NAME¶

std::ranges::binary_search - std::ranges::binary_search

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 (since
<const T*, std::projected<I, Proj>> Comp = C++20)
ranges::less > (until
constexpr bool binary_search( I first, S last, const T& C++26)
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 = (since
ranges::less > 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*, (since
std::projected<ranges::iterator_t<R>, C++20)
Proj>> Comp = (until
ranges::less > 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*, (since
std::projected<ranges::iterator_t<R>, C++26)
Proj>> Comp =
ranges::less >
constexpr bool binary_search( R&& r, const T& value,

Comp comp = {}, Proj proj = {}
);

1) Checks if a projected element equivalent to value appears within the range
[first, last).
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.

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.

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 log
2(last - first) + O(1) comparisons and projections). However, for iterator-sentinel
pair that does not model std::random_access_iterator, 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 Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
algorithms (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

See also¶

ranges::equal_range returns range of elements matching a specific key
(C++20) (niebloid)
ranges::lower_bound returns an iterator to the first element not less than the
(C++20) given value
(niebloid)
ranges::upper_bound returns an iterator to the first element greater than a
(C++20) certain value
(niebloid)
ranges::contains
ranges::contains_subrange checks if the range contains the given element or subrange
(C++23) (niebloid)
(C++23)
determines if an element exists in a partially-ordered
binary_search range
(function template)