What Data Structure Is a Dictionary?


Larry Thompson

A dictionary is a fundamental data structure in programming that stores data in key-value pairs. It is also known as an associative array, map, or hash table. In this article, we will explore what exactly a dictionary is and how it works in various programming languages.

What is a Dictionary?

A dictionary is an unordered collection of elements that are stored as key-value pairs. Unlike other data structures like arrays or lists that are indexed by integers, dictionaries can be indexed by any hashable object. The key serves as the identifier for the value it maps to.

Key Characteristics of Dictionaries:

  • Unordered: Dictionaries do not maintain any specific order for their elements.
  • Mutable: The elements of a dictionary can be modified after creation.
  • Unique Keys: Each key in a dictionary must be unique. If you try to add a duplicate key, the previous value associated with it will be overwritten.

Working with Dictionaries

To understand how dictionaries work, let’s consider an example where we want to store information about students.

students = {
    "John": 20,
    "Alice": 22,
    "Bob": 19

In this example, the keys are the names of the students (“John”, “Alice”, and “Bob”), and the values are their corresponding ages (20, 22, and 19).

We can access the values of specific keys by using square brackets:

john_age = students["John"]
print(john_age) # Output: 20

We can also modify existing values or add new key-value pairs:

students["Alice"] = 23
students["Charlie"] = 21

To check if a key exists in a dictionary, we can use the in keyword:

if "Bob" in students:
    print("Bob is in the dictionary")

Common Operations with Dictionaries

Dictionaries provide a range of useful operations to work with their elements. Some common operations include:

  • Adding or Updating Elements: You can add new key-value pairs or update existing values associated with keys.
  • Removing Elements: You can remove elements by using the del keyword or the pop() method.
  • Getting Keys and Values: You can retrieve all the keys or values from a dictionary using the keys(), values(), or items() methods.
  • Merging Dictionaries:You can combine multiple dictionaries into one using the update() method.

Dictionaries in Different Programming Languages

Dictionaries are available in many programming languages, although they may be called by different names. Here are some examples:

  • In Python:

    Python provides built-in support for dictionaries using curly braces ({}) and colons (:) to define key-value pairs.
  •     students = {
            "John": 20,
            "Alice": 22,
            "Bob": 19
  • In JavaScript:

    JavaScript has a similar data structure called an object, which functions as a dictionary. It uses curly braces to define properties and values.
  •     let students = {
            "John": 20,
            "Alice": 22,
            "Bob": 19


Dictionaries are versatile data structures that allow us to store and retrieve information using unique keys. They provide efficient lookups and offer flexibility in terms of the types of objects that can be used as keys. Understanding how dictionaries work is essential for efficient programming, as they are widely used in various applications.

By using dictionaries, you can organize your data in a way that is both meaningful and easily accessible.

Now that you have learned about dictionaries, start incorporating them into your programs to take advantage of their powerful capabilities!

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