std::nth_element
Defined in header <algorithm>


template< class RandomIt > void nth_element( RandomIt first, RandomIt nth, RandomIt last ); 
(1)  
template< class ExecutionPolicy, class RandomIt > void nth_element( ExecutionPolicy&& policy, RandomIt first, RandomIt nth, RandomIt last ); 
(2)  (since C++17) 
template< class RandomIt, class Compare > void nth_element( RandomIt first, RandomIt nth, RandomIt last, Compare comp ); 
(3)  
template< class ExecutionPolicy, class RandomIt, class Compare > void nth_element( ExecutionPolicy&& policy, RandomIt first, RandomIt nth, RandomIt last, Compare comp ); 
(4)  (since C++17) 
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 newnth
element.
More formally, nth_element
partially sorts the range [first, last)
in ascending order so that the condition !(*j < *i) (for (12), or comp(*j, *i) == false for (34)) 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.
operator<
.comp
.policy
. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is trueParameters
first, last    random access iterators defining the range sort 
nth    random access iterator defining the sort partition point 
policy    the execution policy to use. See execution policy for details. 
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. 
Type requirements  
RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator .
 
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible .

Return value
(none)
Complexity
Exceptions
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
 If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicy
is one of the three standard policies, std::terminate is called. For any otherExecutionPolicy
, the behavior is implementationdefined.  If the algorithm fails to allocate memory, std::bad_alloc is thrown.
Notes
The algorithm used is typically introselect although other selection algorithms with suitable averagecase complexity are allowed.
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'; }
Output:
The median is 5 The second largest element is 7
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) 