std::is_partitioned(3)

🦋 std::is_partitioned(3) — C++ Standard Libary 🦋

okay so bestie, welcome to the C++ Standard Libary page for std::is_partitioned — the algorithm that does a whole vibe check on whether your range is partitioned. no cap, it’s giving very specific energy. 💅

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🌸 NAME 🌸

std::is_partitioned — std::is_partitioned

That’s it. That’s the name. iconic. ✨

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✨ Synopsis ✨

Defined in header <algorithm>.

template< class InputIt, class UnaryPred >
bool is_partitioned( InputIt first, InputIt last,
                     UnaryPred p );

(1) — (since C++11) — (constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
bool is_partitioned( ExecutionPolicy&& policy,
                     ForwardIt first, ForwardIt last,
                     UnaryPred p );

(2) — (since C++17)

1. Checks whether [first, last) is partitioned by the predicate p: all elements that satisfy p appear before all elements that do not.
2. Same as (1), but executed according to policy.

Overload (2) 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)

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🎀 Parameters 🎀

• first, last — the range of elements to check.
• policy — the execution policy to use. See execution policy for details.
• p — unary predicate which returns true for the elements expected to be found in the beginning of the range.

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).

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🦋 Type requirements 🦋

• InputIt must meet the requirements of LegacyInputIterator.
• ForwardIt must meet the requirements of LegacyForwardIterator. and its value type must be convertible to UnaryPred's parameter type.
• UnaryPred must meet the requirements of Predicate.

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💖 Return value 💖

true if the elements e of [first, last) are partitioned with respect to the expression p(e). false otherwise.

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🔥 Complexity 🔥

At most std::distance(first, last) applications of p.

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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 🌈

template<class InputIt, class UnaryPred>
bool is_partitioned(InputIt first, InputIt last, UnaryPred p)
{
    for (; first != last; ++first)
        if (!p(*first))
            break;
    for (; first != last; ++first)
        if (p(*first))
            return false;
    return true;
}

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🍓 Example 🍓

// Run this code

#include <algorithm>
#include <array>
#include <iostream>

int main()
{
    std::array<int, 9> v {1, 2, 3, 4, 5, 6, 7, 8, 9};

    auto is_even = [](int i) { return i % 2 == 0; };
    std::cout.setf(std::ios_base::boolalpha);
    std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';

    std::partition(v.begin(), v.end(), is_even);
    std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';

    std::reverse(v.begin(), v.end());
    std::cout << std::is_partitioned(v.cbegin(), v.cend(), is_even) << ' ';
    std::cout << std::is_partitioned(v.crbegin(), v.crend(), is_even) << '\n';
}

🧚 Output 🧚

false true false true

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👑 See also 👑

• partition — divides a range of elements into two groups (function template)
• partition_point — locates the partition point of a partitioned range (C++11) (function template)
• ranges::is_partitioned — determines if the range is partitioned by the given predicate (C++20) (niebloid)

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that’s the whole page, bestie 🫶 — source: http://cppreference.com, version/date: 2024.06.10, and the manual title is std::is_partitioned(3). periodt. 💅✨