map: Look Things Up by Key

A map stores key → value pairs. Instead of finding things by position number, you find them by a key, like a word or a name:

Think of it like this: A map is like a real dictionary: you look up a word (the key) and find its meaning (the value). Or a phone book: name → phone number.

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    map<string, int> ages;
    ages["Ana"] = 31;
    ages["Ben"] = 25;
    ages["Cleo"] = 40;

    cout << "Ben is " << ages["Ben"] << "\n";
    ages["Ben"] = 26;                         // change a value
    cout << "Ben is now " << ages["Ben"] << "\n";
    cout << "People: " << ages.size() << "\n";
    return 0;
}

Output

Ben is 25
Ben is now 26
People: 3

Does a key exist?

Use count(key): it is 1 if the key is there and 0 if not. (find works too.)

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    map<string, string> capitals = {{"France", "Paris"}, {"Japan", "Tokyo"}};
    string country = "Japan";
    if (capitals.count(country)) {
        cout << "The capital of " << country << " is " << capitals[country] << "\n";
    }
    if (capitals.count("Peru") == 0) {
        cout << "I don't know Peru's capital\n";
    }
    capitals.erase("France");
    cout << capitals.size() << " left\n";
    return 0;
}

Output

The capital of Japan is Tokyo
I don't know Peru's capital
1 left

Common mistake: Reading m[key] for a key that isn't there creates it, with a value of 0 (or an empty string). Check with count first if you only want to look.

Looping over a map

A map keeps its keys sorted. Each item is a pair of key and value:

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    map<string, int> stock = {{"pears", 4}, {"apples", 10}, {"kiwis", 0}};
    for (const auto& [fruit, count] : stock) {
        cout << fruit << ": " << count << "\n";
    }
    return 0;
}

Output

apples: 10
kiwis: 0
pears: 4

The counting trick

Because a missing key starts at 0, m[key]++ is a perfect way to count things:

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    string text = "mississippi";
    map<char, int> counts;
    for (char c : text) {
        counts[c]++;
    }
    for (const auto& [letter, n] : counts) {
        cout << letter << " appears " << n << " times\n";
    }
    return 0;
}

Output

i appears 4 times
m appears 1 times
p appears 2 times
s appears 4 times

Tip: unordered_map (from <unordered_map>) works the same way and is usually faster, but its keys come out in no particular order. Use map when you want them sorted.