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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, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
ranges::binary_search
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutation operations
Numeric operations
Fold operations
Operations on uninitialized storage
Return types
 
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 [firstlast).
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 [firstlast) 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] Notes

std::ranges::binary_search doesn't return an iterator to the found element when an element whose projection equals value is found. If an iterator is desired, std::ranges::lower_bound should be used instead.

[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
    {
        auto x = ranges::lower_bound(first, last, value, comp, proj);
        return (!(x == last) && !(std::invoke(comp, value, std::invoke(proj, *x))));
    }
 
    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::move(comp), std::move(proj));
    }
};
 
inline constexpr binary_search_fn binary_search;

[edit] Example

#include <algorithm>
#include <iostream>
#include <ranges>
 
int main()
{
    constexpr static auto haystack = {1, 3, 4, 5, 9};
    static_assert(std::ranges::is_sorted(haystack));
 
    for (const int needle : std::views::iota(1)
                          | std::views::take(3))
    {
        std::cout << "Searching for " << needle << ": ";
        std::ranges::binary_search(haystack, needle)
            ? std::cout << "found " << needle << '\n'
            : 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]
returns an iterator to the first element not less than the given value
(niebloid)[edit]
returns an iterator to the first element greater than a certain value
(niebloid)[edit]
checks if the range contains the given element or subrange
(niebloid)[edit]
determines if an element exists in a partially-ordered range
(function template) [edit]