std::includes() in C++ STL

C++ defines a function which can be used to recognise if all the numbers in a container, also exists in other container. This task can be achieved by using “includes()” which is defined in “algorithm” header. The implementations 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 final position of first sorted sequence.

beg2 :  Input iterator to initial position of second sorted sequence.
end2 :  Input iterator to final position of second sorted sequence.

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

// C++ code to demonstrate the working of 
// includes() implementation 1

#include<bits/stdc++.h>
using namespace std;

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 final position of first sorted sequence.

beg2 :  Input iterator to initial position of second sorted sequence.
end2 :  Input iterator to final position of second sorted sequence.

comp : The comparator function that returns a boolean
true/false of the each elements compared. This function 
accepts two arguments. This can be function pointer or 
function object and cannot change values.

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

// C++ code to demonstrate the working of 
// includes() implementation 2

#include<bits/stdc++.h>
using namespace std;

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

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";
    
}

Output:

All elements of 2nd container are in 1st container

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

// C++ code to demonstrate the application of 
// includes() 

#include<bits/stdc++.h>
using namespace std;

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 lottey numbers )";
    else 
    cout << "User has not won the lottery";
    
}

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