What Data Structure Underlies a Dictionary?


Scott Campbell

What Data Structure Underlies a Dictionary?

When it comes to storing and retrieving data, dictionaries are one of the most commonly used data structures. In many programming languages, dictionaries are often referred to as maps, hash maps, or associative arrays.

But have you ever wondered what data structure lies beneath the surface of a dictionary? In this article, we’ll explore the inner workings of dictionaries and the data structure that powers them.

Understanding Dictionaries

Dictionaries are key-value pairs that allow you to store and retrieve values based on unique keys. The keys can be of any immutable type such as strings, numbers, or tuples, while the values can be of any type. Dictionaries provide an efficient way to search for and access values without having to iterate over multiple elements.

The Hash Table

Underneath a dictionary’s hood lies a powerful data structure called a hash table. A hash table is an array-based data structure that uses a technique called hashing to store and retrieve values in constant time. Hashing is a process where an input value is transformed into a fixed-size numerical value, known as a hash code.

The hash code is then used as an index in the underlying array to store the associated value. This allows for quick access to values based on their corresponding keys. However, since two different keys could potentially produce the same hash code (known as hash collisions), additional mechanisms are needed to handle such scenarios.

Handling Hash Collisions

In order to handle hash collisions, most programming languages implement various strategies such as chaining or open addressing. Chaining involves storing multiple key-value pairs at each index in the underlying array. Each index contains either a linked list or another data structure to hold the values associated with the same hash code.

Open addressing, on the other hand, aims to find an alternative index for storing the value when a collision occurs. This is typically done by applying a probing technique, such as linear probing or quadratic probing, which iteratively searches for an empty slot in the array.

Benefits of Hash Tables

The use of hash tables as the underlying data structure for dictionaries offers several advantages. First and foremost, it provides constant-time average-case performance for searching, inserting, and deleting values. This makes dictionaries an efficient choice when dealing with large amounts of data.

Additionally, hash tables allow for efficient key lookup by providing a direct mapping from keys to their associated values. This eliminates the need to iterate over all elements in search of a specific key.


In conclusion, dictionaries are built upon the powerful foundation of hash tables. The use of hashing and appropriate collision resolution techniques allows dictionaries to provide efficient storage and retrieval of key-value pairs. Understanding the underlying data structure can help you make informed decisions when working with dictionaries in your programming projects.

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