NAME¶

std::binary_search - std::binary_search

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class T >
bool binary_search( ForwardIt first, ForwardIt (until C++20)
last, const T& value );
template< class ForwardIt, class T >
constexpr bool binary_search( ForwardIt first, (since C++20)
ForwardIt last, const T& value );
template< class ForwardIt, class T, class (1)
Compare > (until C++20)
bool binary_search( ForwardIt first, ForwardIt
last, const T& value, Compare comp ); (2)
template< class ForwardIt, class T, class
Compare > (since C++20)
constexpr bool binary_search( ForwardIt first,
ForwardIt last, const T& value, Compare comp );

Checks if an element equivalent to value appears within the range [first, last).

For std::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 element < value or comp(element, value) (that is,
all elements for which the expression is true precede all elements for which the
expression is false)
* partitioned with respect to !(value < element) or !comp(value, element)
* for all elements, if element < value or comp(element, value) is true then
!(value < element) or !comp(value, element) is also true

A fully-sorted range meets these criteria.

The first version uses operator< to compare the elements, the second version uses
the given comparison function comp.

Parameters¶

first, last - the range of elements to examine
value - value to compare the elements to
binary predicate which returns true if the first argument is less
than (i.e. 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);

comp - 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

Return value¶

true if an element equal to value is found, false otherwise.

Complexity¶

The number of comparisons performed is logarithmic in the distance between first and
last (At most log
2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators,
number of iterator increments is linear.

Possible implementation¶

See also the implementations in libstdc++ and libc++.

First version¶

template<class ForwardIt, class T>
bool binary_search(ForwardIt first, ForwardIt last, const T& value)
{
first = std::lower_bound(first, last, value);
return (!(first == last) && !(value < *first));
}

Second version¶

template<class ForwardIt, class T, 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) && !(comp(value, *first)));
}

Example¶

// Run this code

#include <iostream>
#include <algorithm>
#include <vector>

int main()
{
std::vector<int> haystack {1, 3, 4, 5, 9};
std::vector<int> needles {1, 2, 3};

for (auto needle : needles) {
std::cout << "Searching for " << needle << '\n';
if (std::binary_search(haystack.begin(), haystack.end(), needle)) {
std::cout << "Found " << needle << '\n';
} else {
std::cout << "no dice!\n";
}
}
}

Output:¶

Searching for 1
Found 1
Searching for 2
no dice!
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 be a only a partitioning is needed;
strict weak ordering heterogeneous comparisons permitted

See also¶

equal_range returns range of elements matching a specific key
(function template)
returns an iterator to the first element not less than the
lower_bound given value
(function template)
returns an iterator to the first element greater than a
upper_bound certain value
(function template)
ranges::binary_search determines if an element exists in a partially-ordered range
(C++20) (niebloid)