# std::partial_sort

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< cpp‎ | algorithm

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.

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);

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`.

### 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 three 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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