IntermediatePractice in the editor

C++ Map

A map stores pairs of a key and a value: a word and its meaning, a name and a phone number, a product and its stock. Each key appears once, and looking up a key is fast. std::map keeps the keys sorted; std::unordered_map keeps no order but is usually faster.

Adding and reading

ExampleTry it in the editor

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

int main() {
    std::map<std::string, int> stock;
    stock["mango"] = 12;
    stock["apple"] = 30;
    stock["banana"] += 5;            // a missing key starts at 0
    stock["apple"] -= 4;
    for (const auto& [fruit, count] : stock) {     // sorted by key
        std::cout << fruit << ": " << count << "\n";
    }
    std::cout << stock.size() << " kinds\n";
    return 0;
}

Output

apple: 26
banana: 5
mango: 12
3 kinds

m[key] returns the value for a key, creating the key with a default value (0 for numbers, empty for strings) if it is missing. That is handy for counting, but it means just reading m["kiwi"] adds kiwi to the map.

Looking up safely

To check without adding, use contains (C++20), count or find. at(key) reads an existing key and throws std::out_of_range if it is missing.

ExampleTry it in the editor

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

int main() {
    std::map<std::string, std::string> capitals = {
        {"India", "New Delhi"}, {"Japan", "Tokyo"}, {"France", "Paris"}
    };
    for (std::string country : {"Japan", "Peru"}) {
        auto it = capitals.find(country);
        if (it != capitals.end()) {
            std::cout << country << ": " << it->second << "\n";
        } else {
            std::cout << country << ": not known\n";
        }
    }
    std::cout << std::boolalpha << capitals.contains("India") << "\n";
    return 0;
}

Output

Japan: Tokyo
Peru: not known
true

An iterator to a map element points at a pair: it->first is the key, it->second the value.

Counting words

ExampleTry it in the editor

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

int main() {
    std::string text = "the cat and the dog and the bird";
    std::map<std::string, int> count;
    std::istringstream in(text);
    std::string word;
    while (in >> word) count[word]++;
    for (const auto& [w, n] : count) std::cout << w << " " << n << "\n";
    return 0;
}

Output

and 2
bird 1
cat 1
dog 1
the 3

std::istringstream (from sstream) reads words from a string the way cin reads from the keyboard.

unordered_map

std::unordered_map has the same operations but stores keys in a hash table: lookups are very fast on average, and the order of the keys when looping is unpredictable. Use it for large maps where order does not matter, std::map when you want sorted output.

Practice exercises

Store a small phone book and look up a name the user types.
Count how many times each letter appears in a sentence and print the counts in alphabetical order.
Remove all entries whose value is 0 from a map of stock counts.