In C++ dictionary is a helpful tool for handling data. It lets you connect unique keys with specific values, making it easy to find and organize information. This is handy when you need to match things like usernames with profiles or product codes with details. Unlike arrays that use numbers to access data, a dictionary allows you to use more descriptive keys, like names or custom objects. This makes your code more precise and easier to work with.
In this article, you will learn about the C++ dictionary. How can you create a dictionary using different methods? After going through this article, you can easily create a dictionary and learn about all the details related to the C++ dictionary.
What is a C++ Dictionary?
Table of Contents
In 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 std::map or std::unordered_map to create one.
1. std::map
What It Is: 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.
Key Features:
- Order: Keys are kept in a sorted order.
- Implementation: Uses a type of tree to manage the data.
- Performance: Adding, removing, or finding items takes O(log n) time, where n = number of items.
Example:
#include <iostream>
#include <map>
int main() {
// Create a map with int values and string keys
std::map<std::string, int> dictionary;
// Add key-value pairs
dictionary[“Coke”] = 1;
dictionary[“Sprite”] = 2;
dictionary[“Pepsi”] = 3;
// Access values by key
std::cout << “Coke: ” << dictionary[“Coke”] << std::endl;
std::cout << “Sprite: ” << dictionary[“Sprite”] << std::endl;
// Loop through the map
for (const auto& pair: dictionary) {
std::cout << pair.first << “: ” << pair.second << std::endl;
}
return 0;
}
How to Create a std::map:
Default Constructor: Creates an empty map.
std::map<std::string, int> myMap;
Initialization List: Creates the map with some key-value pairs.
std::map<std::string, int> myMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };
Using insert Method: Adds items to an existing map.
myMap.insert(std::make_pair(“Coke”, 1));
myMap.insert({“Sprite”, 2});
2. std::unordered_map
What It Is: std::unordered_map is a container that stores key-value pairs without any specific order. It uses a hash table to find items quickly.
Key Features:
- Order: Keys are not sorted.
- Implementation: Uses a hash table for fast access.
- Performance: Adding, removing, or finding items usually takes O(1) time, but it can be slower if there are many collisions.
Example:
#include <iostream>
#include <unordered_map>
int main() {
// Create an unordered_map with string keys and int values
std::unordered_map<std::string, int> dictionary;
// Add key-value pairs
dictionary[“Coke”] = 1;
dictionary[“Sprite”] = 2;
dictionary[“Pepsi”] = 3;
// Access values by key
std::cout << “Coke: ” << dictionary[“Coke”] << std::endl;
std::cout << “Sprite: ” << dictionary[“Sprite”] << std::endl;
// Loop through the unordered_map
for (const auto& pair : dictionary) {
std::cout << pair.first << “: ” << pair.second << std::endl;
}
return 0;
}
How to Create a std::unordered_map:
Default Constructor: Creates an empty unordered_map.
std::unordered_map<std::string, int> myUnorderedMap;
Initialization List: Creates the unordered_map with some key-value pairs.
std::unordered_map<std::string, int> myUnorderedMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };
Using the emplace Method: Adds items efficiently.
myUnorderedMap.emplace(“Coke”, 1);
myUnorderedMap.emplace(“Sprite”, 2);
Ways to Create a C++ Dictionary
- Default Constructor:
- Start with an empty dictionary and add key-value pairs later.
std::map<std::string, int> myMap;
std::unordered_map<std::string, int> myUnorderedMap;
- Initialization List:
- Create the dictionary with key-value pairs right away.
std::map<std::string, int> myMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };
std::unordered_map<std::string, int> myUnorderedMap = { {“Coke”, 1}, {“Sprite”, 2}, {“Pepsi”, 3} };
- Using insert Method:
- Add key-value pairs to an existing dictionary.
std::map<std::string, int> myMap;
myMap.insert(std::make_pair(“Coke”, 1));
myMap.insert({“Sprite”, 2});
- Using emplace Method (for std::unordered_map):
- Efficiently add key-value pairs.
std::unordered_map<std::string, int> myUnorderedMap;
myUnorderedMap.emplace(“Coke”, 1);
myUnorderedMap.emplace(“Sprite”, 2);
Key Points
- Key-Value Pair: Each item in the dictionary has a unique key that maps to a value.
- Accessing Values: Get a value by using its key, like a dictionary[“key”].
- Insertion: Add items to the dictionary using = or methods like insert and emplace.
- Iteration: You can loop through both std::map and std::unordered_map, but std::unordered_map does not guarantee the order of items.
Choose std::map if you need to sort items. Choose std::unordered_map if you want faster access times without caring about the order.
Step-by-Step Guide to Creating a Dictionary in C++ Using std::map
Creating a dictionary in C++ is simple when you use the std::map from the Standard Template Library (STL). Here’s an easy guide to help you get started.
Step 1: Include the Needed Headers
First, you need to include some headers in your program to use std::map.
#include <iostream>
#include <map>
#include <string>
Step 2: Declare Your std::map
Next, declare a std::map that will be your dictionary. You’ll need to specify the types of keys and values.
std::map<std::string, int> dictionary;
Here:
- std::string is used for the keys (like the names of fruits).
