std::ranges::binary_search(3)

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

std::ranges::binary_search 💅

okay so bestie, this is the std::ranges::binary_search page from the C++ Standard Library — manual section 3, folded in right here because we're cute like that. It's literally the “does this partitioned/sorted-ish range contain my value?” girlie, and she ate. no cap. 🦋

NAME ✨

std::ranges::binary_search — std::ranges::binary_search

(yes, the name is the name. iconic.)

Synopsis 🎀

Defined in header <algorithm>.

Call signature:

template< std::forward_iterator I, std::sentinel_for<I> S,
          class T, class Proj = std::identity,
          std::indirect_strict_weak_order
              <const T*, std::projected<I, Proj>> Comp =
              ranges::less >
(since C++20)
(until C++26)
constexpr bool binary_search( I first, S last, const T& value,
                              Comp comp = {}, Proj proj = {}
);
template< std::forward_iterator I, std::sentinel_for<I> S,
          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj>,
          std::indirect_strict_weak_order
              <const T*, std::projected<I, Proj>> Comp =
              ranges::less >
(since C++26)
constexpr bool binary_search( I first, S last, const T& value,
                              Comp comp = {}, Proj proj = {}
);
template< ranges::forward_range R,
                                                                     (1)
          class T, class Proj = std::identity,
          std::indirect_strict_weak_order
              <const T*,
              std::projected<ranges::iterator_t<R>,
                                                  Proj>> Comp =
              ranges::less >
(since C++20)
(until C++26)
constexpr bool binary_search( R&& r, const T& value,
                              Comp comp = {}, Proj proj = {}
);
template< ranges::forward_range R,                               (2)
          class Proj = std::identity,
          class T =
std::projected_value_t<ranges::iterator_t<R>, Proj>,
          std::indirect_strict_weak_order
              <const T*,
              std::projected<ranges::iterator_t<R>,
                                                  Proj>> Comp =
          ranges::less >
(since C++26)
constexpr bool binary_search( R&& r, const T& value,
                              Comp comp = {}, Proj proj = {}
);

Checks if a projected element equivalent to value appears within the range [first, last).
Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

For ranges::binary_search to succeed, the range [first, last) must be at least partially ordered with respect to value, i.e. it must satisfy all of the following requirements:

partitioned with respect to std::invoke(comp, std::invoke(proj, element), value) (that is, all projected elements for which the expression is true precedes all elements for which the expression is false).
partitioned with respect to !std::invoke(comp, value, std::invoke(proj, element)).
for all elements, if std::invoke(comp, std::invoke(proj, element), value) is true then !std::invoke(comp, value, std::invoke(proj, element)) is also true.

A fully-sorted range meets these criteria.

bestie, this is the vibe check: the range has to be at least partially ordered with respect to value; a fully-sorted range meets these criteria. 🫶

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 — the range of elements to examine.
r — the range of elements to examine.
value — value to compare the elements to.
comp — comparison function to apply to the projected elements.
proj — projection to apply to the elements.

Return value 💖

true if an element equal to value is found, false otherwise.

Complexity 🔥

The number of comparisons and projections performed is logarithmic in the distance between first and last (at most log₂(last - first) + O(1) comparisons and projections). However, for an iterator-sentinel pair that does not model std::random_access_iterator, the number of iterator increments is linear.

Notes 🧚

std::ranges::binary_search doesn't return an iterator to the found element when an element whose projection equals value is found. If an iterator is desired, std::ranges::lower_bound should be used instead.

Feature-test macro tea ☕:

__cpp_lib_algorithm_default_value_type — value 202403 — (C++26) — feature: “List-initialization for algorithms” — applies to overloads (1,2).

Possible implementation 👑

struct binary_search_fn {
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T =
std::projected_value_t<I, Proj>,
             std::indirect_strict_weak_order
                 <const T*, std::projected<I, Proj>> Comp =
ranges::less>
    constexpr bool operator()(I first, S last, const T& value,
                              Comp comp = {}, Proj proj = {}) const
    {
        auto x = ranges::lower_bound(first, last, value, comp, proj);
        return (!(x == last) && !(std::invoke(comp, value,
std::invoke(proj, *x))));
    }

    template<ranges::forward_range R, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>,
Proj>,
             std::indirect_strict_weak_order
                 <const T*, std::projected<ranges::iterator_t<R>,
                                           Proj>> Comp = ranges::less>
    constexpr bool operator()(R&& r, const T& value, Comp comp = {},
Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value,
                       std::move(comp), std::move(proj));
    } };

inline constexpr binary_search_fn binary_search;

Example 🍓

// Run this code

#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <ranges>
#include <vector>

int main()
{
    constexpr static auto haystack = {1, 3, 4, 5, 9};
    static_assert(std::ranges::is_sorted(haystack));

    for (const int needle : std::views::iota(1)
                          | std::views::take(3))
    {
        std::cout << "Searching for " << needle << ": ";
        std::ranges::binary_search(haystack, needle)
            ? std::cout << "found " << needle << '\n'
            : std::cout << "no dice!\n";
    }

    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 3}, {4, 2}, {4, 3}};
    auto cmpz = [](CD x, CD y){ return abs(x) < abs(y); };
    #ifdef __cpp_lib_algorithm_default_value_type
        assert(std::ranges::binary_search(nums, {4, 2}, cmpz));
    #else
        assert(std::ranges::binary_search(nums, CD{4, 2}, cmpz));
    #endif
}

Output:

Searching for 1: found 1
Searching for 2: no dice!
Searching for 3: found 3

std::ranges::binary_search(3)