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std::is_sorted_until in C++

  • Difficulty Level : Hard
  • Last Updated : 18 Feb, 2021

std::is_sorted_until is used to find out the first unsorted element in the range [first, last). It returns an iterator to the first unsorted element in the range, so all the elements in between first and the iterator returned are sorted.
It can also be used to count the total no. of sorted elements in the range. It is defined inside the header file . In case, the whole range is sorted, it will return an iterator pointing to last.
It can be used in two ways as shown below: 

Comparing elements using “<“: 
Syntax: 

template 
ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last);

first: Forward iterator to the first element in the list.
last: forward iterator to the last element in the list.

Return Value: It returns an iterator to the first 
unsorted element in the list.
It returns last in case if there is only one element in 
the list or if all the elements are sorted.

CPP




// C++ program to demonstrate the use of std::is_sorted_until
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
    int v[] = { 1, 2, 3, 4, 7, 10, 8, 9 }, i;
 
    int* ip;
 
    // Using std::is_sorted_until
    ip = std::is_sorted_until(v, v + 8);
 
    cout << "There are " << (ip - v) << " sorted elements in "
         << "the list and the first unsorted element is " << *ip;
 
    return 0;
}

Output: 

There are 6 sorted elements in the list and the first unsorted element is 8

By comparing using a pre-defined function:
Syntax:

template 
 ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last,
                                  Compare comp);

Here, first and last are the same as previous case.

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: It returns an iterator 
to the first unsorted element in the list.
It returns last in case if there is only one element in 
the list or if all the elements are sorted.

CPP




// C++ program to demonstrate
// the use of std::nth_element
// C++ program to demonstrate the
// use of std::nth_element
#include <algorithm>
#include <iostream>
using namespace std;
 
// Defining the BinaryFunction
bool comp(int a, int b) { return (a < b); }
int main()
{
    int v[] = { 1, 3, 20, 10, 45, 33, 56, 23, 47 }, i;
    int* ip;
 
    // Using std::is_sorted_until
    ip = std::is_sorted_until(v, v + 9, comp);
 
    cout << "There are " << (ip - v)
         << " sorted elements in "
         << "the list and the first unsorted element is "
         << *ip;
 
    return 0;
}

Output:

There are 3 sorted elements in the list and the first unsorted element is 10

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