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 C++20) (until C++26) |
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template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(since C++26) | |
(2) | ||
template< ranges::forward_range R, class T, class Proj = std::identity, |
(since C++20) (until C++26) |
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template< ranges::forward_range R, class Proj = std::identity, |
(since C++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.
Contents |
[edit] 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 |
[edit] Return value
true if an element equal to value is found, false otherwise.
[edit] 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.
[edit] 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) |
[edit] 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; |
[edit] 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
[edit] See also
(C++20) |
returns range of elements matching a specific key (niebloid) |
(C++20) |
returns an iterator to the first element not less than the given value (niebloid) |
(C++20) |
returns an iterator to the first element greater than a certain value (niebloid) |
(C++23)(C++23) |
checks if the range contains the given element or subrange (niebloid) |
determines if an element exists in a partially-ordered range (function template) |