Functions and collections · Lesson 10 of 21

map, set and unordered_map

Look things up by key, count occurrences, and drop duplicates.

10 minutes2 games

std::map stores key → value pairs, like a Python dict or a JavaScript object, always sorted by key. std::set is a collection of unique values, also sorted.

C++

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

int main() {
    std::map<std::string, int> xp;
    xp["Laila"] = 320;
    xp["Karim"] = 150;
    xp["Omar"] += 90;          // missing key: created as 0, then += 90

    for (const auto& entry : xp) {
        std::cout << entry.first << ": " << entry.second << "\n";
    }
    std::cout << "has Nour? " << xp.count("Nour") << "\n";

    std::set<std::string> cities = {"Cairo", "Giza", "Cairo", "Alexandria"};
    std::cout << cities.size() << " unique cities, first " << *cities.begin() << "\n";
    return 0;
}

	std::map	std::unordered_map
Storage	balanced tree	hash table
Order	sorted by key	no order
Lookup	O(log n)	O(1) average
When?	need order or ranges	just need speed

C++

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

int main() {
    std::string text = "to be or not to be";
    std::unordered_map<std::string, int> counts;
    std::istringstream words(text);
    std::string w;
    while (words >> w) counts[w]++;

    std::cout << "to: " << counts["to"] << ", be: " << counts["be"] << ", or: " << counts["or"] << "\n";
    std::cout << counts.size() << " different words\n";
    return 0;
}

What's the output?

#include <iostream>
#include <map>
int main() {
    std::map<int, char> m = {{3, 'c'}, {1, 'a'}, {2, 'b'}};
    m[1] = 'z';
    for (const auto& p : m) std::cout << p.second;
    std::cout << m.size() << "\n";
}

What will this code print?

Exercise

Print each letter of "banana" with its count, alphabetically, one per line as a 3.

Expected output:

a 3
b 1
n 2

C++

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

int main() {
    std::string word = "banana";
    
    return 0;
}

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