nth_element

Header

template<class RandomAccessIterator> inline
void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last)

The nth_element algorithm partitions the sequence [first..last) on the value referenced by nth. All the elements less than or equal to the value are placed before value and all elements greater than value are placed after value in the sequence. The non-predicate version of nth_element uses operator< for comparisons.

template<class RandomAccessIterator, class Compare> inline
void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare compare)

The nth_element algorithm partitions the sequence [first..last) on the value referenced by nth. All the elements less than or equal to the value are placed before value and all elements greater than value are placed after value in the sequence. The predicate version of nth_element uses the compare function for comparisons.

Samples

Sample for Non-Predicate Version

// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)

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

using namespace std ;

void 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) ;

    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] = 100;

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

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

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

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

    // partition the elements by the 4th element
    nth_element(start, start+4, end) ;

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

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

Program Output

Before calling nth_element

Numbers { 4 10 70 30 10 69 96 100  }

After calling nth_element

Numbers { 4 10 10 30 69 70 96 100  }

Sample for Predicate Version

// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)

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

using namespace std;


void 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) ;

    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] = 100;

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

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

    cout << "Before calling nth_element:\n" << endl ;

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

    // partition the elements by the 4th element
    nth_element(start, start+4, end, less<int>()) ;

    cout << "After calling nth_element:\n" << endl ;

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

Program Output

Before calling nth_element:

Numbers { 4 10 70 30 10 69 96 100  }

After calling nth_element:

Numbers { 4 10 10 30 69 70 96 100  }