# 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 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_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 `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)
    {
        while (first != last) {
            if (p(*first)) {
                *d_first_true = *first;
                ++d_first_true;
            } else {
                *d_first_false = *first;
                ++d_first_false;
            }
            ++first;
        }
        return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
    }

### Example

Run this code

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

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

        std::cout << "true_arr: ";
        for (int x : true_arr) {
            std::cout << x << ' ';
        }
        std::cout << '\n';

        std::cout << "false_arr: ";
        for (int x : false_arr) {
            std::cout << x << ' ';
        }
        std::cout << '\n';

        return 0;

    }

Output:

    true_arr: 6 7 8 9 10
    false_arr: 1 2 3 4 5

### 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)

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