std::partition_copy

 

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if

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)

If *first is not writable to d_first_true or d_first_false, the program is ill-formed.

Among the input range and the two output ranges, if any two ranges overlap, the behavior is undefined.

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.
- OutputIt1, OutputIt2 must meet the requirements of LegacyOutputIterator.
- ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.
- UnaryPred 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 (2), 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

partition_copy (1)

template<class InputIt, class OutputIt1, class OutputIt2, class UnaryPred> constexpr //< since C++20 std::pair<OutputIt1, OutputIt2> partition_copy(InputIt first, InputIt last, OutputIt1 d_first_true, OutputIt2 d_first_false, UnaryPred 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, const auto& v)
{
    for (std::cout << rem; const auto& 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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P0896R4	C++11 C++17	1. the value type of InputIt (C++11)/ForwardIt1 (C++17)     was required to be CopyAssignable 2. the two output ranges could overlap	1. not required 2. the behavior is     undefined in this case

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)
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)
ranges::partition_copy (C++20)	copies a range dividing the elements into two groups (niebloid)