Which Data Structure Is Not Mutable?


Angela Bailey

Which Data Structure Is Not Mutable?

When working with data structures in programming, it is important to understand the concept of mutability. Mutability refers to whether a data structure can be modified or changed after it has been created. Some data structures are mutable, meaning they can be altered, while others are immutable and cannot be changed once created.

Mutable Data Structures

Let’s start by looking at some common mutable data structures:

  • List:
  • A list is a collection of elements that can be modified. Elements can be added, removed, or updated within a list.

  • Dictionary:
  • A dictionary is a key-value pair collection where values can be modified by their keys.

  • Set:
  • A set is an unordered collection of unique elements that can be modified by adding or removing elements.

Immutable Data Structures

In contrast to mutable data structures, immutable data structures cannot be changed once they are created. This characteristic brings several advantages such as thread safety and improved performance in certain scenarios.


A tuple is an ordered collection of elements enclosed in parentheses ( ). Tuples are immutable, meaning their values cannot be modified once assigned. However, individual elements within a tuple may still be mutable depending on their types.


A string is a sequence of characters that is also immutable in many programming languages including Python and Java. Once a string is defined, its contents cannot be changed.

Frozen Set

In addition to the mutable set we mentioned earlier, there is also an immutable version called a frozen set. Once a frozen set is created, its elements cannot be modified. This can be useful in scenarios where you need to ensure the integrity of a set.

Choosing the Right Data Structure

Knowing whether a data structure is mutable or immutable is important when designing and implementing algorithms. It helps determine the appropriate data structure to use based on the requirements of your program.

If you need a collection that allows modifications, then mutable data structures like lists, dictionaries, and sets are suitable choices. On the other hand, if you want to ensure data integrity or improve performance in certain scenarios, using immutable data structures like tuples, strings, or frozen sets may be more appropriate.

Understanding the mutability of different data structures empowers you as a programmer to make informed decisions when it comes to designing efficient and reliable code.

So next time you’re working on a project, take a moment to consider whether you need a mutable or immutable data structure based on your specific needs.

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