std::partition in C++ STL

C++ has a class in its STL algorithms library which allows us easy partition algorithms using certain inbuilt functions. Partition refers to act of dividing elements of containers depending upon a given condition. 
Partition operations :
1. partition(beg, end, condition) :- This function is used to partition the elements on basis of condition mentioned in its arguments.
2. is_partitioned(beg, end, condition) :- This function returns boolean true if container is partitioned else returns false.

CPP

// C++ code to demonstrate the working of
// partition() and is_partitioned()
#include<iostream>
#include<algorithm> // for partition algorithm
#include<vector> // for vector
using namespace std;
int main()
{
    // Initializing vector
    vector<int> vect = { 2, 1, 5, 6, 8, 7 };

    // Checking if vector is partitioned
    // using is_partitioned()
    is_partitioned(vect.begin(), vect.end(), [](int x)
    {
        return x%2==0;

    })?

    cout << "Vector is partitioned":
    cout << "Vector is not partitioned";
    cout << endl;

    // partitioning vector using partition()
    partition(vect.begin(), vect.end(), [](int x)
    {
        return x%2==0;

    });

    // Checking if vector is partitioned
    // using is_partitioned()
    is_partitioned(vect.begin(), vect.end(), [](int x)
    {
        return x%2==0;

    })?

    cout << "Now, vector is partitioned after partition operation":
    cout << "Vector is still not partitioned after partition operation";
    cout << endl;

    // Displaying partitioned Vector
    cout << "The partitioned vector is : ";
    for (int &x : vect) cout << x << " ";

    return 0;

}

Output: 

Vector is not partitioned
Now, vector is partitioned after partition operation
The partitioned vector is : 2 8 6 5 1 7

In the above code, partition function partitions the vector depending on whether an element is even or odd, even elements are partitioned from odd elements in no particular order. 
3. stable_partition(beg, end, condition) :- This function is used to partition the elements on basis of condition mentioned in its arguments in such a way that the relative order of the elements is preserved..
4. partition_point(beg, end, condition) :- This function returns an iterator pointing to the partition point of container i.e. the first element in the partitioned range [beg,end) for which condition is not true. The container should already be partitioned for this function to work.

CPP

// C++ code to demonstrate the working of
// stable_partition() and partition_point()
#include<iostream>
#include<algorithm> // for partition algorithm
#include<vector> // for vector
using namespace std;
int main()
{
    // Initializing vector
    vector<int> vect = { 2, 1, 5, 6, 8, 7 };

    // partitioning vector using stable_partition()
    // in sorted order
    stable_partition(vect.begin(), vect.end(), [](int x)
    {
        return x%2 == 0;
    });

    // Displaying partitioned Vector
    cout << "The partitioned vector is : ";
    for (int &x : vect) cout << x << " ";
    cout << endl;

    // Declaring iterator
    vector<int>::iterator it1;

    // using partition_point() to get ending position of partition
    auto it = partition_point(vect.begin(), vect.end(), [](int x)
    {
        return x%2==0;
    });

    // Displaying partitioned Vector
    cout << "The vector elements returning true for condition are : ";
    for ( it1= vect.begin(); it1!=it; it1++)
    cout << *it1 << " ";
    cout << endl;

    return 0;

}

Output: 

The partitioned vector is : 2 6 8 1 5 7 
The vector elements returning true for condition are : 2 6 8

In the above code, even and odd elements are partitioned and in the increasing order (sorted). Not always in increasing order though, here the elements (even and odd) appeared in increased order so is the result after partition. if vect would have been { 2,1,7,8,6,5 } after stable_partition() it would be  { 2,8,6,1,7,5 }. The order of appearance is maintained.
5. partition_copy(beg, end, beg1, beg2, condition) :- This function copies the partitioned elements in the different containers mentioned in its arguments. It takes 5 arguments. Beginning and ending position of container, beginning position of new container where elements have to be copied (elements returning true for condition), beginning position of new container where other elements have to be copied (elements returning false for condition) and the condition. Resizing new containers is necessary for this function.

CPP

// C++ code to demonstrate the working of
// partition_copy()
#include<iostream>
#include<algorithm> // for partition algorithm
#include<vector> // for vector
using namespace std;
int main()
{
    // Initializing vector
    vector<int> vect = { 2, 1, 5, 6, 8, 7 };

    // Declaring vector1
    vector<int> vect1;

    // Declaring vector1
    vector<int> vect2;

    // Resizing vectors to suitable size using count_if() and resize()
    int n = count_if (vect.begin(), vect.end(), [](int x)
    {
        return x%2==0;

    } );
    vect1.resize(n);
    vect2.resize(vect.size()-n);

    // Using partition_copy() to copy partitions
    partition_copy(vect.begin(), vect.end(), vect1.begin(),
                                     vect2.begin(), [](int x)
    {
        return x%2==0;
    });


    // Displaying partitioned Vector
    cout << "The elements that return true for condition are : ";
    for (int &x : vect1)
            cout << x << " ";
    cout << endl;

    // Displaying partitioned Vector
    cout << "The elements that return false for condition are : ";
    for (int &x : vect2)
            cout << x << " ";
    cout << endl;

    return 0;
}

Output: 

The elements that return true for condition are : 2 6 8 
The elements that return false for condition are : 1 5 7

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