std::binary_search

 

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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

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, i.e. it must satisfy all of the following requirements:

• partitioned with respect to element < value or comp(element, value)
• 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
comp	-	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); The signature does not need to have const &, but the function must not modify the objects passed to it. 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 ForwardIterator.
- 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-RandomAccessIterators, number of iterator increments is linear.

Possible implementation

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 strict weak ordering	only a partitioning is needed; heterogeneous comparisons permitted

See also

equal_range

returns range of elements matching a specific key (function template)