std::ranges::binary_search - cppreference.com (original) (raw)

| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | --------------------------- | | Call signature | | | | (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) | | | | template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr bool binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); | | (since C++20) (until C++26) | | 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 binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); | | (since C++26) |

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 (comp, value, std::invoke(proj, element)).
• for all elements, if std::invoke(comp, std::invoke(proj, element), value) is true then (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:

• 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[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.

[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.

[edit] Possible implementation

struct binary_search_fn { template<std::forward_iterator I, std::sentinel_for 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, Proj>, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t, 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 #include #include #include #include #include   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; std::vector 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