C++: How to Iterate the Elements over the Sets (set, unordered_set, multiset, unordered_multiset)

In C++, we use set, multiset, unordered_multiset, unordered_set to store a hash set. C++ set/multiset implements a Red-Black tree that maintains the order of the elements. On the other hand, the unordered_set and unordered_multiset are based on Hashmap/Hashtable thus the elements are not sorted.

The multiset and unordered_multiset allows storing the duplicates in the set. To iterate over the sets, we can just use the simple for loop:

#include <iostream>
#include <unordered_set>

using namespace std;

int main() {
	unordered_set<int> data;
	data.insert(5);
	data.insert(3);
	data.insert(2);
	data.insert(3);
	data.insert(6);
	data.insert(7);
	for (const auto &n: data) {
		cout << n << endl;
	}
	return 0;
}

Duplicates are stored only once. And the unordered_set does not sort the elements. Thus you may see different order e.g. 7 6 2 5 3.

Iterating over the C++ set is similar. The output shows that elements are ordered. The C++ set O(logN) update/insert/delete/query is slower than unordered_set which has O(1) constant complexity.

#include <iostream>
#include <set>

using namespace std;

int main() {
	set<int> data;
	data.insert(5);
	data.insert(3);
	data.insert(2);
	data.insert(3);
	data.insert(6);
	data.insert(7);
	for (const auto &n: data) {
		cout << n << endl; // prints 2 3 5 6 7
	}
	return 0;
}

For multiset and unordered_multiset, iterating over the elements are similar except that the results will print the duplicates.

#include <iostream>
#include <set>

using namespace std;

int main() {
	multiset<int> data;
	data.insert(5);
	data.insert(3);
	data.insert(2);
	data.insert(3);
	data.insert(6);
	data.insert(7);
	for (const auto &n: data) {
		cout << n << endl; // prints 2 3 3 5 6 7
	}
	return 0;
}

Note that the C++ multiset is included in set header and unordered_multiset is provided in unordered_set header.

Alternatively, we can increment the iterator to start from begin() to end() of the sets: (set, multiset, unordered_set, and unordered_multiset). We use *it to de-reference the iterator pointer.

#include <iostream>
#include <set>

using namespace std;

int main() {
	multiset<int> data;
	data.insert(5);
	data.insert(3);
	data.insert(2);
	data.insert(3);
	data.insert(6);
	data.insert(7);
	for (auto it = begin(data); it != end(data); it ++) {
		cout << *it << endl; // prints 2 3 3 5 6 7
	}
	return 0;
}

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