std::binary_search
| Defined in header <algorithm>
|
||
template< class ForwardIt, class T >
bool binary_search( ForwardIt first, ForwardIt last,
const T& value );
|
(1) | (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) | |
template< class ForwardIt, class T, class Compare >
bool binary_search( ForwardIt first, ForwardIt last,
const T& value, Compare comp );
|
(2) | (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 exists in the partitioned source range [first, last). An element is considered equivalent to value if it neither orders before nor orders after value.
operator<(until C++20)std::less{}(since C++20).bool(e < value) and !bool(value < e)(until C++20)bool(std::less{}(e, value)) and !bool(std::less{}(value, e))(since C++20). If any of the following conditions is satisfied, the behavior is undefined:
- The elements
eof the source range are not partitioned with respect to these two expressions at the same time. - There exists an element
ein the source range such that these two expressions yield different values.
comp.bool(comp(e, value)) and !bool(comp(value, e)). If any of the following conditions is satisfied, the behavior is undefined:
- The elements
eof the source range are not partitioned with respect to these two expressions at the same time. - There exists an element
ein the source range such that these two expressions yield different values.
Parameters
| first, last | - | the pair of iterators defining the source range |
| 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:
While the signature does not need to have |
| Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
| ||
-Compare must meet the requirements of BinaryPredicate. It is not required to satisfy Compare.
| ||
Return value
true if an element equivalent to value exists, false otherwise.
Complexity
Given N as std::distance(first, last):
value using operator<(until C++20)std::less{}(since C++20).comp.Notes
Although std::binary_search only requires the source range to be partitioned, this algorithm is usually used in the case where the source range 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.
If ForwardIt is not a LegacyRandomAccessIterator, the number of iterator increments is linear in std::distance(first, last).
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | List-initialization for algorithms (1,2) |
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)));
}
|
Example
#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
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<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::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
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 log2(N)+2 comparisons were allowed | corrected to 2log2(N)+𝓞(1) |
See also
(C++20) |
determines if an element exists in a range using binary search (algorithm function object) |
| finds the range of elements matching the given value using binary search (function template & algorithm function object) | |
(C++20) |
|
| finds the first element not less than the given value using binary search (function template & algorithm function object) | |
(C++20) |
|
| finds the first element greater than the given value using binary search (function template & algorithm function object) | |
(C++20) |