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std::ranges::binary_search

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
ranges::binary_search
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
Defined in header <algorithm>
Call signature
template< std::forward_iterator I, std::sentinel_for<I> S, class T,

          class Proj = std::identity,
          std::indirect_strict_weak_order<
              const T*,
              std::projected<I, Proj>> Comp = ranges::less >
constexpr bool

binary_search( I first, S last, const T& value, Comp comp = {}, Proj proj = {} );
(1) (since C++20)
template< ranges::forward_range R, class T, class Proj = std::identity,

          std::indirect_strict_weak_order<
              const T*,
              std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
constexpr bool

binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} );
(2) (since C++20)
1) Checks if a projected element equivalent to value appears within the range [first, last).
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

For ranges::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 std::invoke(comp, std::invoke(proj, element), value) (that is, all projected elements for which the expression is true precedes all elements for which the expression is false)
  • partitioned with respect to !std::invoke(comp, value, std::invoke(proj, element))
  • for all elements, if std::invoke(comp, std::invoke(proj, element), value) is true then !std::invoke(comp, value, std::invoke(proj, element)) is also true

A fully-sorted range meets these criteria.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

first, last - the range of elements to examine
r - the range of elements to examine
value - value to compare the elements to
comp - comparison function to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

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

[edit] Complexity

The number of comparisons and projections performed is logarithmic in the distance between first and last (At most log
2
(last - first) + O(1)
comparisons and projections). However, for iterator-sentinel pair that does not model std::random_access_iterator, number of iterator increments is linear.

[edit] Possible implementation

struct binary_search_fn {
    template<std::forward_iterator I, std::sentinel_for<I> S, class T,
             class Proj = std::identity,
             std::indirect_strict_weak_order<
                 const T*,
                 std::projected<I, Proj>> Comp = ranges::less>
    constexpr bool
    operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const
    {
        first = std::lower_bound(first, last, value, comp);
        return (!(first == last) && !(comp(value, *first)));
    }
 
    template<ranges::forward_range R, class T, class Proj = std::identity,
             std::indirect_strict_weak_order<
                const T*,
                std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr bool operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value,
                       std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr binary_search_fn binary_search;

[edit] Example

#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 (ranges::binary_search(haystack, 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

[edit] See also

returns range of elements matching a specific key
(niebloid) [edit]
determines if an element exists in a certain range
(function template) [edit]