std::partition_copy

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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.
Type requirements
- `InputIt` must meet the requirements of InputIterator.
- The type of dereferenced `InputIt` must meet the requirements of CopyAssignable.
- `OutputIt1, OutputIt2` must meet the requirements of OutputIterator.
- `ForwardIt1, ForwardIt2, ForwardIt3` must meet the requirements of ForwardIterator. 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 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.

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

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

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)
copycopy_if (C++11)	copies a range of elements to a new location (function template)
remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)