std::partition

Defined in header `<algorithm>`
	(1)
`template< class ForwardIt, class UnaryPredicate > ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );`		(until C++20)
`template< class ForwardIt, class UnaryPredicate > constexpr ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );`		(since C++20)
`template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > ForwardIt partition( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p );`	(2)	(since C++17)

1) Reorders the elements in the range [first, last) in such a way that all elements for which the predicate p returns true precede the elements for which predicate p returns false. Relative order of the elements is not preserved.

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

`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)

Parameters

first, last	-	the range of elements to reorder
policy	-	the execution policy to use. See execution policy for details.
p	-	unary predicate which returns `true` if the element should be ordered before other elements. 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 `ForwardIt`, 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
-`ForwardIt` must meet the requirements of ValueSwappable and LegacyForwardIterator. However, the operation is more efficient if `ForwardIt` also satisfies the requirements of LegacyBidirectionalIterator.
-`UnaryPredicate` must meet the requirements of Predicate.

Return value

Iterator to the first element of the second group.

Complexity

Given N = std::distance(first, last),

1) Exactly N applications of p. At most N / 2 swaps if ForwardIt meets the requirements of LegacyBidirectionalIterator, and at most N swaps otherwise.

2) O(N·log N) swaps and O(N) applications of p.

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

Implements overload (1) preserving C++11 compatibility.

template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
    first = std::find_if_not(first, last, p);
    if (first == last)
        return first;
 
    for (auto i = std::next(first); i != last; ++i)
        if (p(*i))
        {
            std::iter_swap(i, first);
            ++first;
        }
 
    return first;
}

Example

#include <algorithm>
#include <forward_list>
#include <iostream>
#include <iterator>
#include <vector>
 
template<class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return;
 
    auto pivot = *std::next(first, std::distance(first, last) / 2);
    auto middle1 = std::partition(first, last, [pivot](const auto& em)
    {
        return em < pivot;
    });
    auto middle2 = std::partition(middle1, last, [pivot](const auto& em)
    {
        return !(pivot < em);
    });
 
    quicksort(first, middle1);
    quicksort(middle2, last);
}
 
int main()
{
    std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "Original vector: ";
    for (int elem : v)
        std::cout << elem << ' ';
 
    auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
 
    std::cout << "\nPartitioned vector: ";
    std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
    std::cout << "* ";
    std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
 
    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: ";
    for (int n : fl)
        std::cout << n << ' ';
 
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "\nSorted using quicksort: ";
    for (int fi : fl)
        std::cout << fi << ' ';
    std::cout << '\n';
}

Possible output:

Original vector: 0 1 2 3 4 5 6 7 8 9 
Partitioned vector: 0 8 2 6 4 * 5 3 7 1 9 
Unsorted list: 1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92 
Sorted using quicksort: -8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 498	C++98	`std::partition` required `first` and `last` to be LegacyBidirectionalIterator	only required to be LegacyForwardIterator (the complexity requirement for non-bidirectional iterators is weaker)

is_partitioned (C++11)	determines if the range is partitioned by the given predicate (function template)
stable_partition	divides elements into two groups while preserving their relative order (function template)
ranges::partition (C++20)	divides a range of elements into two groups (niebloid)

Docs

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