NAME¶

std::is_sorted_until - std::is_sorted_until

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt > (since C++11)
ForwardIt is_sorted_until( ForwardIt first, ForwardIt (1) (constexpr since C++20)
last );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt is_sorted_until( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, ForwardIt
last );
template< class ForwardIt, class Compare >

ForwardIt is_sorted_until( ForwardIt first, ForwardIt (3) (since C++11)
last, (constexpr since C++20)

Compare comp );
template< class ExecutionPolicy, class ForwardIt, class
Compare >

ForwardIt is_sorted_until( ExecutionPolicy&& policy, (4) (since C++17)
ForwardIt first, ForwardIt
last,

Compare comp );

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

1) Finds the largest range whether elements are sorted with respect to
operator<
(until C++20)
std::less{}
(since C++20).
3) Finds the largest range whether elements are sorted with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

Parameters¶

first, last - the range of elements to examine
policy - the execution policy to use. See execution policy for details.
comparison function object (i.e. an object that satisfies the
requirements of Compare) which returns true if the first argument is
less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the
following:

bool cmp(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
ForwardIt can be dereferenced and then implicitly converted to both of
them.

Type requirements¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
Compare must meet the requirements of Compare.

Return value¶

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

Returns last for empty ranges and ranges of length one.

Complexity¶

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) \(\scriptsize O(N)\)O(N) comparisons using
operator<
(until C++20)
std::less{}
(since C++20).
3,4) \(\scriptsize O(N)\)O(N) applications of the comparator comp.

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

See also the implementations in libstdc++ and libc++.

is_sorted_until (1)
template<class ForwardIt>
constexpr //< since C++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
{
return std::is_sorted_until(first, last, std::less<>());
}
is_sorted_until (2)
template<class ForwardIt, class Compare>
constexpr //< since C++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last, Compare comp)
{
if (first != last)
{
ForwardIt next = first;
while (++next != last)
{
if (comp(*next, *first))
return next;
first = next;
}
}
return last;
}

Example¶

// Run this code

#include <algorithm>
#include <cassert>
#include <iostream>
#include <iterator>
#include <random>
#include <string>

int main()
{
std::random_device rd;
std::mt19937 g(rd());
const int N = 6;
int nums[N] = {3, 1, 4, 1, 5, 9};

const int min_sorted_size = 4;

for (int sorted_size = 0; sorted_size < min_sorted_size;)
{
std::shuffle(nums, nums + N, g);
int *const sorted_end = std::is_sorted_until(nums, nums + N);
sorted_size = std::distance(nums, sorted_end);
assert(sorted_size >= 1);

for (const auto i : nums)
std::cout << i << ' ';
std::cout << ": " << sorted_size << " initial sorted elements\n"
<< std::string(sorted_size * 2 - 1, '^') << '\n';
}
}

Possible output:¶

4 1 9 5 1 3 : 1 initial sorted elements
^
4 5 9 3 1 1 : 3 initial sorted elements
^^^^^
9 3 1 4 5 1 : 1 initial sorted elements
^
1 3 5 4 1 9 : 3 initial sorted elements
^^^^^
5 9 1 1 3 4 : 2 initial sorted elements
^^^
4 9 1 5 1 3 : 2 initial sorted elements
^^^
1 1 4 9 5 3 : 4 initial sorted elements
^^^^^^^

See also¶

is_sorted checks whether a range is sorted into ascending order
(C++11) (function template)
ranges::is_sorted_until finds the largest sorted subrange
(C++20) (niebloid)