How do I get a value from a dictionary in C++?<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n\n\n\n\nIn C++, a dictionary is a way to store data in pairs, each pair consisting of a unique key and a value. This makes it easy to find a value if you know the key. C++ doesn’t have a built-in dictionary, but you can use <\/strong>std::map<\/strong> or std::unordered_map<\/strong> to create one.<\/p>\n\n\n\n<\/span>1. <\/strong>std::map<\/strong><\/span><\/h3>\n\n\n\nWhat It Is<\/strong>: std::map is a container that keeps key-value pairs sorted by the key. This means that when you add items, they will be organized in order.<\/p>\n\n\n\nKey Features<\/strong>:<\/p>\n\n\n\n\n- Order<\/strong>: Keys are kept in a sorted order.<\/li>\n\n\n\n
- Implementation<\/strong>: Uses a type of tree to manage the data.<\/li>\n\n\n\n
- Performance<\/strong>: Adding, removing, or finding items takes O(log n) time, where n = number of items.<\/li>\n<\/ul>\n\n\n\n
Example<\/strong>:<\/p>\n\n\n\n#include <iostream><\/p>\n\n\n\n
#include <map><\/p>\n\n\n\n
int main() {<\/p>\n\n\n\n
\/\/ Create a map with int values and string keys<\/p>\n\n\n\n
std::map<std::string, int> dictionary;<\/p>\n\n\n\n
\/\/ Add key-value pairs<\/p>\n\n\n\n
dictionary[“Coke”] = 1;<\/p>\n\n\n\n
dictionary[“Sprite”] = 2;<\/p>\n\n\n\n
dictionary[“Pepsi”] = 3;<\/p>\n\n\n\n
\/\/ Access values by key<\/p>\n\n\n\n
std::cout << “Coke: ” << dictionary[“Coke”] << std::endl;<\/p>\n\n\n\n
std::cout << “Sprite: ” << dictionary[“Sprite”] << std::endl;<\/p>\n\n\n\n
\/\/ Loop through the map<\/p>\n\n\n\n
for (const auto& pair: dictionary) {<\/p>\n\n\n\n
std::cout << pair.first << “: ” << pair.second << std::endl;<\/p>\n\n\n\n
}<\/p>\n\n\n\n
return 0;<\/p>\n\n\n\n
}<\/p>\n\n\n\n
<\/span>How to Create a <\/strong>std::map<\/strong>:<\/strong><\/span><\/h3>\n\n\n\nDefault Constructor<\/strong>: Creates an empty map.
std::map<std::string, int> myMap;<\/p>\n\n\n\nInitialization List<\/strong>: Creates the map with some key-value pairs.
std::map<std::string, int> myMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };<\/p>\n\n\n\nUsing <\/strong>insert<\/strong> Method<\/strong>: Adds items to an existing map.
myMap.insert(std::make_pair(“Coke”, 1));<\/p>\n\n\n\nmyMap.insert({“Sprite”, 2});<\/p>\n\n\n\n
<\/span>2. <\/strong>std::unordered_map<\/strong><\/span><\/h3>\n\n\n\nWhat It Is<\/strong>: std::unordered_map is a container that stores key-value pairs without any specific order. It uses a hash table to find items quickly.<\/p>\n\n\n\nKey Features<\/strong>:<\/p>\n\n\n\n\n- Order<\/strong>: Keys are not sorted.<\/li>\n\n\n\n
- Implementation<\/strong>: Uses a hash table for fast access.<\/li>\n\n\n\n
- Performance<\/strong>: Adding, removing, or finding items usually takes O(1) time, but it can be slower if there are many collisions.<\/li>\n<\/ul>\n\n\n\n
Example<\/strong>:<\/p>\n\n\n\n#include <iostream><\/p>\n\n\n\n
#include <unordered_map><\/p>\n\n\n\n
int main() {<\/p>\n\n\n\n
\/\/ Create an unordered_map with string keys and int values<\/p>\n\n\n\n
std::unordered_map<std::string, int> dictionary;<\/p>\n\n\n\n
\/\/ Add key-value pairs<\/p>\n\n\n\n
dictionary[“Coke”] = 1;<\/p>\n\n\n\n
dictionary[“Sprite”] = 2;<\/p>\n\n\n\n
dictionary[“Pepsi”] = 3;<\/p>\n\n\n\n
\/\/ Access values by key<\/p>\n\n\n\n
std::cout << “Coke: ” << dictionary[“Coke”] << std::endl;<\/p>\n\n\n\n
std::cout << “Sprite: ” << dictionary[“Sprite”] << std::endl;<\/p>\n\n\n\n
\/\/ Loop through the unordered_map<\/p>\n\n\n\n
for (const auto& pair : dictionary) {<\/p>\n\n\n\n
std::cout << pair.first << “: ” << pair.second << std::endl;<\/p>\n\n\n\n
}<\/p>\n\n\n\n
return 0;<\/p>\n\n\n\n
}<\/p>\n\n\n\n
<\/span>How to Create a <\/strong>std::unordered_map<\/strong>:<\/span><\/h3>\n\n\n\nDefault Constructor<\/strong>: Creates an empty unordered_map.
