C++ Tutorial/STL Algorithms Sorting/nth element

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Illustrating the generic nth_element algorithm

#include <iostream>
#include <cassert>
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
  vector<int> v(7);
  v[0] = 25; 
  v[1] = 7; 
  v[2] = 9;
  v[3] = 2; 
  v[4] = 0; 
  v[5] = 5; 
  v[6] = 21;
  const int N = 4;
  // Use nth_element to place the Nth smallest 
  // element in v in the Nth position, v.begin() + N:
  nth_element(v.begin(), v.begin() + N, v.end());
  
  for (int i = 0; i < N; ++i)
    assert (v[N] >= v[i]);
  
  for (int i = N + 1; i < 7; ++i)
     cout << v[N];
  return 0;
}
99

Use nth_element to extract the four highest elements

/* 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
    nth_element (coll.begin(),     // beginning of range
                 coll.end()-4,     // element that should be sorted correctly
                 coll.end());      // end of range
    // print them
    cout << "the four highest elements are: ";
    copy (coll.end()-4, coll.end(),
          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: 5 6 7 6

Use nth_element to extract the four lowest elements

/* 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 lowest elements
    nth_element (coll.begin(),     // beginning of range
                 coll.begin()+3,   // element that should be sorted correctly
                 coll.end());      // end of range
    // print them
    cout << "the four lowest 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 lowest elements are:  2 1 2 3

Use nth_element with custom function to extract the four highest elements

/* 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