المجموعات (Set) في C++17: دليل مفصل

What is a Set?

Imagine a box for storing unique toys. If you try to put two identical balls in it, the second ball simply won't fit — the box already has one like it. In C++, such a "box" for data is std::set. It is a collection that stores elements only once and automatically sorts them in ascending order.

Why is this needed? For example, to:

• Remove duplicates from a list (keep only unique items).
• Check if an element exists in the collection (if a contact is saved in the address book).
• Merge two lists without duplicates (a shopping list from two friends).

How to include and declare a set?

To use a set, you need to include the #include <set> library.

#include <iostream>
#include <set> // Include sets
#include <string>

int main() {
    // Create a set of strings
    std::set<std::string> ingredients;
    
    // Create a set of integers
    std::set<int> numbers;
    
    return 0;
}

How to add an element?

Use the .insert() method. If the element already exists, it will not be added again.

#include <iostream>
#include <set>

int main() {
    std::set<int> mySet;
    
    mySet.insert(10);
    mySet.insert(5);
    mySet.insert(10); // This element will not be added because 10 already exists!
    mySet.insert(3);
    
    // Print all elements (they will be sorted)
    for(int num : mySet) {
        std::cout << num << " ";
    }
    // Output: 3 5 10
    return 0;
}

How to check if an element exists?

The .count() method returns 1 (if the element exists) or 0 (if it does not).

#include <iostream>
#include <set>
#include <string>

int main() {
    std::set<std::string> contacts = {"Anna", "Boris", "Vika"};
    
    std::string name;
    std::cout << "Enter a name to search for: ";
    std::cin >> name;
    
    if(contacts.count(name) == 1) {
        std::cout << "Contact found!";
    } else {
        std::cout << "Contact not in the address book.";
    }
    return 0;
}

How to delete an element?

The .erase() method removes an element by value.

#include <iostream>
#include <set>

int main() {
    std::set<std::string> shopping = {"milk", "bread", "eggs", "cheese"};
    
    std::string item;
    std::cout << "What do you want to remove from the list? ";
    std::cin >> item;
    
    shopping.erase(item); // Remove the element
    
    std::cout << "Remaining in the list: ";
    for(const auto& thing : shopping) {
        std::cout << thing << " ";
    }
    return 0;
}

How to find the size of a set?

The .size() method returns the number of elements.

#include <iostream>
#include <set>

int main() {
    std::set<int> numbers = {1, 2, 3, 4, 5, 5, 5}; // Duplicates are not counted
    std::cout << "Set size: " << numbers.size(); // Output: 5
    return 0;
}

Analysis of Typical Problems

Problem 1. Unique Pizza Ingredients

The user enters ingredients separated by commas. You need to output only the unique ones.

#include <iostream>
#include <set>
#include <sstream> // For working with strings
#include <string>

int main() {
    std::string input;
    std::cout << "Enter ingredients separated by commas: ";
    std::getline(std::cin, input);
    
    std::set<std::string> uniqueIngredients;
    std::stringstream ss(input);
    std::string ingredient;
    
    // Split the string by commas
    while(std::getline(ss, ingredient, ',')) {
        // Remove leading spaces
        if(!ingredient.empty() && ingredient[0] == ' ') {
            ingredient = ingredient.substr(1);
        }
        if(!ingredient.empty()) {
            uniqueIngredients.insert(ingredient);
        }
    }
    
    std::cout << "Unique ingredients: ";
    for(const auto& item : uniqueIngredients) {
        std::cout << item << " ";
    }
    return 0;
}

Problem 2. Merging Shopping Lists

There are two shopping lists. You need to merge them without duplicates.

#include <iostream>
#include <set>
#include <string>

int main() {
    std::set<std::string> mainList = {"cola", "chips", "pizza"};
    std::set<std::string> guestList = {"juice", "chips", "nuts"};
    
    // Add elements from the second list to the first
    for(const auto& item : guestList) {
        mainList.insert(item);
    }
    
    std::cout << "Final shopping list: ";
    for(const auto& product : mainList) {
        std::cout << product << " ";
    }
    // Output: cola nuts pizza juice chips (all unique and sorted)
    return 0;
}

Common Beginner Mistakes

• Confusing set with vector. You cannot access elements in a set by index (mySet[0] is an error). Use a for loop or iterators.
• Forgetting #include <set>. Without this line, the compiler will not understand what std::set is.
• Thinking insert will add an element even if it exists. No, insert does not create duplicates. If you need to store duplicates, use std::multiset.
• Trying to modify an element in a set. Elements in a set are constant. You cannot write *it = 5;. First delete the old one, then add the new one.

Final Example: Contact Address Book

Let's create a program that stores contact names (only unique