# std::partial_sort_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class RandomIt > RandomIt partial_sort_copy( InputIt first, InputIt last, RandomIt d_first, RandomIt d_last ); |  | (until C++20)
template< class InputIt, class RandomIt > constexpr RandomIt partial_sort_copy( InputIt first, InputIt last, RandomIt d_first, RandomIt d_last ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class RandomIt > RandomIt partial_sort_copy( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, RandomIt d_first, RandomIt d_last ); | (2) | (since C++17)
| (3) |
template< class InputIt, class RandomIt, class Compare > RandomIt partial_sort_copy( InputIt first, InputIt last, RandomIt d_first, RandomIt d_last, Compare comp ); |  | (until C++20)
template< class InputIt, class RandomIt, class Compare > constexpr RandomIt partial_sort_copy( InputIt first, InputIt last, RandomIt d_first, RandomIt d_last, Compare comp ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class RandomIt, class Compare > RandomIt partial_sort_copy( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, RandomIt d_first, RandomIt d_last, Compare comp ); | (4) | (since C++17)

Sorts some of the elements in the range `[first, last)` in ascending order, storing the result in the range `[d_first, d_last)`.

At most d_last - d_first of the elements are placed sorted to the range `[d_first, d_first + n)`. `n` is the number of elements to sort (n = min(last - first, d_last - d_first)). The order of equal elements is not guaranteed to be preserved.

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`. This overload only participates in overload resolution if std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true

### Parameters

first, last | - | the range of elements to sort
d_first, d_last | - | random access iterators defining the destination range
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
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `RandomIt` must meet the requirements of ValueSwappable and LegacyRandomAccessIterator.
- The type of dereferenced `RandomIt` must meet the requirements of MoveAssignable and MoveConstructible.

### Return value

an iterator to the element defining the upper boundary of the sorted range, i.e. d_first + min(last - first, d_last - d_first).

### Complexity

O(N·log(min(D,N)), where N = std::distance (first, last), D = std::distance (d_first, d_last) 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

The following code sorts a vector of integers and copies them into a smaller and a larger vector.

Run this code

    #include <algorithm>
    #include <vector>
    #include <functional>
    #include <iostream>

    int main()
    {
        std::vector<int> v0{4, 2, 5, 1, 3};
        std::vector<int> v1{10, 11, 12};
        std::vector<int> v2{10, 11, 12, 13, 14, 15, 16};
        std::vector<int>::iterator it;

        it = std::partial_sort_copy(v0.begin(), v0.end(), v1.begin(), v1.end());

        std::cout << "Writing to the smaller vector in ascending order gives: ";
        for (int a : v1) {
            std::cout << a << " ";
        }
        std::cout << '\n';
        if(it == v1.end())
            std::cout << "The return value is the end iterator\n";

        it = std::partial_sort_copy(v0.begin(), v0.end(), v2.begin(), v2.end(),
                                    std::greater<int>());

        std::cout << "Writing to the larger vector in descending order gives: ";
        for (int a : v2) {
            std::cout << a << " ";
        }
        std::cout << '\n' << "The return value is the iterator to " << *it << '\n';
    }

Output:

    Writing to the smaller vector in ascending order gives: 1 2 3
    The return value is the end iterator
    Writing to the larger vector in descending order gives: 5 4 3 2 1 15 16
    The return value is the iterator to 15

### See also

partial_sort | sorts the first N elements of a range (function template)
sort | sorts a range into ascending order (function template)
stable_sort | sorts a range of elements while preserving order between equal elements (function template)

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