Use nth_element with custom function to extract the four highest elements : nth_element « STL Algorithms Sorting « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Sorting
3. nth_element

/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */

#include <iostream>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
#include <numeric>

/* PRINT_ELEMENTS()
 * - prints optional C-string optcstr followed by
 * - all elements of the collection coll
 * - separated by spaces
 */
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
    typename T::const_iterator pos;

    std::cout << optcstr;
    for (pos=coll.begin(); pos!=coll.end(); ++pos) {
        std::cout << *pos << ' ';
    }
    std::cout << std::endl;
}

/* INSERT_ELEMENTS (collection, first, last)
 * - fill values from first to last into the collection
 * - NOTE: NO half-open range
 */
template <class T>
inline void INSERT_ELEMENTS (T& coll, int first, int last)
{
    for (int i=first; i<=last; ++i) {
        coll.insert(coll.end(),i);
    }
}

using namespace std;

int main()
{
    deque<int> coll;

    INSERT_ELEMENTS(coll,3,7);
    INSERT_ELEMENTS(coll,2,6);
    INSERT_ELEMENTS(coll,1,5);
    PRINT_ELEMENTS(coll);

    // extract the four highest elements (second version)
    nth_element (coll.begin(),     // beginning of range
                 coll.begin()+3,   // element that should be sorted correctly
                 coll.end(),       // end of range
                 greater<int>());  // sorting criterion

    // print them
    cout << "the four highest elements are: ";
    copy (coll.begin(), coll.begin()+4,
          ostream_iterator<int>(cout," "));
    cout << endl;

}

3 4 5 6 7 2 3 4 5 6 1 2 3 4 5
the four highest elements are: 6 7 6 5