std::binary_search(3)

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)