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.
To use a map, you have to include the <map> header file:
// 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> people
If 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: 50
However, 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: 50
Add Elements
To add elements to a map, it is ok to use square brackets []:
Example
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:
Example
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:
Example
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:
Example
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 3
Check if a Map is Empty
Use the .empty() function to find out if a map is empty or not.
The .empty() function returns
10 (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
10 (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
autokeyword (introduced in C++ version 11) inside theforloop. 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
.firstto access the keys, and.secondto 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";
}
The output will be :
Adele is: 45 Bo is: 29 John is: 32
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";
}
The output will be:
John is: 32 Bo is: 29 Adele is: 45