vimwiki

C++ hash maps

A hash map or map is a data structure that stores a collection of elements with a key-value pair. Like a dictionary in other language.

Create a map

There are two types of maps

• unordered_map
• map

Include the map library and create a map with:

std::unordered_map<key_type, value_map> [name];

#include <unordered_map>
#include <map>

std::unordered_map<std::string, int> breiz_dep;

You can initialize a hash at creation with an initializer list.

std::unordered_map<std::string, int> breiz_dep = {
        {"Morbihan", 56},
        {"Finister", 29},
        {"Cote d'Armor", 22}
    };

!! the key-value are separated by a comma , not a colon :

Hash maps methods

Add elements

Use .insert() or the [] operator. The last is also used to access elements, but will create it if not existing.

breiz_dep.insert({"Cote d'Armor", 22});
breiz_dep["Ille et Vilaine"] = 35;

Remove elements

Use .erase() with the key.

breiz_dep.erase("Pays de la Loire");

Check for an element

Use .count() to check if a key is in the map.

breiz_dep.count("Pays de la Loire");

Access an elements

Use the [] operator or .at()

.at() should be preferred because it does not insert the element if not present.

std::cout << breiz_dep["Morbihan"];

std::cout << breiz_dep.at("Morbihan");

Size of a hash map

Use .size() to get the size of a map. Or .empty() to know if it’s empty or not(return 1 or 0).

breiz_dep.size();
breiz_dep.empty();

Iterating through a hash map

Use a for-each loop. The counter variable is the hash map itslef, which is an object of type std::pair. It has two member variables .first which is the key variable. And .second which is the value variable. Both can be used inside the loop.

for(auto mp: breiz_dep) {
    std::cout << "key: " << mp.first << " value: " << mp.second;
}

Order vs unordered map

unordered_map stores elements in no particular order. Inserting, deleting and searching is faster in a unordered_map.

map store elements in order.

Note: only use map when elements need to be sorted.