std::is_partitioned in C++

 bool is_partitioned (InputIterator first, 
                      InputIterator last, UnaryPredicate pred);

first: Input iterator to the first element in the range.
last: Input iterator to the last element in the range.
pred: Unary function that accepts an element in the 
range as argument, and returns a value convertible to bool. 
The value returned indicates whether the element belongs to
the first group (if true, the element is expected before all
the elements for which it returns false).
The function shall not modify its argument.
This can either be a function pointer or a function object.


Returns: It returns true if all the elements in the range [first, last)
for which pred returns true precede those for which it returns false.
Otherwise it returns false.
If the range is empty, the function returns true.

// C++ program to demonstrate the use of std::is_partitioned
#include <iostream>
#include <algorithm>
#include <vector>

// Defining the BinaryFunction
bool pred(int a)
{
    return (a % 3 == 0);
}

using namespace std;
int main()
{
    // Declaring first vector
    vector<int> v1 = { 3, 6, 9, 10, 11, 13 };

    // Using std::is_partitioned
    bool b = std::is_partitioned(v1.begin(), v1.end(), pred);

    if (b == 1) {
        cout << "It is partitioned";
    } else {
        cout << "It is not partitioned.";
    }
    return 0;
}

It is partitioned

Another Example

• To check whether all the odd and even elements are partitioned CPP

  // C++ program to demonstrate the use of std::is_partitioned
  #include <iostream>
  #include <algorithm>
  #include <vector>
  
  // Defining the BinaryFunction
  bool pred(int a)
  {
      return (a % 2 == 0);
  }
  
  using namespace std;
  int main()
  {
      // Declaring first vector
      vector<int> v1 = { 2, 4, 6, 3, 5, 7, 9 };
  
      // Using std::is_partitioned
      bool b = std::is_partitioned(v1.begin(), v1.end(), pred);
  
      if (b == 1) {
          cout << "All the even no. are present before odd no.";
      } else {
          cout << "All the even no. are not present before odd no.";
      }
  
      // Inserting an even no. at the end of v1
      // so std::is_partitioned returns false
      v1.push_back(16);
  
      // Now again using std::is_partitioned
      b = std::is_partitioned(v1.begin(), v1.end(), pred);
  
      if (b == 1) {
          cout << "\nAll the even no. are present before odd no.";
      } else {
          cout << "\nAll the even no. are not present before odd no.";
      }
  
      return 0;
  }
  

  Output:

  All the even no. are present before odd no.
  All the even no. are not present before odd no.