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std::ranges::equal_range

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::equal_range
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 ranges::subrange<I>

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

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

equal_range(R&& r, const T& value, Comp comp = {}, Proj proj = {});
(2) (since C++20)
1) Returns a view containing all elements equivalent to value in the range [first, last).

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 precedes 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 returned view is constructed from two iterators, one pointing to the first element that is not less than value and another pointing to the first element greater than value. The first iterator may be alternatively obtained with std::ranges::lower_bound(), the second - with std::ranges::upper_bound().

2) Same as (1), but uses r as the source range, as if using the range ranges::begin(r) as first and ranges::end(r) as last.

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 the elements to examine
value - value to compare the elements to
comp - if the first argument is less than (i.e. is ordered before) the second
proj - projection to apply to the elements

[edit] Return value

std::ranges::subrange containing a pair of iterators defining the wanted range, the first pointing to the first element that is not less than value and the second pointing to the first element greater than value.

If there are no elements not less than value, the last iterator (iterator that is equal to last or ranges::end(r)) is returned as the first element. Similarly if there are no elements greater than value, the last iterator is returned as the second element.

[edit] Complexity

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

[edit] Possible implementation

struct equal_range_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 ranges::subrange<I>
    operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const
    {
        return ranges::subrange(
            ranges::lower_bound(first, last, value, std::ref(comp), std::ref(proj)),
            ranges::upper_bound(first, last, value, std::ref(comp), std::ref(proj)));
    }
 
    template< ranges::forward_range R, class T, class Proj = std::identity,
              std::indirect_strict_weak_order<
                  const T*,
                  std::projected<std::ranges::iterator_t<R>, Proj>> Comp = ranges::less >
    constexpr ranges::borrowed_subrange_t<R>
    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 equal_range_fn equal_range;

[edit] Example

#include <algorithm>
#include <compare>
#include <vector>
#include <iostream>
 
struct S
{
    int number;
    char name;
    // note: name is ignored by these comparison operators
    friend bool operator==  ( const S s1, const S s2 ) { return s1.number == s2.number; }
    friend auto operator<=> ( const S s1, const S s2 ) { return s1.number <=> s2.number; }
};
 
int main()
{
    // note: not ordered, only partitioned w.r.t. S defined below
    std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4, 'D'}, {4,'G'}, {3,'F'} };
 
    S value = {2, '?'};
 
    namespace ranges = std::ranges;
 
    {
        auto p = ranges::equal_range(vec, value);
 
        for ( auto i : p )
            std::cout << i.name << ' ';
    }
    {
        auto p = ranges::equal_range(vec.begin(), vec.end(), value);
 
        for ( auto i = p.begin(); i != p.end(); ++i )
            std::cout << i->name << ' ';
    }
 
    std::cout << '\n';
 
    {
        auto p = ranges::equal_range(vec, 'D', ranges::less{}, &S::name);
 
        for ( auto i : p )
            std::cout << i.name << ' ';
    }
    {
        auto p = ranges::equal_range(vec.begin(),vec.end(), 'D', ranges::less{}, &S::name);
 
        for ( auto i = p.begin(); i != p.end(); ++i )
            std::cout << i->name << ' ';
    }
}

Output:

B C D B C D
D D D D

[edit] See also

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