What Are Dictionaries in Data Structure?


Larry Thompson

Dictionaries in Data Structure

Dictionaries are a fundamental data structure in computer science that allow for efficient storage and retrieval of key-value pairs. Similar to a physical dictionary that maps words to their definitions, a dictionary data structure maps keys to their corresponding values.

What is a Dictionary?

A dictionary is an unordered collection of elements, where each element is stored as a key-value pair. The key serves as an identifier or label for the value it maps to.

The value can be of any data type, such as numbers, strings, lists, or even other dictionaries. Dictionaries are also known by other names in different programming languages, such as associative arrays or hash maps.

Creating a Dictionary

In most programming languages, dictionaries are represented using curly braces ({}) or the dict keyword. To create a dictionary, you can simply enclose the key-value pairs within the curly braces and separate them using colons (:). Let’s look at an example:

{ "name": "John", "age": 25 }

In this example, we have created a dictionary with two key-value pairs. The keys are “name” and “age”, and their corresponding values are “John” and 25 respectively.

Accessing Values in a Dictionary

To access the value associated with a specific key in a dictionary, you can use square brackets ([ ]) along with the key name. For instance:

dictionary = { "name": "John", "age": 25 }
print(dictionary["name"])  # Output: John

In this example, we access the value associated with the key “name” from the dictionary and print it on the console.

Dictionary Methods

Dictionaries provide various methods for manipulating their contents. Here are some commonly used methods:

  • get(): Retrieves the value associated with a key. Returns None if the key is not found.
  • keys(): Returns a list of all the keys in the dictionary.
  • values(): Returns a list of all the values in the dictionary.
  • items(): Returns a list of all the key-value pairs in the dictionary as tuples.
  • update(): Updates the dictionary with new key-value pairs from another dictionary or iterable.

Iterating Over a Dictionary

You can iterate over a dictionary using a loop to perform operations on each key-value pair. Here’s an example that demonstrates how to iterate over a dictionary and print its contents:

dictionary = { "name": "John", "age": 25 }

for key, value in dictionary.items():
    print(key + ": " + str(value))

# Output:
# name: John
# age: 25

In this example, we use the items() method to retrieve all the key-value pairs from the dictionary. The loop then iterates over each pair and prints them on separate lines.


Dictionaries are versatile data structures that provide efficient storage and retrieval of key-value pairs. They offer a convenient way to organize and access data based on unique identifiers (keys). By understanding dictionaries and their various methods, you can effectively utilize them in your programs to handle complex data structures and algorithms.

To summarize, dictionaries are essential tools for any programmer working with large amounts of data or needing efficient lookup operations. They provide an intuitive way to access values based on specific keys, making them invaluable when dealing with structured information in computer science.

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