Which Data Structure Stores Key-Value Pairs?


Heather Bennett

A key-value pair is a fundamental data structure that allows you to associate a value with a unique key. This data structure is commonly used in various programming languages and databases to efficiently store and retrieve information. In this article, we will explore the different data structures that can be used to store key-value pairs and discuss their characteristics and use cases.

1. Arrays

Arrays are one of the simplest data structures used to store key-value pairs.

In an array, each element has an index associated with it, and the value can be accessed using this index. In the context of key-value pairs, the keys can be treated as indices, and their corresponding values can be stored at those indices.

However, arrays have some limitations when it comes to storing key-value pairs:

  • Limited Key Types: Arrays usually require integer indices, so if your keys are not integers or need custom string-based keys, arrays may not be suitable.
  • Fixed Size: Arrays have a fixed size that needs to be defined upfront. If you don’t know the exact number of key-value pairs in advance or if it may change dynamically, arrays may not be an ideal choice.

2. Objects

In many programming languages like JavaScript or Python, objects are commonly used to store key-value pairs. An object is a collection of properties where each property has a name (key) and a value associated with it.

Objects offer several advantages when it comes to storing key-value pairs:

  • Flexible Key Types: Objects allow you to use various types of keys such as strings or symbols instead of just integers.
  • No Fixed Size: Objects can dynamically grow and shrink as key-value pairs are added or removed.

For example, in JavaScript:

const person = {
  name: 'John',
  age: 30,
  profession: 'Software Engineer'

3. Hash Tables/Hash Maps

Hash tables, also known as hash maps, are widely used data structures to store key-value pairs.

They provide efficient insertion, deletion, and retrieval operations based on the keys.

A hash table uses a hash function to convert the key into an index or a hash code. This index is then used to store the value associated with that key. The hash function ensures that each key is mapped to a unique index in the underlying array-like structure.

Hash tables offer the following benefits:

  • Fast Access: Hash tables have constant-time average access complexity, making them ideal for scenarios where quick access to values based on their keys is required.
  • No Fixed Size: Similar to objects, hash tables can dynamically resize themselves as key-value pairs are added or removed.

4. Databases

In addition to in-memory data structures, databases also provide efficient ways to store and query key-value pairs. NoSQL databases like MongoDB or Apache Cassandra are specifically designed for storing and retrieving data using flexible schema formats like documents or columns.

Databases offer several advanced features when it comes to handling key-value pairs:

  • Distributed Storage: Databases can distribute the key-value pairs across multiple machines for scalability and fault tolerance.
  • Data Indexing and Querying: Databases can create indexes on specific keys to optimize querying and searching for key-value pairs.


There are various data structures available for storing key-value pairs, each with its own advantages and limitations. Arrays and objects are simple options, while hash tables and databases provide more advanced features for efficient storage and retrieval. The choice of the appropriate data structure depends on the specific requirements of your application, such as the type of keys, expected size, and required performance.

By understanding these different options, you can make informed decisions when it comes to storing and accessing key-value pairs in your programs or databases.

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