C++ · STL: map and unordered_map

C++ has two kinds of dictionary:

	std::map	std::unordered_map
structure	balanced binary tree	hash table
lookup	O(log n)	O(1) average
iteration order	sorted by key	unspecified

The trap: m[key] inserts a default-constructed value if the key is missing. To just check, use find() or count() (or contains() in C++20). That’s the opposite of Python, where d[k] raises an error instead.

#include <iostream>
#include <map>
#include <sstream>
#include <string>
#include <unordered_map>

int main() {
    std::map<std::string, int> marks{{"python", 95}, {"maths", 88}, {"cpp", 81}};
    for (const auto& [subject, m] : marks)           // sorted by key
        std::cout << subject << ": " << m << '\n';

    std::cout << "size before: " << marks.size();
    int x = marks["physics"];                        // inserts physics=0!
    std::cout << ", after operator[]: " << marks.size() << " (value " << x << ")\n";

    if (auto it = marks.find("chemistry"); it == marks.end())
        std::cout << "chemistry not found, size still " << marks.size() << '\n';

    std::unordered_map<std::string, int> freq;
    std::istringstream text("to be or not to be");
    for (std::string w; text >> w;) ++freq[w];       // here insert-on-miss is handy
    std::cout << "to=" << freq["to"] << " be=" << freq["be"] << " or=" << freq["or"] << '\n';
}

Output:

cpp: 81
maths: 88
python: 95
size before: 3, after operator[]: 4 (value 0)
chemistry not found, size still 4
to=2 be=2 or=1