std::partition_copy (3) - Linux Manuals

std::partition_copy: std::partition_copy

NAME

std::partition_copy - std::partition_copy

Synopsis

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

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.
                unary predicate which returns true if the element should be placed in d_first_true.
p - 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