< cpp‎ | algorithm
Defined in header <algorithm>
template< class RandomIt >
void nth_element( RandomIt first, RandomIt nth, RandomIt last );
template< class RandomIt, class Compare >
void nth_element( RandomIt first, RandomIt nth, RandomIt last, Compare comp );

nth_element is a partial sorting algorithm that rearranges elements in [first, last) such that:

  • The element pointed at by nth is changed to whatever element would occur in that position if [first, last) was sorted.
  • All of the elements before this new nth element are less than or equal to the elements after the new nth element.

More formally, nth_element partially sorts the range [first, last) in ascending order so that the condition !(*j < *i) (for the first version, or comp(*j, *i) == false for the second version) is met for any i in the range [first, nth) and for any j in the range [nth, last). The element placed in the nth position is exactly the element that would occur in this position if the range was fully sorted.

nth may be the end iterator, in this case the function has no effect.


[edit] Parameters

first, last - random access iterators defining the range sort
nth - random access iterator defining the sort partition point
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

 bool cmp(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them. ​

Type requirements
RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator.
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.

[edit] Return value


[edit] Complexity

Linear in std::distance(first, last) on average.

[edit] Notes

The algorithm used is typically introselect although other selection algorithms with suitable average-case complexity are allowed.

[edit] Example

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
int main()
    std::vector<int> v{5, 6, 4, 3, 2, 6, 7, 9, 3};
    std::nth_element(v.begin(), v.begin() + v.size()/2, v.end());
    std::cout << "The median is " << v[v.size()/2] << '\n';
    std::nth_element(v.begin(), v.begin()+1, v.end(), std::greater<int>());
    std::cout << "The second largest element is " << v[1] << '\n';


The median is 5
The second largest element is 7

[edit] See also

copies and partially sorts a range of elements
(function template)
sorts a range of elements while preserving order between equal elements
(function template)
sorts a range into ascending order
(function template)
parallelized version of std::nth_element
(function template)