std::ranges..._partitioned(3)

🎀 NAME 🎀

Okay so bestie, welcome to the std::ranges..._partitioned(3) page from the C++ Standard Libary manual 💖 The name up top is std::ranges::is_partitioned — literally std::ranges::is_partitioned, no cap.

✨ Synopsis ✨

Defined in header <algorithm>. Call signature vibes:

template< std::input_iterator I, std::sentinel_for<I> S,
          class Proj = std::identity,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr bool
    is_partitioned( I first, S last, Pred pred, Proj proj = {} );

(1) (since C++20)

template< ranges::input_range R, class Proj = std::identity,
          std::indirect_unary_predicate<
              std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr bool
    is_partitioned( R&& r, Pred pred, Proj proj = {} );

(2) (since C++20)

1. Returns true if all elements in the range [first, last) that satisfy the predicate pred after projection appear before all elements that don't. Also returns true if [first, last) is empty.
2. Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

💅 Parameters 💅

• first, last — iterator-sentinel pair denoting the range of elements to examine
• r — the range of elements to examine
• pred — predicate to apply to the projected elements
• proj — projection to apply to the elements

🌸 Return value 🌸

true if the range [first, last) is empty or is partitioned by pred, false otherwise.

🔥 Complexity 🔥

At most ranges::distance(first, last) applications of pred and proj.

👀 Possible implementation 👀

If you're nosy, here's one possible implementation, bestie:

struct is_partitioned_fn {
    template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr bool operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (!std::invoke(pred, std::invoke(proj, *first)))
                break;

        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first)))
                return false;

        return true;
    }

    template<ranges::input_range R, class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>,
                                                          Proj>> Pred>
    constexpr bool operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred),
                       std::ref(proj));
    } };

inline constexpr auto is_partitioned = is_partitioned_fn();

🍓 Example 🍓

// Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <numeric>
#include <utility>

int main()
{
    std::array<int, 9> v;

    auto print = [&v](bool o)
    {
        for (int x : v)
            std::cout << x << ' ';
        std::cout << (o ? "=> " : "=> not ") << "partitioned\n";
    };

    auto is_even = [](int i) { return i % 2 == 0; };

    std::iota(v.begin(), v.end(), 1); // or std::ranges::iota(v, 1);
    print(std::ranges::is_partitioned(v, is_even));

    std::ranges::partition(v, is_even);
    print(std::ranges::is_partitioned(std::as_const(v), is_even));

    std::ranges::reverse(v);
    print(std::ranges::is_partitioned(v.cbegin(), v.cend(), is_even));
    print(std::ranges::is_partitioned(v.crbegin(), v.crend(),
                                      is_even));
}

Output:

1 2 3 4 5 6 7 8 9 => not partitioned
2 4 6 8 5 3 7 1 9 => partitioned
9 1 7 3 5 8 6 4 2 => not partitioned
9 1 7 3 5 8 6 4 2 => partitioned

🦋 See also 🦋

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

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Source/version/date: http://cppreference.com · 2024.06.10 · std::ranges..._partitioned(3) — mwah, bestie 💅💖