Which Data Structure Is Immutable?


Scott Campbell

Data structures are an essential part of programming. They help in organizing and storing data efficiently, allowing us to perform various operations on it. One important aspect to consider when working with data structures is whether they are mutable or immutable.

What is immutability?

Immutability refers to the property of an object that cannot be modified after it is created. In other words, once an immutable object is created, its state remains constant throughout its lifetime.

Why use immutable data structures?

Immutable data structures provide several benefits:

  • Thread safety: Immutability ensures thread safety by eliminating the need for locks or synchronization mechanisms. Multiple threads can safely access and share immutable data without any risk of inconsistencies.
  • Predictable behavior: Immutable objects have a predictable state at all times, making debugging and testing easier.

    Since their state cannot change, we can reason about their behavior more confidently.

  • Caching: Immutable objects can be cached efficiently as they never change. This caching can lead to significant performance improvements in certain scenarios.
  • Functional programming: Immutable data structures are a fundamental concept in functional programming paradigms. Functional programming emphasizes immutability and avoids side effects, leading to more robust and maintainable code.

Which data structure is immutable?

Not all data structures are immutable by default. Some commonly used immutable data structures include:


Tuples are ordered collections of elements that cannot be modified once created. They can contain elements of different types and are often used to represent a group of related values.


In many programming languages, strings are immutable. Once a string is created, its content cannot be changed. Any operation on a string results in the creation of a new string.

Lists in functional programming languages

In functional programming languages like Haskell, lists are immutable by default. Operations on lists return new lists without modifying the original list.

Immutable collections

Several libraries and frameworks provide immutable data structures like maps, sets, and stacks. These data structures can be used to store and manipulate data immutably.


Immutability is an important concept in programming, especially when working with data structures. Immutable data structures have numerous benefits like thread safety, predictable behavior, caching, and support for functional programming paradigms.

Tuples, strings, lists in functional programming languages, and various immutable collections are examples of data structures that exhibit immutability. Understanding when and how to use these data structures can lead to more robust and efficient code.

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