STL List

Notes

📚 Detailed Notes: Understanding STL list Container in C++

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🧱 What is a list in C++?

Definition:

• A list in C++ STL is a doubly linked list.

• Each element (node) contains:

  • The actual value

  • A pointer to the next node

  • A pointer to the previous node

Key Characteristics:

Feature	List (STL)
Type	Doubly Linked List
Size	Dynamic
Fast for	Insertions & Deletions
Slow for	Searching
Memory Allocation	Non-contiguous

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⚙️ How to Use list in C++

1. Include the header

#include <list>

2. Creating a list

std::list<int> myList;

This creates an empty list of integers named myList.

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➕ Inserting Elements

Two Common Insertion Methods:

Method	Description	Example
push_back()	Adds element at the end	myList.push_back(10);
push_front()	Adds element at the beginning	myList.push_front(30);

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📤 Printing Elements of the List

Using Iterators:

for (std::list<int>::iterator it = myList.begin(); it != myList.end(); ++it) {
    std::cout << *it << std::endl;
}

• it is an iterator (like a pointer).

• *it dereferences the iterator to access the actual value.

• Iterators are essential for traversing through STL containers.

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❌ Deleting Elements

Erasing an Element:

myList.erase(myList.begin()); // Deletes the first element

• You pass an iterator to the erase() function.

• Can delete any element if you pass the correct iterator.

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🕹️ Real-World Example: Matchmaking System in Games

Imagine a Battle Royale game lobby (like Fortnite, PUBG, Warzone) where:

• Players have a skill rating (1–10)

• You want to match similar skilled players to make it fair

1. Setup

std::list<int> allPlayers = {2, 9, 6, 7, 3, 1, 4, 8, 3, 2, 9};
std::list<int> beginners;
std::list<int> pros;

2. Grouping Players

for (std::list<int>::iterator it = allPlayers.begin(); it != allPlayers.end(); ++it) {
    int rating = *it;
    if (rating >= 1 && rating <= 5)
        beginners.push_back(rating);
    else if (rating >= 6 && rating <= 10)
        pros.push_back(rating);
}

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🖨️ Displaying Ratings Function

Function to Display Player Ratings:

void displayRating(const std::list<int>& playerRatings) {
    for (std::list<int>::const_iterator it = playerRatings.begin(); it != playerRatings.end(); ++it) {
        std::cout << *it << std::endl;
    }
}

Notes:

• Uses const_iterator to ensure read-only access

• Passes list by reference (&) to avoid unnecessary copying

• Uses const for safety — to prevent modification

Example Usage:

std::cout << "Beginners:" << std::endl;
displayRating(beginners);
 
std::cout << "Pros:" << std::endl;
displayRating(pros);

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🔁 Ordering Players by Rating

Problem:

We want the players in each group to be ordered by their rating in ascending order.

Solution: Custom Insert Function

void insertPlayerIntoOrderedList(int rating, std::list<int>& orderedPlayers) {
    for (auto it = orderedPlayers.begin(); it != orderedPlayers.end(); ++it) {
        if (*it > rating) {
            orderedPlayers.insert(it, rating);
            return;
        }
    }
    orderedPlayers.push_back(rating);
}

Use in Matchmaking Logic:

if (rating >= 1 && rating <= 5)
    insertPlayerIntoOrderedList(rating, beginners);
else if (rating >= 6 && rating <= 10)
    insertPlayerIntoOrderedList(rating, pros);

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✅ Summary: Advantages and Disadvantages of list

Advantages	Disadvantages
Fast Insertion and Deletion (O(1))	Slow Traversal and Searching (O(n))
Dynamic size	More memory per element (due to pointers)
Good for queues, editing text buffers	Not cache-friendly

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💡 Analogy: Friends Holding Hands (Linked List)

Imagine:

• A group of friends holding hands.

• Each friend knows who is in front and who is behind.

• If you join in the middle, you just link hands between two friends.

• If you leave, the others just hold hands again.

This is how a list grows or shrinks dynamically.

But… If you’re trying to find Joe, you’d have to go person by person and ask for his name — very slow!

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🔁 Comparison with vector

Feature	list	vector
Access Time	O(n) (need to traverse)	O(1) (random access by index)
Insertion/Removal	Fast at any position (O(1) with iterator)	Slow (due to reallocation)
Memory Layout	Non-contiguous (more overhead)	Contiguous (better cache performance)
Best For	Frequent insert/delete	Frequent access/search

Vector Analogy:

• Like a classroom with fixed seats.

• Easy to find a student by their seat number (index).

• But hard to add/remove students — need a new room (reallocation).

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🎓 Practical Programming Tip

• If your app frequently adds/removes elements, use a list.

• If your app frequently searches or accesses by index, use a vector.

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📦 Conclusion

• STL containers like list help organize data efficiently.

• Use the right container based on the operation requirements.

• Understanding their underlying structure helps you make better coding decisions.

• Combine your knowledge of data structures to build real-world applications like a matchmaking system in games.

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References

• Video
• Code
• C++ STL Introduction