std::partition_copy

 

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Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
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lower_bound	upper_bound	binary_search	equal_range
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merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
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is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
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)
C library
qsort	bsearch

 

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

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)