std::unordered_map<std::string, int> myUnorderedMap;<\/p>\n\n\n\nInitialization List<\/strong>: Creates the unordered_map with some key-value pairs.
std::unordered_map<std::string, int> myUnorderedMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };<\/p>\n\n\n\nUsing the <\/strong>emplace<\/strong> Method<\/strong>: Adds items efficiently.
myUnorderedMap.emplace(“Coke”, 1);<\/p>\n\n\n\nmyUnorderedMap.emplace(“Sprite”, 2);<\/p>\n\n\n\n
<\/span>Ways to Create a C++ Dictionary<\/strong><\/span><\/h3>\n\n\n\n\n- Default Constructor<\/strong>:\n
\n- Start with an empty dictionary and add key-value pairs later.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n
std::map<std::string, int> myMap;<\/p>\n\n\n\n
std::unordered_map<std::string, int> myUnorderedMap;<\/p>\n\n\n\n
\n- Initialization List<\/strong>:\n
\n- Create the dictionary with key-value pairs right away.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n
std::map<std::string, int> myMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };<\/p>\n\n\n\n
std::unordered_map<std::string, int> myUnorderedMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };<\/p>\n\n\n\n
\n- Using <\/strong>insert<\/strong> Method<\/strong>:\n
\n- Add key-value pairs to an existing dictionary.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n
std::map<std::string, int> myMap;<\/p>\n\n\n\n
myMap.insert(std::make_pair(“Coke”, 1));<\/p>\n\n\n\n
myMap.insert({“Sprite”, 2});<\/p>\n\n\n\n
\n- Using <\/strong>emplace<\/strong> Method<\/strong> (for std::unordered_map):\n
\n- Efficiently add key-value pairs.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n
std::unordered_map<std::string, int> myUnorderedMap;<\/p>\n\n\n\n
myUnorderedMap.emplace(“Coke”, 1);<\/p>\n\n\n\n
myUnorderedMap.emplace(“Sprite”, 2);<\/p>\n\n\n\n
<\/span>Key Points<\/strong><\/span><\/h3>\n\n\n\n\n- Key-Value Pair<\/strong>: Each item in the dictionary has a unique key that maps to a value.<\/li>\n\n\n\n
- Accessing Values<\/strong>: Get a value by using its key, like a dictionary[“key”].<\/li>\n\n\n\n
- Insertion<\/strong>: Add items to the dictionary using = or methods like insert and emplace.<\/li>\n\n\n\n
- Iteration<\/strong>: You can loop through both std::map and std::unordered_map, but std::unordered_map does not guarantee the order of items.<\/li>\n<\/ul>\n\n\n\n
Choose std::map if you need to sort items. Choose std::unordered_map if you want faster access times without caring about the order.<\/p>\n\n\n\n
<\/span>Step-by-Step Guide to Creating a Dictionary in C++ Using <\/strong>std::map<\/strong><\/span><\/h3>\n\n\n\nCreating a dictionary in C++ is simple when you use the std::map from the Standard Template Library (STL). Here\u2019s an easy guide to help you get started.<\/p>\n\n\n\n