Defined in header <algorithm>  

(1)  
template< class RandomIt > void nth_element( RandomIt first, RandomIt nth, RandomIt last );  (until C++20)  
template< class RandomIt > constexpr void nth_element( RandomIt first, RandomIt nth, RandomIt last );  (since C++20)  
template< class ExecutionPolicy, class RandomIt > void nth_element( ExecutionPolicy&& policy, RandomIt first, RandomIt nth, RandomIt last );  (2)  (since C++17) 
(3)  
template< class RandomIt, class Compare > void nth_element( RandomIt first, RandomIt nth, RandomIt last, Compare comp );  (until C++20)  
template< class RandomIt, class Compare > constexpr void nth_element( RandomIt first, RandomIt nth, RandomIt last, Compare comp );  (since C++20)  
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:
nth
is changed to whatever element would occur in that position if [first, last)
were sorted. 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 (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>>
(until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.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:
While the signature does not need to have 
Type requirements  
RandomIt must meet the requirements of ValueSwappable and LegacyRandomAccessIterator. 

The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible. 
(none).
std::distance(first, last)
on average.The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. The algorithm used is typically introselect although other selection algorithms with suitable averagecase complexity are allowed.
See also the implementations in libstdc++ and libc++.
#include <vector> #include <cassert> #include <numeric> #include <iostream> #include <algorithm> #include <functional> void printVec(const std::vector<int> &vec) { std::cout << "v= {"; for (int i : vec) std::cout << i << ", "; std::cout << "}\n"; } int main() { std::vector<int> v{5, 10, 6, 4, 3, 2, 6, 7, 9, 3}; printVec(v); auto m = v.begin() + v.size()/2; std::nth_element(v.begin(), m, v.end()); std::cout << "\nThe median is " << v[v.size()/2] << '\n'; // The consequence of the inequality of elements before/after the Nth one: assert(std::accumulate(v.begin(), m, 0) < std::accumulate(m, v.end(), 0)); printVec(v); // Note: comp function changed std::nth_element(v.begin(), v.begin()+1, v.end(), std::greater{}); std::cout << "\nThe second largest element is " << v[1] << '\n'; std::cout << "The largest element is " << v[0] << '\n'; printVec(v); }
Possible output:
v= {5, 10, 6, 4, 3, 2, 6, 7, 9, 3, } The median is 6 v= {3, 2, 3, 4, 5, 6, 10, 7, 9, 6, } The second largest element is 9 The largest element is 10 v= {10, 9, 6, 7, 6, 3, 5, 4, 3, 2, }
returns the largest element in a range (function template) 

returns the smallest element in a range (function template) 

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) 

(C++20)  partially sorts the given range making sure that it is partitioned by the given element (niebloid) 
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