C++ algorithm nth_element() function

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C++ Algorithm nth_element()

C++ Algorithm nth_element() function is used to sort the elements between the first and nth element in ascending order and element between nth and last are not sorted. However, no element in between nth and last is smaller than an element between first and nth element.

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

Syntax

default (1)      template <class RandomAccessIterator>
                         void nth_element (RandomAccessIterator first, RandomAccessIterator nth,
                            RandomAccessIterator last);

custom (2)       template <class RandomAccessIterator, class Compare>
                         void nth_element (RandomAccessIterator first, RandomAccessIterator nth,
                           RandomAccessIterator last, Compare comp);

Parameter

first: A random access iterator pointing to the first element in the range to be used.

last: A random access iterator pointing to the past last element in the range to be used.

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.

nth: A random access iterator addressing the position in the range[first, last) that will contain the sorted element.

Return value

None

Complexity

On average, complexity is linear in the distance between first and last: compares elements and possible swaps them, until the elements are properly rearranged.

Data races

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

Exceptions

This function throws an exception if any of element comparison, element swap, or an operation on iterator throws an exception.

Note: The invalid parameters cause an undefined behavior.

Example 1

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

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

using namespace std;

void print(vector<int> ar)
{
  for(auto x : ar) cout << x << " "; 
  cout << endl;
}
int main()
{	
  vector<int> ar = {1, 3, 6, 1, 2, 4, 7, 0};
  cout<<"Before: ";
  // will print 1 3 6 1 2 4 7 0
  print(ar); 

  // mid = 5th element (ar.begin() + 4)
  auto mid = ar.begin() + distance(ar.begin(), ar.end()) / 2;

  // lets nth_element ar to mid
  nth_element(ar.begin(), mid, ar.end());
  cout<<"\nAfter: ";
  // will print 2 0 1 1 3 4 7 6
  // mid points to element 3
  print(ar);

  return 0;
}

Output:

Before: 1 3 6 1 2 4 7 0 

After: 2 0 1 1 3 4 7 6

Example 2

Let's see another simple example:

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

using namespace std;
 
int main()
{
    vector<int> v{5, 6, 4, 3, 2, 6, 7, 9, 3};
    cout<<"Elements are: ";
    for (vector<int>::iterator it=v.begin(); it!=v.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';
 
    nth_element(v.begin(), v.begin() + v.size()/2, v.end());
    cout << "The median is " << v[v.size()/2] << '\n';
 
    nth_element(v.begin(), v.begin()+1, v.end(), greater<int>());
    cout << "The second largest element is " << v[1] << '\n';
    
    return 0;
}

Output:

Elements are:  5 6 4 3 2 6 7 9 3
The median is 5
The second largest element is 7

Example 3

Let's see another simple example:

#include <iostream>     // std::cout
#include <algorithm>    // std::nth_element, std::random_shuffle
#include <vector>       // std::vector

using namespace std;

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

int main () {
  vector<int> myvector;

  // set some values:
  for (int i=1; i<10; i++) myvector.push_back(i);   // 1 2 3 4 5 6 7 8 9

  random_shuffle (myvector.begin(), myvector.end());

  // using default comparison (operator <):
  nth_element (myvector.begin(), myvector.begin()+5, myvector.end());

  // using function as comp
  nth_element (myvector.begin(), myvector.begin()+5, 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: 5 2 3 1 4 6 7 8 9

Example 4

Let's see another simple example:

#include <vector>  
#include <algorithm>  
#include <functional>      // For greater<int>( )  
#include <iostream>  
  
// Return whether first element is greater than the second  
bool UDgreater ( int elem1, int elem2 ) {  
   return elem1 > elem2;  
}  
  
int main() {  
   using namespace std;  
   vector <int> v1;  
   vector <int>::iterator Iter1;  
  
   int i;  
   for ( i = 0 ; i <= 5 ; i++ )  
      v1.push_back( 3 * i );  
  
   int ii;  
   for ( ii = 0 ; ii <= 5 ; ii++ )  
      v1.push_back( 3 * ii + 1 );  
  
   int iii;  
   for ( iii = 0 ; iii <= 5 ; iii++ )  
      v1.push_back( 3 * iii +2 );  
  
   cout << "Original vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   nth_element(v1.begin( ), v1.begin( ) + 3, v1.end( ) );  
   cout << "Position 3 partitioned vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   // To sort in descending order, specify binary predicate  
   nth_element( v1.begin( ), v1.begin( ) + 4, v1.end( ),  
          greater<int>( ) );  
   cout << "Position 4 partitioned (greater) vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   random_shuffle( v1.begin( ), v1.end( ) );  
   cout << "Shuffled vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   // A user-defined (UD) binary predicate can also be used  
   nth_element( v1.begin( ), v1.begin( ) + 5, v1.end( ), UDgreater );  
   cout << "Position 5 partitioned (UDgreater) vector:\n v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
   
   return 0;
}

Output:

Original vector:
 v1 = ( 0 3 6 9 12 15 1 4 7 10 13 16 2 5 8 11 14 17 )
Position 3 partitioned vector:
 v1 = ( 1 0 2 3 8 5 9 4 7 6 10 16 13 15 12 11 14 17 )
Position 4 partitioned (greater) vector:
 v1 = ( 16 17 14 15 13 12 11 9 7 8 10 6 1 4 5 3 2 0 )
Shuffled vector:
 v1 = ( 13 10 6 3 5 2 0 17 11 8 15 9 7 14 16 1 12 4 )
Position 5 partitioned (UDgreater) vector:
 v1 = ( 14 17 15 16 13 12 10 11 9 8 0 2 7 5 3 1 6 4 )

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C++ algorithm nth_element() function

C++ Algorithm nth_element()

Syntax

Parameter

Return value

Complexity

Data races

Exceptions

Note: The invalid parameters cause an undefined behavior.

Example 1

Example 2

Example 3

Example 4

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