Difference between revisions of "cpp/container/map"

#include <iostream>
#include <map>
#include <string>
#include <string_view>
 
void print_map(std::string_view comment, const std::map<std::string, int>& m)
{
    std::cout << comment << "\n";
 
    // Demonstrate multiple ways to iterate over the `std::map`
 
    // ----------------------------
    // Legacy iterators
    // ----------------------------
 
    // 1. Use explicit iterator
    for (std::map<std::string, int>::const_iterator it = m.begin(); it != m.end(); it++) {
        std::cout << it->first << " = " << it->second << "; ";
    }
    std::cout << "\n";
 
    // 2. Use `auto` for the iterator; notice we use `cbegin()` and `cend()` 
    // here to force `auto` to be a `const_iterator`
    for (auto it = m.cbegin(); it != m.cend(); it++) {
        std::cout << it->first << " = " << it->second << "; ";
    }
    std::cout << "\n";
 
    // ----------------------------
    // Range-based for loops
    // ----------------------------
 
    // 1. Explicitly specify the `std::pair` iterator 
    for (const std::pair<const std::string, int>& n : m) {
        std::cout << n.first << " = " << n.second << "; ";
    }
    std::cout << "\n";
 
    // 2. Use `auto` for the `std::pair` iterator 
    for (const auto& n : m) {
        std::cout << n.first << " = " << n.second << "; ";
    }
    std::cout << "\n";
 
    // 3. Iterate using automatic "structured binding" to `key`, `value` variables
    //    (since C++17)
    for (const auto& [key, value] : m) {
        std::cout << key << " = " << value << "; ";
    }
    std::cout << "\n";
}
 
int main()
{
    // Create a map of three (strings, int) pairs
    std::map<std::string, int> m { {"CPU", 10}, {"GPU", 15}, {"RAM", 20}, };
 
    print_map("Initial map: ", m);
    std::cout << "\n";
 
    m["CPU"] = 25;  // update an existing value
    m["SSD"] = 30;  // insert a new value
    print_map("Updated map: ", m);
    std::cout << "\n";
 
    // using operator[] with non-existent key always performs an insert
    std::cout << "m[new_key] = " << m["new_key"] << "\n";
    print_map("Updated map: ", m);
}