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std::ranges::is_sorted_until

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
ranges::is_sorted_until
Binary search operations
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 Proj = std::identity,

          std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less >

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

          std::indirect_strict_weak_order<
              std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
  constexpr ranges::borrowed_iterator_t<R>

is_sorted_until( R&& r, Comp comp = {}, Proj proj = {} );
(2) (since C++20)

Examines the range [first, last) and finds the largest range beginning at first in which the elements are sorted in non-descending order.

A sequence is sorted with respect to a comparator comp if for any iterator it pointing to the sequence and any non-negative integer n such that it + n is a valid iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj, *(it + n)), std::invoke(proj, *it)) evaluates to false.

1) Elements are compared using the given binary comparison function comp.
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.

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 - iterator-sentinel defining the range to find its sorted upper bound
r - the range to find its sorted upper bound
comp - comparison function to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

The upper bound of the largest range beginning at first in which the elements are sorted in non-descending order. That is, the last iterator it for which range [first, it) is sorted.

[edit] Complexity

Linear in the distance between first and last.

[edit] Possible implementation

struct is_sorted_until_fn {
  template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
           std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
  constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
  {
      if (first == last) {
          return first;
      }
 
      auto next = first;
      while (++next != last) {
          if (std::invoke(comp, std::invoke(proj, *next), std::invoke(proj, *first))) {
              return next;
          }
          first = next;
      }
 
      return first;
  }
 
  template< ranges::forward_range R, class Proj = std::identity,
          std::indirect_strict_weak_order<
              std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
  constexpr ranges::borrowed_iterator_t<R>
  operator()( R&& r, Comp comp = {}, Proj proj = {} ) const
  {
      return (*this)(ranges::begin(r), ranges::end(r),
                     std::ref(comp), std::ref(proj));
  }
};
 
inline constexpr is_sorted_until_fn is_sorted_until;

[edit] Notes

ranges::is_sorted_until returns an iterator equal to last for empty ranges and ranges of length one.

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
 
int main()
{
    std::random_device rd;
    std::mt19937 g{rd()};
    std::array nums {3, 1, 4, 1, 5, 9};
 
    constexpr int min_sorted_size = 4;
    int sorted_size = 0;
    do {
        std::ranges::shuffle(nums, g);
        const auto sorted_end = std::ranges::is_sorted_until(nums);
        sorted_size = std::ranges::distance(nums.begin(), sorted_end);
 
        std::ranges::copy(nums, std::ostream_iterator<int>(std::cout, " "));
        std::cout << " : " << sorted_size << " leading sorted element(s)\n";
    } while (sorted_size < min_sorted_size);
}

Possible output:

4 1 9 5 1 3  : 1 leading sorted element(s)
4 5 9 3 1 1  : 3 leading sorted element(s)
9 3 1 4 5 1  : 1 leading sorted element(s)
1 3 5 4 1 9  : 3 leading sorted element(s)
5 9 1 1 3 4  : 2 leading sorted element(s)
4 9 1 5 1 3  : 2 leading sorted element(s)
1 1 4 9 5 3  : 4 leading sorted element(s)

[edit] See also

checks whether a range is sorted into ascending order
(niebloid) [edit]
finds the largest sorted subrange
(function template) [edit]