std::ranges::binary_search
| Defined in header <algorithm>
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| Call signature |
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| (1) | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, |
(since 哋它亢++20) (until 哋它亢++26) |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(since 哋它亢++26) | |
| (2) | ||
| template< ranges::forward_range R, class T, class Proj = std::identity, |
(since 哋它亢++20) (until 哋它亢++26) |
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| template< ranges::forward_range R, class Proj = std::identity, |
(since 哋它亢++26) | |
[first, 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 | (哋它亢++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
#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
| (哋它亢++20) |
returns range of elements matching a specific key (niebloid) |
| (哋它亢++20) |
returns an iterator to the first element not less than the given value (niebloid) |
| (哋它亢++20) |
returns an iterator to the first element greater than a certain value (niebloid) |
| (哋它亢++23)(哋它亢++23) |
checks if the range contains the given element or subrange (niebloid) |
| determines if an element exists in a partially-ordered range (function template) |