C++ Algorithm Library - is_sorted_until() Function


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


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


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


  • first − Forward iterator to the initial position.

  • last − Forward iterator to the final position.

  • comp − A binary function which accepts two arguments and returns bool.

Return value

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


Throws exception if either the binary function or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Time complexity



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

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

using namespace std;

bool ignore_case(char a, char b) {
   return (tolower(a) == tolower(b));

int main(void) {
   vector<char> v = {'A', 'b', 'C', 'd', 'E'};

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

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

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

   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 = C
Entire vector is sorted.
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