What is the difference between Set vs Hashset in C++?

In C++, both set and HashSet(also called unordered_set) are used to store elements but they have different properties and use cases. In this article, we will learn the key differences between a set and a HashSet in C++.

Set in C++

In C++, a std::set is a container that stores only the unique elements in a sorted fashion. We can add duplicate elements in any random order but when we retrieve the elements we will get the unique elements in a sorted manner only. Following are the important points regarding set in C++:

• Set are implemented using self-balancing Binary Search Trees, therefore they are ordered.
• Elements in a set are always sorted in increasing order.
• The time complexity for searching in a set is O(log(n)).

Syntax to Declare Set

set <dataType> setName;

Example

The below example demonstrates the use of a set in C++.

// C++ program to demonstrate the use of set container

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

int main()
{
    // creating a set of integer type
    set<int> st;

    // Inserting values in random order and with duplicates
    // in a set
    st.insert(10);
    st.insert(5);
    st.insert(10);
    st.insert(15);

    // printing the element in a set
    for (auto it : st) {
        cout << it << ' ';
    }
    return 0;
}

5 10 15 

Time Complexity: O(log n), where n is the number of elements in the set,
Auxilliary Space: O(n)

HashSet in C++

In C++ , a HashSet or std::unordered_set both are same. It is a container which is also used to store elements but unlike set it does not store elements in a sorted order. Following are some important points about HashSet:

• HashSet is not part of the C++ standard. It’s an extension and its equivalent is unordered_set in the C++ standard library.
• HashSet is implemented using hash tables.
• The key element, is hashed into an index of the hash table and gets stored at that particular index.
• The time complexity to search an element in a HashSet is O(1), which is faster than a set.
• When we iterate through a HashSet, the order of elements will be random.

Syntax to Declare HashSet

unordered_set<dataType> unordered_set_name;

Example

The below example demonstrates the use of a HashSet in C++.

// C++ program to demonstrate the use of HashSet container

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

int main()
{
    // creating a HashSet of integer type
    unordered_set<int> ust;

    // Inserting values in random order and with duplicates
    // in a HashSet
    ust.insert(10);
    ust.insert(5);
    ust.insert(10);
    ust.insert(15);

    // printing the element in a set
    for (auto it : ust) {
        cout << it << ' ';
    }
    return 0;
}

15 5 10 

Time Complexity: O(1), but can degrade to O(n) in the worst case due to hash collisions.
Auxilliary Space: O(n)

Difference Between Set and HashSet in C++

To understand the difference between Set and HashSet in C++, we need to know what each container does.

Conclusion

In conclusion, to choose between Set and HashSet depends on our specific needs. If we need the elements to be sorted, we can use a set. If we need faster access and searching and don’t care about the order of elements, we can use a HashSet instead. The basic understanding of the key differences between these two containers is important for implementing them.