std::partition - cppreference.com

From cppreference.com

Defined in header `<algorithm>`
`template< class ForwardIt, class UnaryPred > ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );`	(1)	(constexpr since C++20)
`template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt partition( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred 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 all 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 participates in overload resolution only if all following conditions are satisfied:

`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 the type of *first is not Swappable(until C++11)ForwardIt is not ValueSwappable(since C++11), the behavior is undefined.

Parameters

first, last	-	the pair of iterators defining the range of elements to reorder
policy	-	the execution policy to use
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 LegacyForwardIterator.
-`UnaryPred` must meet the requirements of Predicate.

Return value

Iterator to the first element of the second group.

Complexity

Given N as 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) applications of p.

O(N·log(N)) swaps.

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 UnaryPred>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred 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
LWG 2150	C++98	`std::partition` was only required to place one element satisfying `p` before one element not satisfying `p`	corrected the requirement

	determines if the range is partitioned by the given predicate (function template) [edit]
	divides elements into two groups while preserving their relative order (function template) [edit]
	divides a range of elements into two groups (algorithm function object)[edit]