C++ Map
A map stores elements in " key/value " pairs.
Elements in a map are
- Accessible by keys (not index), and each key is unique.
- Automatically sorted in ascending order by their keys.
// Include the map library #include <map>Create a Map
To create a map, use the map keyword, and specify the type of both the key and the value it should store within angle brackets <> . At last, specify the name of the map, like: map< keytype, valuetype > mapName :
Example
// Create a map called people that will store strings as keys and integers as values
map<string, int> peopleIf you want to add elements at the time of declaration, place them in a comma-separated list, inside curly braces {} :
Example
// Create a map that will store the name and age of different people
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };Access a Map
You cannot access map elements by referring to index numbers, like you would with arrays and vectors .
Instead, you can access a map element by referring to its key inside square brackets [] :
Example
// Create a map that will store the name and age of different people map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} }; // Get the value associated with the key "John" cout << "John is: " << people["John"] << "\n"; // Get the value associated with the key "Adele" cout << "Adele is: " << people["Adele"] << "\n";You can also access elements with the .at() function:
Example
// Create a map that will store the name and age of different people map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} }; // Get the value associated with the key "Adele" cout << "Adele is: " << people.at("Adele") << "\n"; // Get the value associated with the key "Bo" cout << "Bo is: " << people.at("Bo") << "\n";Note
The .at() function is often preferred over square brackets [] because it throws an error message if the element does not exist:
Example
// Create a map that will store the name and age of different people map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} }; // Try to access an element that does not exist (will throw an exception) cout << people.at("Jenny");Change Values
You can also change the value associated with a key:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Change John's value to 50 instead of 32 people["John"] = 50; cout << "John is: " << people["John"]; // Now outputs John is: 50However, it is safer to use the .at() function:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Change John's value to 50 instead of 32 people.at("John") = 50; cout << "John is: " << people.at("John"); // Now outputs John is: 50Add Elements
To add elements to a map, it is ok to use square brackets [] :
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Add new elements
people["Jenny"] = 22;
people["Liam"] = 24;
people["Kasper"] = 20;
people["Anja"] = 30;But you can also use the .insert() function:
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Add new elements people.insert({"Jenny", 22});
people.insert({"Liam", 24});
people.insert({"Kasper", 20});
people.insert({"Anja", 30});Elements with Equal Keys
A map cannot have elements with equal keys.
For example, if we try to add "Jenny" two times to the map, it will only keep the first one:
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Trying to add two elements with equal keys people.insert({"Jenny", 22});
people.insert({"Jenny", 30});To sum up; values can be equal, but keys must be unique.
Remove Elements
To remove specific elements from a map, you can use the .erase() function:
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Remove an element by key
people.erase("John");To remove all elements from a map, you can use the .clear() function:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
// Remove all elements
people.clear();Find the Size of a Map
To find out how many elements a map has, use the .size() function:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
cout
<< people.size(); // Outputs 3Check if a Map is Empty
Use the .empty() function to find out if a map is empty or not.
The .empty() function returns 1 ( true ) if the map is empty and 0 ( false ) otherwise:
Example
map<string, int> people;
cout << people.empty();
// Outputs 1 (The map is empty)Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
cout
<< people.empty();
// Outputs 0 (not empty)Note
You can also check if a specific element exists, by using the .count( key ) function.
It returns 1 ( true ) if the element exists and 0 ( false ) otherwise:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
cout
<< people.count("John");
// Outputs 1 (John exists)Loop Through a Map
You can loop through a map with the for-each loop. However, there are a couple of things to be aware of:
- You should use the auto keyword (introduced in C++ version 11) inside the for loop. This allows the compiler to automatically determine the correct data type for each key-value pair.
- Since map elements consist of both keys and values, you have to include .first to access the keys, and .second to access values in the loop.
- Elements in the map are sorted automatically in ascending order by their keys:
Example
map<string, int> people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
for ( auto
person : people) {
cout << person.first
<< " is: " << person.second
<< "\n";
}If you want to reverse the order, you can use the greater< type > functor inside the angle brackets, like this:
Example
map<string, int, greater<string> > people = { {"John", 32}, {"Adele", 45}, {"Bo", 29} };
for (auto person : people) {
cout << person.first << " is: " << person.second << "\n";
}Tip
It is also possible to loop through maps with an iterator , which you will learn more about in the next chapter.