std::ranges::adjacent_find(3)

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

✨ std::ranges::adjacent_find ✨

omg bestie, okay so this is literally one of those C++ Standard Library pages — std::ranges::adjacent_find(3) — and it's giving "find me two consecutive besties who are exactly the same" energy 💅 no cap.

🌸 NAME 🌸

std::ranges::adjacent_find — okay so this is the iconic algorithm that finds the first two adjacent elements in a range that are equal (or satisfy some predicate you give it). it's lowkey obsessed with consecutive pairs, fr fr.

🎀 Synopsis 🎀

So this is defined in the header <algorithm> (iykyk), and there are two call signatures — both since C++20, periodt.

template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
          std::indirect_binary_predicate<
              std::projected<I, Proj>,
              std::projected<I, Proj>> Pred = ranges::equal_to >
constexpr I
    adjacent_find( I first, S last, Pred pred = {}, Proj proj = {} );

(this one's numbered (1) btw, since C++20)

template< ranges::forward_range R, class Proj = std::identity,
          std::indirect_binary_predicate<
              std::projected<ranges::iterator_t<R>, Proj>,
              std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to >
constexpr ranges::borrowed_iterator_t<R>
    adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );

(and this one's (2), also since C++20)

Okay so what does it actually do? It searches the range [first, last) for two consecutive equal elements. Like, it's literally looking for twin vibes in your data 💖

  1. Elements are compared using pred — but wait, there's a glow-up first: they're projected with the projection proj before comparison. so it's pred on the projected elements, got it?
  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. same energy, different syntax ✨
🫶 Also important bestie: the function-like entities described on this page are niebloids, which means: - 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. (the compiler girlies are doing their thing behind the scenes 😭)

🌈 Parameters 🌈

  • first, last — the range of elements to examine (like, the whole squad you're checking)
  • r — the range of the elements to examine (same thing, but as a range object instead of iterators)
  • pred — predicate to apply to the projected elements (the vibe check function)
  • proj — projection to apply to the elements (the glow-up filter before the vibe check)

💜 Return value 💜

An iterator to the first of the first pair of identical elements — that is, the first iterator it such that:

bool(std::invoke(pred, std::invoke(proj1, *it),
                 std::invoke(proj, *(it + 1)))) is true.

(yeah, it says proj1 in the original, that's not a typo on my part — copy-pasting the exact thing bestie, don't come for me 🙈)

If no such elements are found, an iterator equal to last is returned. so basically "no twin vibes found, here's the end, bye 💀"

🦋 Complexity 🦋

Exactly min((result - first) + 1, (last - first) - 1) applications of the predicate and projection — where result is the return value. so it's efficient, it's not doing extra work, it's literally that girl who gets in and gets out 🔥

🧚 Possible implementation 🧚

okay so here's how it might be implemented under the hood — bestie this is the whole struct, verbatim, don't @ me:

struct adjacent_find_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_binary_predicate<
                 std::projected<I, Proj>,
                 std::projected<I, Proj>> Pred = ranges::equal_to>
    constexpr I operator()(I first, S last, Pred pred = {}, Proj proj = {}) const
    {
        if (first == last)
            return first;
        auto next = ranges::next(first);
        for (; next != last; ++next, ++first)
            if (std::invoke(pred, std::invoke(proj, *first), std::invoke(proj, *next)))
                return first;
        return next;
    }

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

inline constexpr adjacent_find_fn adjacent_find;

like, it's literally walking through the range two-at-a-time and checking if the projected values satisfy pred. iconic behavior honestly 👑

🌟 Example 🌟

okay bestie, here's a whole example — you can literally run this code, it's giving "demo energy":

#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>

constexpr bool some_of(auto&& r, auto&& pred) // some but not all
{
    return std::ranges::cend(r) != std::ranges::adjacent_find(r,
        [&pred](auto const& x, auto const& y)
        {
            return pred(x) != pred(y);
        });
}

// test some_of
constexpr auto a = {0, 0, 0, 0}, b = {1, 1, 1, 0}, c = {1, 1, 1, 1};
auto is_one = [](auto x){ return x == 1; };
static_assert(!some_of(a, is_one) && some_of(b, is_one) && !some_of(c, is_one));

int main()
{
    const auto v = {0, 1, 2, 3, 40, 40, 41, 41, 5}; /*
                                ^^          ^^       */
    namespace ranges = std::ranges;

    if (auto it = ranges::adjacent_find(v.begin(), v.end()); it == v.end())
        std::cout << "No matching adjacent elements\n";
    else
        std::cout << "The first adjacent pair of equal elements is at ["
                  << ranges::distance(v.begin(), it) << "] == " << *it << '\n';

    if (auto it = ranges::adjacent_find(v, ranges::greater()); it == v.end())
        std::cout << "The entire vector is sorted in ascending order\n";
    else
        std::cout << "The last element in the non-decreasing subsequence is at ["
                  << ranges::distance(v.begin(), it) << "] == " << *it << '\n';
}

And the output is literally:

The first adjacent pair of equal elements is at [4] == 40
The last element in the non-decreasing subsequence is at [7] == 41

see how it found the 40, 40 pair first? and then with ranges::greater() it found where the non-decreasing streak breaks? chef's kiss 🔥👑

👑 See also 👑

  • ranges::unique — removes consecutive duplicate elements in a range. (C++20) (niebloid) — like, it's the one that actually removes the duplicates, not just finds them 💅
  • adjacent_find — finds the first two adjacent items that are equal (or satisfy a given predicate). (function template) — the OG non-ranges version, still iconic in its own way ✨

okay bestie that's literally the whole page 💖 you're gonna slay with adjacent_find now, no cap. this was std::ranges::adjacent_find(3) from the C++ Standard Library, brought to you by the girlies at http://cppreference.com — page dated 2024.06.10 🌸 go forth and find those adjacent twins, periodt 🫶

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