std::partition(3)

💖 std::partition — NAME 💖

okay so this is std::partition(3), folded in near the top from the C++ Standard Libary (manual name kept exactly, bestie, typo and all). The name itself is literally: std::partition — std::partition. That’s the whole NAME section, and honestly? Iconic minimalism. 💅

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✨ Synopsis ✨

Defined in header <algorithm>

template< class ForwardIt, class UnaryPred >
ForwardIt partition( ForwardIt first, ForwardIt last,    (1)
(constexpr since C++20)
UnaryPred p );

template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
ForwardIt partition( ExecutionPolicy&& policy,           (2) (since C++17)
                     ForwardIt first, ForwardIt last,
                     UnaryPred p );

Two overloads, bestie, and they are giving overload-resolution drama:

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 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)

And bestie, if the type of *first is not Swappable (until C++11) or ForwardIt is not ValueSwappable (since C++11), the behavior is undefined. Not cute. Do not do that. 💀

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🎀 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).

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🦋 Type requirements 🦋

• ForwardIt must meet the requirements of LegacyForwardIterator.
• UnaryPred must meet the requirements of Predicate.

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🌸 Return value 🌸

Iterator to the first element of the second group. That’s the boundary girlie. 🫶

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💅 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.

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🫶 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.

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🔥 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;
}

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😭 Example 😭

Run this code, bestie 👇

#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';
}

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👑 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

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💀 Defect reports 💀

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

• DR LWG 498 — Applied to: C++98. Behavior as published: std::partition required first and last to be LegacyBidirectionalIterator. Correct behavior: only required to be LegacyForwardIterator.
• DR LWG 2150 — Applied to: C++98. Behavior as published: std::partition was only required to place one element satisfying p before one element not satisfying p. Correct behavior: corrected the requirement.

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🌈 See also 🌈

• 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).

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okay that’s the whole page, bestie 🫶 cute little sign-off time: source is http://cppreference.com, version/date is 2024.06.10, and this was the std::partition(3) page. byeeee 💖✨🎀