Difference Between set, multiset, unordered_set, unordered_multiset in C++ (original) (raw)

Last Updated : 26 Feb, 2024

In C++ Standard Template Library, set, multiset, unordered_set, unordered_multiset are used to store elements. Although they are similar but differ from each other in some functionalities. The differences are discussed below:

**1. Set: Sets are associative containers that store unique elements following a specific order.Following are the properties of sets:

• Stores the values in sorted order.
• Stores only unique values.
• Elements can only be inserted or deleted but cannot be modified.
• We can erase more than 1 element by giving the start iterator and end iterator position.
• Traversal using iterators.
• Sets are implemented as Binary Search Tree.

**Syntax:

set setname;

The following example demonstrates the application of set.

CPP `

// CPP program to demonstrate insert and // delete in set #include <bits/stdc++.h> using namespace std;

// Driver code int main() { // set declare set s;

// Elements added to set
s.insert(12);
s.insert(10);
s.insert(2);
// duplicate added
s.insert(10);
s.insert(90);
s.insert(85);
s.insert(45);

// Iterator declared to traverse
// set elements
set<int>::iterator it, it1, it2;
cout << "Set elements (Note : sorted "
        "and duplcayes removed) \n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';
cout << '\n';

it1 = s.find(10);
it2 = s.find(90);

// elements from 10 to elements before
// 90 erased
s.erase(it1, it2);
cout << "Set Elements after erase:\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';

return 0;

}

`

Output

Set elements (Note : sorted and duplcayes removed) 2 10 12 45 85 90 Set Elements after erase: 2 90

**2. Multisets: Multisets are associative containers that store multiple elements having equivalent values following a specific order. Following are the properties of multisets:

• Stores elements in sorted order.
• It allows the storage of multiple elements.
• We can erase more than 1 element by giving the start iterator and end iterator.

**Note: All other properties are similar to the set.

**Syntax:

multiset multisetName;

The following example demonstrates the application of Multiset.

CPP `

// CPP program to demonstrate insert and // delete in set #include <bits/stdc++.h> using namespace std;

// Driver Code int main() { // multiset declare multiset s;

// Elements added to set
s.insert(12);
s.insert(10);
s.insert(2);
// duplicate added
s.insert(10);
s.insert(90);
s.insert(85);
s.insert(45);

// Iterator declared to traverse
// set elements
multiset<int>::iterator it, it1, it2;
cout << "Multiset elements (Note : Sorted and "
  " duplicates allowed) \n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';
cout << '\n';

it1 = s.find(10);
it2 = s.find(90);

// elements from 10 to elements before 90
// erased
s.erase(it1, it2);

cout << "Multiset Elements after erase:\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';

return 0;

}

`

Output

Multiset elements (Note : Sorted and duplicates allowed) 2 10 10 12 45 85 90 Multiset Elements after erase: 2 90

**3. unordered_set: unordered_set are associative containers that store unique elements in no particular order. Following are the properties of Unordered_sets:

• Elements can be stored in any order. ( no sorted order )
• Stores only unique values.
• Hash-table used to store elements.
• We can erase only the element for which the iterator position is given.

**Note: All other properties are similar to the set.

**Syntax:

unordered_set setname;

The following example demonstrates the application of Unordered set.

CPP `

// CPP program to demonstrate insert and // delete in unordered_set #include <bits/stdc++.h> using namespace std; int main() { // unordered_set declare unordered_set s;

// Elements added to set
s.insert(12);
s.insert(10);
s.insert(2);
// duplicate added
s.insert(10);
s.insert(90);
s.insert(85);
s.insert(45);
s.insert(12);
s.insert(70);

// Iterator declared to traverse
// set elements
unordered_set<int>::iterator it, it1;
cout << "Unordered_set elements (Note : "
       "Not sorted):\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';
cout << '\n';

it1 = s.find(10);

// element 10 erased
s.erase(it1);
cout << "Unordered_set Elements after erase:\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';

return 0;

}

`

Output

Unordered_set elements (Note : Not sorted): 70 85 45 12 10 2 90 Unordered_set Elements after erase: 70 85 45 12 2 90

**4. Unordered_multiset: Unordered_multiset is an associative container that contains a set of non-unique elements in unsorted order. Following are the properties of Unordered_multiset:

• Elements can be stored in any order.
• Duplicate elements can be stored.
• Hash-table used to store elements.
• We can erase only the element for which the iterator position is given.

**Note: All other properties are similar to the set.

**Syntax:

unordered_multiset multisetName;

The following example demonstrates the application of Unordered multiset.

CPP `

// CPP program to demonstrate insert and // delete in unordered_multiset #include <bits/stdc++.h> using namespace std; int main() { // unordered_multiset declare unordered_multiset s;

// Elements added to set
s.insert(12);
s.insert(10);
s.insert(2);
// duplicate added
s.insert(10);
s.insert(90);
s.insert(85);
s.insert(45);

// Iterator declared to traverse
// set elements
unordered_multiset<int>::iterator it, it1;
cout << "Unordered-Multiset (Note : Elements are "
  " not sorted and duplciates are allowed :\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';
cout << '\n';

it1 = s.find(10);

// element 10 trained
s.erase(it1);

cout << "Unordered-Multiset Elements after "
        "erase:\n";
for (it = s.begin(); it != s.end(); it++)
    cout << *it << ' ';

return 0;

}

`

Output

Unordered-Multiset (Note : Elements are not sorted and duplciates are allowed : 85 45 12 90 2 10 10 Unordered-Multiset Elements after erase: 85 45 12 90 2 10

**Difference between set, multiset, unordered_set, unordered_multiset:

• In simple words, **set is a container that stores **sorted and unique elements. If **unordered is added means elements are **not sorted.
• If **multiset is added means **duplicate elements storage is allowed.