# std::partial_sort

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class RandomIt > void partial_sort( RandomIt first, RandomIt middle, RandomIt last ); |  | (until C++20)
template< class RandomIt > constexpr void partial_sort( RandomIt first, RandomIt middle, RandomIt last ); |  | (since C++20)
template< class ExecutionPolicy, class RandomIt > void partial_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt middle, RandomIt last ); | (2) | (since C++17)
| (3) |
template< class RandomIt, class Compare > void partial_sort( RandomIt first, RandomIt middle, RandomIt last, Compare comp ); |  | (until C++20)
template< class RandomIt, class Compare > constexpr void partial_sort( RandomIt first, RandomIt middle, RandomIt last, Compare comp ); |  | (since C++20)
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.

1) Elements are compared using `operator<`.
3) Elements are compared using the given binary comparison function `comp`.
2,4) Same as (1,3), but executed according to `policy`. These overloads do not participate in overload resolution unless std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true

### Parameters

first, 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:

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

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for `Type1` a move is equivalent to a copy (since C++11)).
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 LegacyRandomAccessIterator.
- The type of dereferenced `RandomIt` must meet the requirements of MoveAssignable and MoveConstructible.

### Return value

(none)

### Complexity

Approximately (last-first)log(middle-first) applications of `cmp`

### 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 standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Example

Run this code

    #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

### See also

nth_element | partially sorts the given range making sure that it is partitioned by the given element (function template)
partial_sort_copy | copies and partially sorts a range of elements (function template)
stable_sort | sorts a range of elements while preserving order between equal elements (function template)
sort | sorts a range into ascending order (function template)

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