📁 C++ Sets – Store Sorted Unique Elements with std::set

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🧲 Introduction – Why Use std::set in C++

In C++ applications where duplicates are not allowed and fast search, insertion, and deletion are required, std::set is the ideal choice. It stores elements in a sorted order and provides logarithmic time complexity for all operations, thanks to its balanced tree structure.

🎯 In this guide, you’ll learn:

• What std::set is and how it works
• Key operations like insert(), find(), erase()
• How to use iterators and custom sort orders
• Best practices and real-world use cases

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📘 What Is a C++ Set?

A std::set is an ordered associative container that contains only unique keys. It stores the keys in ascending order by default using a Red-Black Tree (balanced BST).

Include the header:

#include <set>

Declare a set:

set<int> s;

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💻 Code Examples – With Output

✅ Insert and Traverse

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

int main() {
    set<int> s;
    s.insert(30);
    s.insert(10);
    s.insert(20);
    s.insert(10); // Duplicate ignored

    for (int x : s) cout << x << " ";
    return 0;
}

🟢 Output:

10 20 30

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✅ Search and Erase

if (s.find(20) != s.end())
    s.erase(20);

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🧩 Set Member Functions

Function	Description
insert(val)	Adds a new element (if not present)
erase(val)	Removes element
find(val)	Returns iterator to element if found
count(val)	Returns 1 if element exists, else 0
size()	Returns number of elements
clear()	Removes all elements
begin()/end()	Start/end iterators for traversal

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🔀 Custom Sort Order with Comparator

struct Desc {
    bool operator()(int a, int b) const {
        return a > b; // Descending
    }
};

set<int, Desc> descSet;

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🔁 Iterating a Set

for (set<int>::iterator it = s.begin(); it != s.end(); ++it)
    cout << *it << " ";

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🆚 Set vs Multiset vs Unordered Set

Feature	set	multiset	unordered_set
Duplicates allowed?	❌ No	✅ Yes	❌ No
Order	✅ Sorted	✅ Sorted	❌ Unordered
Lookup complexity	O(log n)	O(log n)	O(1) average
Underlying structure	Balanced tree	Balanced tree	Hash table

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💡 Best Practices & Tips

📘 Use count() if you only need to check for existence
💡 For heavy insert/delete workloads, consider unordered_set
⚠️ Avoid modifying set elements via iterators—sets reorder elements automatically
📦 Use a custom comparator for descending order or complex types

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🛠️ Use Cases for Sets

🧮 Unique Element Tracking – Email lists, usernames, identifiers
📊 Sorted Data Retrieval – Maintain live sorted datasets
🔍 Efficient Searching – Membership tests and quick lookups
📥 Event Logs – Avoid duplicate event IDs
🧠 Mathematical Sets – Implement union, intersection, difference

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📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• std::set stores unique, automatically sorted elements
• Efficient for searching, insertion, and deletion (O(log n))
• Supports iterators and custom comparators

⚙️ Real-World Relevance:
C++ sets are used in simulations, indexes, validation systems, constraint checkers, and anywhere uniqueness and order are critical.

✅ Next Steps:
Learn about C++ Maps to associate keys with values in sorted order.

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❓FAQ – C++ Sets

❓ Are duplicates allowed in std::set?
❌ No. Use multiset if duplicates are needed.

❓ Can I change an element in a set directly?
⚠️ No. You must remove and re-insert to modify value.

❓ How is set sorted?
By default in ascending order using < operator.

❓ Can I sort a set in descending order?
✅ Yes. Use a custom comparator.

❓ Is find() faster than looping through the set?
✅ Yes. find() uses binary search (O(log n)).

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