function template
<algorithm>

std::is_sorted_until

default (1)
template <class ForwardIterator>
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last);
custom (2)
template <class ForwardIterator, class Compare>
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last,
                                   Compare comp);
Find first unsorted element in range
Returns an iterator to the first element in the range [first,last) which does not follow an ascending order.

The range between first and the iterator returned is sorted.

If the entire range is sorted, the function returns last.

The elements are compared using operator< for the first version, and comp for the second.

The behavior of this function template is equivalent to:
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template <class ForwardIterator>
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last)
{
  if (first==last) return first;
  ForwardIterator next = first;
  while (++next!=last) {
    if (*next<*first) return next;
    ++first;
  }
  return last;
}


Parameters

first, last
Forward iterators to the initial and final positions in a sequence. The range checked is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
comp
Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return value

An iterator to the first element in the range which does not follow an ascending order, or last if all elements are sorted or if the range contains less than two elements.

Example

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// is_sorted_until example
#include <iostream>     // std::cout
#include <algorithm>    // std::is_sorted_until, std::prev_permutation
#include <array>        // std::array

int main () {
  std::array<int,4> foo {2,4,1,3};
  std::array<int,4>::iterator it;

  do {
    // try a new permutation:
    std::prev_permutation(foo.begin(),foo.end());

    // print range:
    std::cout << "foo:";
    for (int& x:foo) std::cout << ' ' << x;
    it=std::is_sorted_until(foo.begin(),foo.end());
    std::cout << " (" << (it-foo.begin()) << " elements sorted)\n";

  } while (it!=foo.end());

  std::cout << "the range is sorted!\n";

  return 0;
}


Output:
foo: 2 3 4 1 (3 elements sorted)
foo: 2 3 1 4 (2 elements sorted)
foo: 2 1 4 3 (1 elements sorted)
foo: 2 1 3 4 (1 elements sorted)
foo: 1 4 3 2 (2 elements sorted)
foo: 1 4 2 3 (2 elements sorted)
foo: 1 3 4 2 (3 elements sorted)
foo: 1 3 2 4 (2 elements sorted)
foo: 1 2 4 3 (3 elements sorted)
foo: 1 2 3 4 (4 elements sorted)
the range is sorted!

Complexity

Up to linear in the distance between first and last: Calls comp for each element until a mismatch is found.

Data races

Some (or all) of the objects in the range [first,last) are accessed (once at most).

Exceptions

Throws if either comp or an operation on an iterator throws.
Note that invalid parameters cause undefined behavior.

See also

is_sorted
Check whether range is sorted (function template )
sort
Sort elements in range (function template )
find_if
Find element in range (function template )

C++