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Showing content from https://cplusplus.com/is_sorted below:

function template

<algorithm>

std::is_sorted default (1) 
template <class ForwardIterator>  bool is_sorted (ForwardIterator first, ForwardIterator last);
custom (2) 
template <class ForwardIterator, class Compare>  bool is_sorted (ForwardIterator first, ForwardIterator last, Compare comp);

Check whether range is sorted

Returns true if the range [first,last) is sorted into ascending order.

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>
  bool is_sorted (ForwardIterator first, ForwardIterator last)
{
  if (first==last) return true;
  ForwardIterator next = first;
  while (++next!=last) {
    if (*next<*first)     // or, if (comp(*next,*first)) for version (2)
      return false;
    ++first;
  }
  return true;
}

Parameters
first, last
Forward iterators to the initial and final positions of the 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 valuetrue if the range [first,last) is sorted into ascending order, false otherwise.

If the range [first,last) contains less than two elements, the function always returns true.



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

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

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

    // print range:
    std::cout << "foo:";
    for (int& x:foo) std::cout << ' ' << x;
    std::cout << '\n';

  } while (!std::is_sorted(foo.begin(),foo.end()));

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

  return 0;
}

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


Complexity Up to linear in one less than the distance between first and last: Compares pairs of elements until a mismatch is found.

Data races The objects in the range [first,last) are accessed.

Exceptions Throws if either an element comparison or an operation on an iterator throws.
Note that invalid arguments cause undefined behavior.

See also
is_sorted_until
Find first unsorted element in range (function template)
sort
Sort elements in range (function template)
partial_sort
Partially sort elements in range (function template)

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