# std::partition_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class OutputIt1, class OutputIt2, class UnaryPredicate > std::pair<OutputIt1, OutputIt2> partition_copy( InputIt first, InputIt last, OutputIt1 d_first_true, OutputIt2 d_first_false, UnaryPredicate p ); |  | (since C++11) (until C++20)
template< class InputIt, class OutputIt1, class OutputIt2, class UnaryPredicate > constexpr std::pair<OutputIt1, OutputIt2> partition_copy( InputIt first, InputIt last, OutputIt1 d_first_true, OutputIt2 d_first_false, UnaryPredicate p ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class UnaryPredicate > std::pair<ForwardIt2, ForwardIt3> partition_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first_true, ForwardIt3 d_first_false, UnaryPredicate p ); | (2) | (since C++17)

1) Copies the elements from the range `[`first`,` last`)` to two different ranges depending on the value returned by the predicate p. The elements that satisfy the predicate p are copied to the range beginning at d_first_true. The rest of the elements are copied to the range beginning at d_first_false.
The behavior is undefined if the input range overlaps either of the output ranges.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to copy from
d_first_true | - | the beginning of the output range for the elements that satisfy p
d_first_false | - | the beginning of the output range for the elements that do not satisfy p
policy | - | the execution policy to use. See execution policy for details.
p | - | unary predicate which returns ​true if the element should be placed in d_first_true. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy (since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- The type of dereferenced `InputIt` must meet the requirements of CopyAssignable.
- `OutputIt1, OutputIt2` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2, ForwardIt3` must meet the requirements of LegacyForwardIterator. ForwardIt1's value type must be CopyAssignable, writable to ForwardIt2 and ForwardIt3, and convertible to UnaryPredicate's argument type
- `UnaryPredicate` must meet the requirements of Predicate.

### Return value

A std::pair constructed from the iterator to the end of the d_first_true range and the iterator to the end of the d_first_false range.

### Complexity

Exactly std::distance(first, last) applications of p.

For the overload with an ExecutionPolicy, there may be a performance cost if ForwardIt's value type is not CopyConstructible.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports 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.

### Possible implementation

    template<class InputIt, class OutputIt1,
             class OutputIt2, class UnaryPredicate>
    std::pair<OutputIt1, OutputIt2>
        partition_copy(InputIt first, InputIt last,
                       OutputIt1 d_first_true, OutputIt2 d_first_false,
                       UnaryPredicate p)
    {
        for (; first != last; ++first)
        {
            if (p(*first))
            {
                *d_first_true = *first;
                ++d_first_true;
            }
            else
            {
                *d_first_false = *first;
                ++d_first_false;
            }
        }

        return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <utility>

    void print(auto rem, auto const& v)
    {
        for (std::cout << rem; auto const& x : v)
            std::cout << x << ' ';
        std::cout << '\n';
    }

    int main()
    {
        int arr[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        int true_arr[5] = {0};
        int false_arr[5] = {0};

        std::partition_copy(std::begin(arr), std::end(arr),
                            std::begin(true_arr), std::begin(false_arr),
                            [](int i) { return 4 < i; });

        print("true_arr:  ", true_arr);
        print("false_arr: ", false_arr);
    }

Output:

    true_arr:  5 6 7 8 9
    false_arr: 0 1 2 3 4

### See also

partition | divides a range of elements into two groups (function template)
stable_partition | divides elements into two groups while preserving their relative order (function template)
copy copy_if (C++11) | copies a range of elements to a new location (function template)
remove_copy remove_copy_if | copies a range of elements omitting those that satisfy specific criteria (function template)
ranges::partition_copy (C++20) | copies a range dividing the elements into two groups (niebloid)

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