# C++ Algorithm Library - is_sorted_until() Function

## Description

The C++ function std::algorithm::is_sorted_until() finds first unsorted element from the sequence. It uses operator< for comparison.

## Declaration

Following is the declaration for std::algorithm::is_sorted_until() function form std::algorithm header.

### C++11

```template <class ForwardIterator>
ForwardIterator is_sorted_until(ForwardIterator first, ForwardIterator last);
```

## Parameters

• first − Forward iterator to the initial position.

• last − Forward iterator to the final position.

## Return value

Returns an iterator to the first unsorted element. If entire range is sorted then it returns last.

## Exceptions

Throws exception if either the element comparison or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Linear.

## Example

The following example shows the usage of std::algorithm::is_sorted_until() function.

```#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(void) {
vector<int> v = {1, 2, 3, 5, 4};
auto it = is_sorted_until(v.begin(), v.end());

cout << "First unsorted element = " << *it << endl;

v[3] = 4;

it = is_sorted_until(v.begin(), v.end());

if (it == end(v))
cout << "Entire vector is sorted." << endl;

return 0;
}
```

Let us compile and run the above program, this will produce the following result −

```First unsorted element = 4
Entire vector is sorted.
```
algorithm.htm