How to Create a Dictionary in C++?

A dictionary is a data structure in python that enables users to store data in the form of key-value pairs, where each key must be unique. In this article, we will learn how to create a dictionary-like data structure in C++.

Examples

Input: {{1 : "C++"}, {3: "Python"}, {2: "Java"}}
Output: 1 : C++
2 : Python
3 : Java
Explanation: Sorted order dictionary is created using std::map

Input: {{45 : "Geeks"}, {11 : "For" }, {9: "Geeks"}}
Output: 9 : Geeks
11 : For
45 : Geeks
Explanation: Sorted order dictionary is created using std::map

In C++, there are two data structures that allows users to store data in the form of key value pairs: map and unordered_map. So, we can implement the dictionary-like data structure using two different ways in C++:

Using std::map

We can use the std::mapto create the dictionary in C++. A map is a container from the Standard Template Library (STL) that stores key-value pairs in ascending order of key. As C++ is statically typed language, we have to declare the type of key and value while creating a map.

Syntax to create std::map

std::map<key_type, value_type> name;

Example

// C++ Program to Create a Dictionary 
// using std::map
#include <bits/stdc++.h>
using namespace std;
int main() {
    // Create an empty dictionary using std::map
    map<int, string> dict;
    // Insert key-value pairs
    dict[1] = "C++";
    dict[3] = "Python";
    dict[2] = "Java";
    for (auto it : dict)
        cout << it.first << ": " << it.second << endl;
    return 0;
}

1: C++
2: Java
3: Python

In C++, std::map is implemented as a balanced binary tree (typically a Red-Black Tree) so it is a bit less efficient than the python dictionaries. But

Using std::unordered_map

STLstd::unordered_map container provides the closest inbuilt thing you can to the Python dictionary in C++. It stores the data in key-value pair. It internally uses a hash table for implementation, so it is as fast as dictionary.

Syntax to create std::unordered_map

std::unordered_map<key_type, value_type> name;

Example

// C++ Program to Create a dictionary using
// unordered_map
#include <bits/stdc++.h>
using namespace std;
int main() {
    // Create an empty dictionary using unordered_map
    unordered_map<int, string> dict;
    // Insert key-value pairs
    dict[1] = "C++";
    dict[3] = "Python";
    dict[2] = "Java";
    for (auto it : dict)
        cout << it.first << ": " << it.second << endl;
    return 0;
}

2: Java
3: Python
1: C++

Time Complexity: O(1), worst case O(n), where n is the number of keys
Auxiliary Space: O(n)

C++ map vs unordered_map vs Python Dictionary

The following table lists the major differences between C++ map, C++ unordered_map and Python Dictionary: