted.

last: A random access iterator pointing to the past last element in the source range to be partially sorted.

result_first: A random access iterator pointing to the first element in the sorted destination range.

result_last: A random access iterator pointing to the past last element in the sorted destination range.

comp: A user-defined binary predicate function that accepts two arguments and returns true if the two arguments are in order and false otherwise. It follows the strict weak ordering to order the elements.

Return value

It returns an iterator addressing to the element that follows the last element written in the result sequence.

Complexity

Average Complexity is less than linearithmic in the distance between first and last. Performs up to N*log (min (N, M)) element comparisons where N = last - first and M = middle - first.

Data races

The objects in the range [first, last) are altered.

Exceptions

This function throws an exception if any of element comparisons, the element swaps (or moves) or an operation on iterator throws an exception.

Please note that invalid parameters cause an undefined behavior.

Example 1

Let's see the simple example to demonstrate the use of partial_sort_copy():

#include <iostream>     // std::cout
#include <algorithm>    // std::partial_sort_copy
#include <vector>       // std::vector

using namespace std;

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

int main () {
  int myints[] = {9,8,7,6,5,4,3,2,1};
  vector<int> myvector (5);

  // using default comparison (operator <):
  partial_sort_copy (myints, myints+9, myvector.begin(), myvector.end());

  // using function as comp
  partial_sort_copy (myints, myints+9, myvector.begin(), 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;
}

Output:

myvector contains: 1 2 3 4 5

Example 2

Let's see another simple example for the default version:

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

using namespace std ;

int main()
{
    const int VECTOR_SIZE = 8 ;

    // Define a template class vector of int
    typedef vector<int> IntVector ;

    //Define an iterator for template class vector of strings
    typedef IntVector::iterator IntVectorIt ;

    IntVector Numbers(VECTOR_SIZE) ;
    IntVector Result(4) ;

    IntVectorIt start, end, it ;

    // Initialize vector Numbers
    Numbers[0] = 4 ;
    Numbers[1] = 10;
    Numbers[2] = 70 ;
    Numbers[3] = 30 ;
    Numbers[4] = 10;
    Numbers[5] = 69 ;
    Numbers[6] = 96 ;
    Numbers[7] = 7;

    start = Numbers.begin() ;   // location of first
                                // element of Numbers

    end = Numbers.end() ;       // one past the location
                                // last element of Numbers

    cout << "Before calling partial_sort_copy\n" << endl ;

    // print content of Numbers
    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    // sort the smallest 4 elements in the Numbers
    // and copy the results in Result
    partial_sort_copy(start, end, Result.begin(), Result.end()) ;

    cout << "After calling partial_sort_copy\n" << endl ;

    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    cout << "Result { " ;
    for(it = Result.begin(); it != Result.end(); it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;
    
    return 0;
}

Output:

Before calling partial_sort_copy

Numbers { 4 10 70 30 10 69 96 7  }

After calling partial_sort_copy

Numbers { 4 10 70 30 10 69 96 7  }

Result { 4 7 10 10  }

Example 3

Let's see a simple example for custom (predicate) version:

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

using namespace std ;

int main()
{
    const int VECTOR_SIZE = 8 ;

    // Define a template class vector of int
    typedef vector<int> IntVector ;

    //Define an iterator for template class vector of strings
    typedef IntVector::iterator IntVectorIt ;

    IntVector Numbers(VECTOR_SIZE) ;
    IntVector Result(4) ;

    IntVectorIt start, end, it ;

    // Initialize vector Numbers
    Numbers[0] = 4 ;
    Numbers[1] = 10;
    Numbers[2] = 70 ;
    Numbers[3] = 30 ;
    Numbers[4] = 10;
    Numbers[5] = 69 ;
    Numbers[6] = 96 ;
    Numbers[7] = 7;

    start = Numbers.begin() ;   // location of first
                                // element of Numbers

    end = Numbers.end() ;       // one past the location
                                // last element of Numbers

    cout << "Before calling partial_sort_copy\n" << endl ;

    // print content of Numbers
    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    // sort the smallest 4 elements in the Numbers
    // and copy the results in Result
    partial_sort_copy(start, end, Result.begin(),Result.end(),less<int>());

    cout << "After calling partial_sort_copy\n" << endl ;

    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    cout << "Result { " ;
    for(it = Result.begin(); it != Result.end(); it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;
    
    return 0;
}

Output:

Before calling partial_sort_copy

Numbers { 4 10 70 30 10 69 96 7  }

After calling partial_sort_copy

Numbers { 4 10 70 30 10 69 96 7  }

Result { 4 7 10 10  }

Example 4

Let's see another example: 

#include <algorithm>
#include <vector>
#include <functional>
#include <iostream>
using namespace std; 
 
vector<int>::iterator it;

void print(vector<int> & v)
{ 
    for (it = v.begin(); it != v.end(); it++) {
        cout << *it << ' ';
    }
    cout << '\n';
}
 
int main()
{
    vector<int> v0{4, 2, 5, 1, 3};
    vector<int> v1{10, 11, 12};
    vector<int> v2{10, 11, 12, 13, 14, 15, 16};
 
    cout << "v0 : ";
    print(v0);
 
    cout << "v1 : ";
    print(v1);
 
    cout << "v2 : ";
    print(v2);
 
    it = partial_sort_copy(v0.begin(), v0.end(), v1.begin(), v1.end());
 
    cout << "Writing v0 to v1 in ascending order gives: ";
    print(v1);
 
    it = partial_sort_copy(v0.begin(), v0.end(), v2.begin(), v2.end(), 
                                std::greater<int>());
 
    cout << "Writing v0 to v2 in descending order gives: ";
    print(v2);
    
    return 0;
}

Output:

v0 : 4 2 5 1 3 
v1 : 10 11 12 
v2 : 10 11 12 13 14 15 16 
Writing v0 to v1 in ascending order gives: 1 2 3 
Writing v0 to v2 in descending order gives: 5 4 3 2 1 15 16

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