- Int is used for the values (like the quantities).
Step 3: Add Items to Your Dictionary
You can add items (key-value pairs) to your std::map using either the [] operator or the insert() method.
Using the [] Operator:
dictionary[“Pepsi”] = 10;
dictionary[“coke”] = 5;
dictionary[“Sprite”] = 20;
Using the insert() Method:
dictionary.insert(std::make_pair(“Pepsi”, 10));
dictionary.insert({“coke”, 5});
Step 4: Get Values by Key
To get the value associated with a key, just use the [] operator.
int PepsiQuantity = dictionary[“Pepsi”];
std::cout << “Pepsi quantity: ” << PepsiQuantity << std::endl;
Step 5: Loop Through Your Dictionary
To see all the items in your dictionary, use a simple for loop.
for (const auto& pair : dictionary) {
std::cout << pair.first << “: ” << pair.second << std::endl;
}
This loop will print out each key and its matching value.
Step 6: Update Values
If you want to change the value for a specific key, just use the [] operator again.
dictionary[“coke”] = 7; // Update the quantity of coke
Step 7: Remove Items
To delete an item from your dictionary, use the erase() method.
dictionary.erase(“Sprite”); // Remove the “Sprite” item
Step 8: Check If a Key Exists
To find out if a particular key is in your dictionary, use the find() method.
if (dictionary.find(“Pepsi”) != dictionary.end()) {
std::cout << “Pepsi is in the dictionary.” << std::endl;
} else {
std::cout << “Pepsi is not in the dictionary.” << std::endl;
}
Step 9: Clear the Dictionary
If you want to remove everything from your dictionary, use the precise () method.
dictionary.clear();
Step 10: Compile and Run Your Code
Once your code is ready, compile it and run the program to see the results.
g++ your_program.cpp -o your_program
./your_program
Reading these steps will allow you to create and manage a dictionary in C++ using std::mly easily. This guide gives you the basics to start building more complex programs as you become more comfortable with C++.
Choosing Between map and unordered_map in C++
When you work with key-value pairs in C++, you often need to decide between using a map or an unordered map. Both are useful but in different ways. Here’s a simple guide to help you choose the right one.
1. How They Store Data
- map:
- How It Works: It uses a Balanced Binary Tree (like a Red-Black Tree).
- Order: It keeps everything sorted by the key.
- Speed: Adding, removing, and searching take O(log n) time.
- unordered_map:
- How It Works: It uses a Hash Table.
- Order: There’s no specific order; it’s all about being fast.
- Speed: Usually, it’s very fast with O(1) time for most operations. But sometimes, if things get complicated, it can slow down to O(n).
2. When to Use map
- Need for Order: If you need your data sorted (like keeping names in alphabetical order), choose the map.
- Range-Based Operations: If you need to find items within a specific range, the map is the way to go.
- Memory Efficiency: map generally uses less memory because it doesn’t need extra space for handling hash problems.
3. When to Use unordered_map
- Fast Lookups: If you want quick searches and don’t care about order, unordered_map is your best option.
- Frequent Updates: Great for situations where you’re often adding or removing items.
- Large Datasets: Perfect for handling large amounts of data where speed is critical and order isn’t necessary.
4. Memory Usage
- map: Uses a tree structure, which is usually more memory-efficient.
- unordered_map: Needs more memory because of the hash table, which includes extra space to manage possible collisions.
5. Performance
- map: Always gives you O(log n) time for all operations, making it reliable no matter how many elements you have.
- unordered_map: Usually fast with O(1) time, but can slow down to O(n) if many keys hash to the same value.
6. Examples of Use
- map:
- Use it when you need to keep data in order, like a phone book sorted by name.
- It’s also good for situations where you need to go through items in order.
- unordered_map:
- Ideal for tasks like counting how many times a word appears where order doesn’t matter.
- It is great for quick access to data, like in a caching system, where speed is more important than order.
7. Conclusion: Which Should You Use?
- Choose map if:
- You need the items to stay in order.
- You want consistent performance without worrying about how the data is stored.
- Your task requires ordered data or needs to find items within a range.
- Choose unordered_map if:
- You need the fastest searches possible and don’t care about the order.
- You’re working with large datasets and want efficient performance.
- The order isn’t necessary, and you’re okay with handling hash collisions if they occur.
Understanding the differences between map and unordered_map can help you choose the correct method for your needs and ensure that your code runs smoothly.
Also Read
Final Words
In C++, there isn’t a built-in “dictionary” type, but you can still manage key-value pairs effectively. A C++ dictionary lets you store and access data using keys. The way you handle this data depends on your needs, whether it’s keeping things in order or accessing data quickly.
Knowing how to use these tools can help you handle key-value data efficiently and keep your programs running smoothly.
How do I create a dictionary in C++?
You create a dictionary using std::map or std::unordered_map. Both let you store pairs of keys and values, but they do things a bit differently.
How do I add items to a dictionary in C++?
To add items:
For map: myMap[key] = value;
For unordered_map: myUnorderedMap[key] = value;
How do I get a value from a dictionary in C++?
To get a value:
For map: value = myMap[key];
For unordered_map: value = myUnorderedMap[key];
