C++ Set

Sets are STL containers that store unique elements of the same type in a sorted manner.

As the value of every element in a set is unique, the value itself acts as the key for identifying the element.

Set Properties

C++ set has the following properties.

1. Unique Elements

No two elements in a set can be equal.

2. Immutable

We can add or remove elements from a set but we can't change the values of existing elements.

3. Sorted Order

Elements in a set are sorted following a specific strict weak ordering.

By default, C++ sets are sorted in ascending order but we have the option to change that.

4. Associative

Elements of a set are referenced using their key, not by their position in the container.

This is unlike arrays where elements are accessed using indexes.

Create a Set

To implement a set in C++, we must include the <set> header file in our program.

#include <set>

We can create a set in C++ using the following syntax:

// declare a set
std::set<data_type> set_name = {key1, key2, key3, ...};

Here,

• std::set - declares an STL container of type set.
• <data_type> - the data type of the values to be stored in the set.
• set_name - a unique name given to the set.
• key1, key2, key3, ... - key/value to be stored in the set.

For example,

// initialize set with elements
std::multiset<int> my_set1 = {5, 3, 8, 1, 3};

// create an empty set
std::multiset<int> my_set2;

Note: Moving forward, we will be using the std namespace so we can omit std:: from our syntax.

Example 1: Create a Set

#include <iostream>
#include <set>
using namespace std;

int main() {

    set<int> my_set = {5, 3, 8, 1, 3};
    
    for(int val : my_set) {
        cout << val << " ";
    }

    return 0;
}

Output

1 3 5 8 

Here, we have created a set of type int.

As you can see, the set returned the values in a sorted manner and ignored the repeated instance of 3.

// this is not allowed in a set
my_set[4] = 4

Sort the Set in Descending Order

To get the elements of set sorted in descending order, we can modify our syntax as:

set<int, greater<int>> my_set;

For example,

#include <iostream>
#include <set>
using namespace std;

int main() {

    set<int, greater<int>> my_set = {5, 3, 8, 1, 3};
    
    for(int val : my_set) {
        cout << val << " ";
    }

    return 0;
}

Output

8 5 3 1 

As can be seen from the output, the set now returns the elements sorted in descending order.

C++ Set Methods

In C++, the set class provides various methods to perform various operations in a set.

Example 2: Add Values to a Set

#include <iostream>     
#include <set>          
using namespace std;
   
int main () {

    set<int> my_set;

    // add values to the set
    my_set.insert(10);
    my_set.insert(30);
    my_set.insert(20);
    my_set.insert(50);
    my_set.insert(40);
    my_set.insert(50);
    
    // print the set elements
    for (int i : my_set) {
        cout << i << "  ";
    }
    
    return 0;
}

Output

10  20  30  40  50  

Here, we have created an empty set my_set of type int and added values to it using the insert() method.

Notice that the duplicate insertions are removed, and the set is sorted in ascending order despite the order of insertion being different.

Example 3: Delete Elements From a Set

#include <iostream>     
#include <set>          
using namespace std;
   
int main () {

    set<int> my_set = {10, 20, 30, 40};
    
    // set before deletion
    cout << "The set before deletion: ";
    for (int i : my_set) {
        cout << i << "  ";
    }

    // delete values from the set
    my_set.erase(10);
    my_set.erase(20);
    
    // set after deletion
    cout << "\nThe set after deletion: ";
    for (int i : my_set) {
        cout << i << "  ";
    }

    // delete all elements from the set
    my_set.clear();

    // set after clearing all elements
    cout << "\nThe set after clearing all elements: ";
    for (int i : my_set) {
        cout << i << "  ";
    }
    
    return 0;
}

Output

The set before deletion: 10  20  30  40  
The set after deletion: 30  40
The set after clearing all elements:   

Here, we have used the erase() method to delete individual set elements.

Then, we used the clear() method to delete all elements of the set.

Example 4: C++ Set empty() and size() Methods

In C++, the empty() and size() methods are called capacity methods.

The empty() method returns

• 1 (true) - if the set is empty
• 0 (false) - if the set is not empty

For example,

#include <iostream>     
#include <set>          
using namespace std;
   
int main () {

    set<int> my_set = {10, 20, 30};

    // set before clear
    cout << "The set before clear: ";
    for (int i : my_set) {
        cout << i << "  ";
    }
    
    // check if the set is empty
    cout << "\nEmpty: " << my_set.empty() << endl;

    // check the size of the set
    cout << "Size: " << my_set.size() << endl;
    
    // clear values from the set
    my_set.clear();
    
    // set after clear
    cout << "\nThe set after clear: ";
    for (int i : my_set) {
        cout << i << "  ";
    }
    
    // use the capacity methods again
    cout << "\nEmpty: " << my_set.empty() << endl;
    cout << "Size: " << my_set.size() << endl;

    return 0;
}

Output

The set before clear: 10  20  30  
Empty: 0
Size: 3

The set after clear: 
Empty: 1
Size: 0

Here, we have used the empty() and size() methods before and after clearing the contents of the set.

Before clearing the contents:

• empty() returns 0, indicating that the set isn't empty.
• size() returns 3, indicating that there are three elements in the set.

After clearing the contents:

• empty() returns 1, indicating that the set is empty.
• size() returns 0, indicating that there are no elements in the set.