| Permutation operations |
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| Fold operations |
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| Operations on uninitialized storage |
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| Return types |
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(1)
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 >
constexpr bool binary_search( I first, S last, const T& value,
Comp comp = {}, Proj proj = {} );
(since C++20) (until C++26)
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 binary_search( I first, S last, const T& value,
Comp comp = {}, Proj proj = {} );
(since C++26)
(2)
(since C++20) (until C++26)
1) Checks if a projected element equivalent to value appears within the range [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 algorithm function objects (informally known as niebloids), that is:
[edit] Parameters
first, last
-
the iterator-sentinel pair defining 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 log2(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.
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.
[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
returns range of elements matching a specific key
(algorithm function object)[edit]
returns an iterator to the first element
not less than the given value
(algorithm function object)[edit]
returns an iterator to the first element
greater than a certain value
(algorithm function object)[edit]
checks if the range contains the given element or subrange
(algorithm function object)[edit]
determines if an element exists in a partially-ordered range
(function template) [edit]