std::ranges::includes(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💅 std::ranges::includes — the <algorithm> bestie you didn't know you needed

okay so this one lives in the C++ Standard Library's <algorithm> header and honestly? it's giving "does this fit inside that" energy. it's std::ranges::includes, and yes it's a niebloid (we'll get to that, don't worry 🫶).

✨ NAME ✨

std::ranges::includes — literally just std::ranges::includes, no notes, that's the whole name bestie 💖

🌸 SYNOPSIS 🌸

Defined in header <algorithm>. Call signature goes a lil something like this:

template< std::input_iterator I1, std::sentinel_for<I1> S1,
          std::input_iterator I2, std::sentinel_for<I2> S2,
          class Proj1 = std::identity, class Proj2 = std::identity,
          std::indirect_strict_weak_order<
              std::projected<I1, Proj1>,
              std::projected<I2, Proj2>> Comp = ranges::less >
(1) (since C++20)
constexpr bool
    includes( I1 first1, S1 last1, I2 first2, S2 last2,
              Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} )
template< ranges::input_range R1, ranges::input_range R2,
          class Proj1 = std::identity, class Proj2 = std::identity,
          std::indirect_strict_weak_order<
              std::projected<ranges::iterator_t<R1>, Proj1>,
              std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less >
(2) (since C++20)
constexpr bool
    includes( R1&& r1, R2&& r2, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} )
  1. Returns true if the projections of the sorted range [first2, last2) is a subsequence of the projections of the sorted range [first1, last1).
  2. Same as (1), but uses r1 and r2 as the source ranges, as if by using ranges::begin(r1) and ranges::begin(r2) as first1 and first2 respectively, and ranges::end(r1) and ranges::end(r2) as last1 and last2 respectively.

🦋 DESCRIPTION 🦋

so. both ranges must be sorted with the given comparison function comp. no cap, that's non-negotiable — the whole algorithm hinges on it. and a subsequence doesn't have to be contiguous, which is lowkey the best part: {a, c} totally counts as a subsequence of {a, b, c} because the b is just... vibing in between, iykyk.

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. main character energy either way 👑

🎀 PARAMETERS 🎀

  • first1, last1 — the sorted range of elements to examine
  • r1 — the sorted range of elements to examine
  • first2, last2 — the sorted range of elements to search for
  • r2 — the sorted range of elements to search for
  • comp — comparison function to apply to the projected elements
  • proj1 — projection to apply to the elements in the first range
  • proj2 — projection to apply to the elements in the second range

⭐ RETURN VALUE ⭐

true if [first2, last2) is a subsequence of [first1, last1); otherwise false. iconic binary behavior, we love to see it 💖

🔥 COMPLEXITY 🔥

at most 2·(N1+N2−1) comparisons, where N1 is ranges::distance(r1) and N2 is ranges::distance(r2). not too shabby, not too spicy 🌶️

🧚 POSSIBLE IMPLEMENTATION 🧚

for the girlies who need to see the source, here's how it could go:

struct includes_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             class Proj1 = std::identity, class Proj2 = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<I1, Proj1>,
                 std::projected<I2, Proj2>> Comp = ranges::less>
    constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                              Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; first2 != last2; ++first1)
        {
            if (first1 == last1 || comp(*first2, *first1))
                return false;
            if (!comp(*first1, *first2))
                ++first2;
        }
        return true;
    }

    template<ranges::input_range R1, ranges::input_range R2,
             class Proj1 = std::identity, class Proj2 = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R1>, Proj1>,
                 std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less>
    constexpr bool operator()(R1&& r1, R2&& r2, Comp comp = {},
                              Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::ref(comp), std::ref(proj1), std::ref(proj2));
    }
};

inline constexpr auto includes = includes_fn {};

look at that inline constexpr auto includes = includes_fn {}; — periodt. 💅

🌈 EXAMPLE 🌈

// Run this code

#include <algorithm>
#include <cctype>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <locale>
#include <string>

template<class T>
std::ostream& operator<<(std::ostream& os, std::initializer_list<T> const& list)
{
    for (os << "{ "; auto const& elem : list)
        os << elem << ' ';
    return os << "} ";
}

struct true_false : std::numpunct<char>
{
    std::string do_truename() const { return "? Yes\n"; }
    std::string do_falsename() const { return "? No\n"; }
};

int main()
{
    std::cout.imbue(std::locale(std::cout.getloc(), new true_false));

    auto ignore_case = [](char a, char b) { return std::tolower(a) < std::tolower(b); };

    const auto
        a = {'a', 'b', 'c'},
        b = {'a', 'c'},
        c = {'a', 'a', 'b'},
        d = {'g'},
        e = {'a', 'c', 'g'},
        f = {'A', 'B', 'C'},
        z = {'a', 'b', 'c', 'f', 'h', 'x'};

    std::cout
        << z << "includes\n" << std::boolalpha
        << a << std::ranges::includes(z.begin(), z.end(), a.begin(), a.end())
        << b << std::ranges::includes(z, b)
        << c << std::ranges::includes(z, c)
        << d << std::ranges::includes(z, d)
        << e << std::ranges::includes(z, e)
        << f << std::ranges::includes(z, f, ignore_case);
}

✨ Output ✨

{ a b c f h x } includes
{ a b c } ? Yes
{ a c } ? Yes
{ a a b } ? No
{ g } ? No
{ a c g } ? No
{ A B C } ? Yes

not the { a a b } ? No catching people off guard 😭 yeah bestie, the second range has two as and the first only has one — subsequence rules are strict like that. and the last one slays because of the custom ignore_case comparator, we love a case-insensitive queen 💜

💀 SEE ALSO 💀

  • ranges::set_difference (C++20) — computes the difference between two sets (niebloid)
  • ranges::search (C++20) — searches for a range of elements (niebloid)
  • ranges::contains (C++23) / ranges::contains_subrange (C++23) — checks if the range contains the given element or subrange (niebloid)
  • includes — returns true if one sequence is a subsequence of another (function template)

okay that's the whole page, go forth and check your subsequences 🦋💖

source: http://cppreference.com, dated 2024.06.10, from the std::ranges::includes(3) page. mwah, stan cppreference for me 💅✨
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