std::binary_search - cppreference.com (original) (raw)
| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | ------------------------------------- | | (1) | | | | template< class ForwardIt, class T > bool binary_search( ForwardIt first, ForwardIt last, const T& value ); | | (constexpr since C++20) (until C++26) | | template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > constexpr bool binary_search( ForwardIt first, ForwardIt last, const T& value ); | | (since C++26) | | (2) | | | | template< class ForwardIt, class T, class Compare > bool binary_search( ForwardIt first, ForwardIt last, const T& value, Compare comp ); | | (constexpr since C++20) (until C++26) | | template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type, class Compare > constexpr bool binary_search( ForwardIt first, ForwardIt last, const T& value, Compare comp ); | | (since C++26) |
Checks if an element equivalent to value appears within the partitioned range [first, last).
- The equivalence is checked using operator<:
| If !bool(*iter < value) && !bool(value < *iter) is true for some iterator iter in [first, last), returns true. Otherwise returns false.If any of the following conditions is satisfied, the behavior is undefined: For any element elem of [first, last), bool(elem < value) does not imply !bool(value < elem). The elements elem of [first, last) are not partitioned with respect to expressions bool(elem < value) and !bool(value < elem). | (until C++20) |
|---|---|
| Equivalent to std::binary_search(first, last, value, std::less{}). | (since C++20) |
- The equivalence is checked using comp:
If !bool(comp(*iter, value)) && !bool(comp(value, *iter)) is true for some iterator iter in [first, last), returns true. Otherwise returns false.
If any of the following conditions is satisfied, the behavior is undefined:
- For any element elem of
[first,last), bool(comp(elem, value)) does not imply !bool(comp(value, elem)). - The elements elem of
[first,last)are not partitioned with respect to expressions bool(comp(elem, value)) and !bool(comp(value, elem)).
Contents
- 1 Parameters
- 2 Return value
- 3 Complexity
- 4 Notes
- 5 Possible implementation
- 6 Example
- 7 Defect reports
- 8 See also
[edit] Parameters
| first, last | - | the pair of iterators defining the partitioned range of elements to examine |
|---|---|---|
| value | - | value to compare the elements to |
| comp | - | binary predicate which returns true if the first argument is ordered before the second. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).The types Type1 and Type2 must be such that an object of type T can be implicitly converted to both Type1 and Type2, and an object of type ForwardIt can be dereferenced and then implicitly converted to both Type1 and Type2. |
| Type requirements | ||
| -ForwardIt must meet the requirements of LegacyForwardIterator. | ||
| -Compare must meet the requirements of BinaryPredicate. It is not required to satisfy Compare. |
[edit] Return value
true if an element equivalent to value is found, false otherwise.
[edit] Complexity
Given \(\scriptsize N\)N as std::distance(first, last):
At most \(\scriptsize \log_{2}(N)+O(1)\)log2(N)+O(1) comparisons with value using operator<(until C++20)std::less{}(since C++20).
At most \(\scriptsize \log_{2}(N)+O(1)\)log2(N)+O(1) applications of the comparator comp.
However, if ForwardIt is not a LegacyRandomAccessIterator, the number of iterator increments is linear in \(\scriptsize N\)N.
[edit] Notes
Although std::binary_search only requires [first, last) to be partitioned, this algorithm is usually used in the case where [first, last) is sorted, so that the binary search is valid for any value.
std::binary_search only checks whether an equivalent element exists. To obtain an iterator to that element (if exists), std::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
See also the implementations in libstdc++ and libc++.
| binary_search (1) |
|---|
| template<class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type> bool binary_search(ForwardIt first, ForwardIt last, const T& value) { return std::binary_search(first, last, value, std::less{}); } |
| binary_search (2) |
| template<class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type, class Compare> bool binary_search(ForwardIt first, ForwardIt last, const T& value, Compare comp) { first = std::lower_bound(first, last, value, comp); return (!(first == last) and !(comp(value, *first))); } |
[edit] Example
#include #include #include #include #include int main() { const auto haystack = {1, 3, 4, 5, 9}; for (const auto needle : {1, 2, 3}) { std::cout << "Searching for " << needle << '\n'; if (std::binary_search(haystack.begin(), haystack.end(), needle)) std::cout << "Found " << needle << '\n'; else std::cout << "Not found!\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::binary_search(nums.cbegin(), nums.cend(), {4, 2}, cmpz)); #else assert(std::binary_search(nums.cbegin(), nums.cend(), CD{4, 2}, cmpz)); #endif }
Output:
Searching for 1 Found 1 Searching for 2 Not found! Searching for 3 Found 3
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 270 | C++98 | Compare was required to satisfy Compare and T was requiredto be LessThanComparable (strict weak ordering required) | only a partitioning is required;heterogeneous comparisons permitted |
| LWG 787 | C++98 | at most \(\scriptsize \log_{2}(N)+2\)log2(N)+2 comparisons were allowed | corrected to \(\scriptsize \log_{2}(N)+O(1)\)log2(N)+O(1) |
[edit] See also
| | returns range of elements matching a specific key (function template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | returns an iterator to the first element not less than the given value (function template) [edit] | | | returns an iterator to the first element greater than a certain value (function template) [edit] | | | determines if an element exists in a partially-ordered range(algorithm function object)[edit] |