Defined in header <algorithm>  

template< class RandomIt > void partial_sort( RandomIt first, RandomIt middle, RandomIt last );  (1)  
template< class ExecutionPolicy, class RandomIt > void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last );  (2)  (since C++17) 
template< class RandomIt, class Compare > void partial_sort( RandomIt first, RandomIt middle, RandomIt last, Compare comp );  (3)  
template< class ExecutionPolicy, class RandomIt, class Compare > void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last, Compare comp );  (4)  (since C++17) 
Rearranges elements such that the range [first, middle)
contains the sorted middle  first
smallest elements in the range [first, last)
.
The order of equal elements is not guaranteed to be preserved. The order of the remaining elements in the range [middle, last)
is unspecified.
operator<
.comp
.policy
. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
is truefirst, last    the range of elements to sort 
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:
The signature does not need to have 
Type requirements  
RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator . 

The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible . 
(none).
Approximately (lastfirst)log(middlefirst)) applications of cmp
.
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the three standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. #include <algorithm> #include <functional> #include <array> #include <iostream> int main() { std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3}; std::partial_sort(s.begin(), s.begin() + 3, s.end()); for (int a : s) { std::cout << a << " "; } }
Possible output:
0 1 2 7 8 6 5 9 4 3
partially sorts the given range making sure that it is partitioned by the given element (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) 
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