std::includes() in C++ STL

includes() is a C++ function that can be used to recognize if all the numbers in a container, also exist in other containers. It helps to check whether a set is a subset of another set or not considering the set is ordered. The elements are expected to be in sorted order. This function is defined in <algorithm> header file. 

There are two implementations possible and are explained below:

Syntax 1: Using operator “<“

 Template:

template 
  bool includes (initer1 beg1, initer1 end1,
                        initer2 beg2, initer2 end2,)

Parameters:

• beg1:  Input iterator to initial position of first sorted sequence.
• end1:  Input iterator to the final position of the first sorted sequence.
• beg2:  Input iterator to initial position of the second sorted sequence.
• end2:  Input iterator to the final position of the second sorted sequence.

Return value: True, if every element of 2nd container lies in 1st container.

It has linear time complexity.

CPP

// C++ code to demonstrate the working of includes()
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
    // initializing 1st container
    vector<int> arr1 = { 1, 4, 6, 3, 2 };

    // initializing 2nd container
    vector<int> arr2 = { 1, 2, 4 };

    // sorting initial containers
    sort(arr1.begin(), arr1.end());
    sort(arr2.begin(), arr2.end());

    // using include() check if all elements
    // of arr2 lie in arr1
    if (includes(arr1.begin(), arr1.end(), arr2.begin(),
                 arr2.end()))
        cout << "All elements of 2nd container are in 1st "
                "container";
    else
        cout << "All elements of 2nd container are not in "
                "1st container";
}

Output

All elements of 2nd container are in 1st container

Syntax 2: Using Comparator

Template:

template 
  bool includes (initer1 beg1, initer1 end1,
                        initer2 beg2, initer2 end2, Compare comp)

Parameters:

• beg1:  Input iterator to initial position of first sorted sequence.
• end1:  Input iterator to the final position of the first sorted sequence.
• beg2:  Input iterator to initial position of the second sorted sequence.
• end2:  Input iterator to the final position of the second sorted sequence.
• comp: The comparator function that returns a boolean true/false of each element compared. This function accepts two arguments. This can be a function pointer or function object and cannot change values.

Return value: True if every element of the 2nd container lies in the 1st container.

CPP

// C++ code to demonstrate the working of
// includes()
#include <bits/stdc++.h>
using namespace std;

// comparator function
bool comp(int i, int j) { return i < j; }

// Driver Code
int main()
{
    // initializing 1st container
    vector<int> arr1 = { 1, 4, 6, 3, 2 };

    // initializing 2nd container
    vector<int> arr2 = { 1, 2, 4 };

    // sorting initial containers
    sort(arr1.begin(), arr1.end());
    sort(arr2.begin(), arr2.end());

    // using include() check if all elements
    // of arr2 lie in arr1
    // using comparator function
    if (includes(arr1.begin(), arr1.end(), arr2.begin(),
                 arr2.end(), comp))
        cout << "All elements of 2nd container are in 1st "
                "container";
    else
        cout << "All elements of 2nd container are not in "
                "1st container";
    return 0;
}

Output

All elements of 2nd container are in 1st container

Application: This function can be potentially used to find if a set is a subset of another or can be used to determine the lottery winner, in a system in which a person who has all the lottery numbers in his card wins the lottery. The latter one is explained below through code.

CPP

// C++ code to demonstrate the application of
// includes()
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
    // lottery numbers
    vector<int> lottery = { 1, 4, 6, 3, 2, 54, 32 };

    // Numbers in user's card
    vector<int> user = { 1, 2, 4, 6 };

    // sorting initial containers
    sort(lottery.begin(), lottery.end());
    sort(user.begin(), user.end());

    // using include() check if all elements
    // of user are present as lottery numbers
    if (includes(lottery.begin(), lottery.end(),
                 user.begin(), user.end()))
        cout << "User has won lottery ( all numbers are "
                "lottery numbers )";
    else
        cout << "User has not won the lottery";
}

Output

User has won lottery ( all numbers are lottery numbers )