Lære Associative Containers: set, map, multiset, multimap

Definition

Associative containers in C++ store and retrieve elements efficiently using keys. Unlike sequence containers, they keep elements automatically sorted (usually with the less-than operator) and include types like std::set, std::map, std::multiset, and std::multimap.

Associative containers guarantee logarithmic time complexity for lookup, insertion, and removal by using balanced binary search trees internally. Each container has unique characteristics:

In a std::set, each key is unique, ensuring no duplicates;
A std::map associates each unique key with a single value;
A std::multiset allows duplicate keys, making it useful when multiple identical elements are needed;
A std::multimap lets you associate multiple values with the same key.

set_example.cpp

map_example.cpp


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#include <iostream>
#include <set>

int main()
{
    std::set<int> s;
    s.insert(3);
    s.insert(1);
    s.insert(2);
    s.insert(2); // Duplicate, will be ignored
}

While std::set and std::map enforce key uniqueness, you sometimes need to store multiple identical keys. Use std::multiset and std::multimap for these situations.

main.cpp


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#include <iostream>
#include <map>
#include <string>

int main()
{
    // Create a multimap to store names and their grades
    std::multimap<std::string, int> grades;

    // Insert multiple grades for the same names
    grades.insert({"Alice", 85});
    grades.insert({"Bob", 90});
    grades.insert({"Alice", 92});
    grades.insert({"Bob", 78});

    // Print all grades for Alice
    std::cout << "All grades for Alice:\n";
    auto range = grades.equal_range("Alice");  // Get range of elements with key "Alice"
    
    for (auto it = range.first; it != range.second; ++it)
        std::cout << it->first << " => " << it->second << "\n";

    // Print all entries in the multimap
    std::cout << "\nAll entries in multimap:\n";
    for (const auto& [name, score] : grades)
        std::cout << name << " => " << score << "\n";
}

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