The Ultimate Guide to Swift Interview Questions: Mastering Data Structures and Protocols

In the ever-evolving world of mobile app development, staying ahead of the curve is crucial. As an iOS developer, you need to be well-versed in Swift, Apple’s powerful programming language. One of the best ways to demonstrate your expertise and secure that dream job is by acing the interview process. That’s where this ultimate guide comes in – we’ll dive deep into the most commonly asked Swift interview questions, with a special focus on data structures and protocols.

Understanding Data Structures

Data structures are the backbone of any programming language, and Swift is no exception. These fundamental building blocks are essential for efficient data organization, manipulation, and retrieval. Let’s explore some of the most commonly asked questions in this domain.

How is a dictionary different from an array?

An array and a dictionary are both collection types in Swift, but they differ in their underlying structure and use cases.

  • Array: An array is an ordered collection of values of the same type. It stores elements in a linear fashion, with each element having a specific index. Arrays are ideal for situations where you need to access elements by their position or perform operations that require preserving the order of elements.

  • Dictionary: A dictionary is an unordered collection of key-value pairs. Each key is unique within the dictionary and is used to access the corresponding value. Dictionaries are great when you need to associate values with unique identifiers (keys) and retrieve them efficiently based on those keys, rather than their position in the collection.

What are the main differences between classes and structs in Swift?

Classes and structs are both used to create custom types in Swift, but they have distinct characteristics:

  • Classes are reference types, meaning that instances share a single instance in memory. Changes made to an instance of a class affect all other instances pointing to the same memory location. Classes support inheritance, allowing you to create more specialized types based on existing ones.

  • Structs are value types, meaning that each instance has its own copy of data in memory. When you pass a struct to a function or assign it to a new variable, a new copy is created. Structs do not support inheritance, but they can conform to protocols and benefit from protocol extensions.

What are tuples and why are they useful?

Tuples are finite, ordered collections of values of different types. They are useful when you need to group together related values of different types without creating a custom type. Tuples are particularly handy for returning multiple values from a function or temporarily grouping related data.

What does the Codable protocol do?

The Codable protocol in Swift is used for encoding and decoding custom data types to and from external representations, such as JSON, property lists, or XML. It combines the functionality of the Encodable and Decodable protocols, allowing you to easily convert your custom types to and from these external formats. This is particularly useful when working with network requests, file operations, and data persistence.

What is the difference between an array and a set?

Both arrays and sets are collection types in Swift, but they differ in their underlying structure and behavior:

  • Array: As mentioned earlier, an array is an ordered collection of values of the same type. It allows duplicate elements and provides efficient access to elements by index.

  • Set: A set is an unordered collection of unique values of the same type. Sets do not allow duplicate elements, and the order of elements is not guaranteed. Sets are useful when you need to keep track of unique values or perform set operations like unions, intersections, and subtractions.

What is the difference between the Float, Double, and CGFloat data types?

These data types represent floating-point numbers in Swift, but they differ in their precision and range:

  • Float: A 32-bit floating-point number, providing a precision of around 6-7 decimal digits. It has a smaller range than Double but requires less memory.
  • Double: A 64-bit floating-point number, offering a precision of around 15-16 decimal digits. It has a larger range than Float but requires more memory.
  • CGFloat: This is a type alias for either Float or Double, depending on the platform and architecture. It is used extensively in Core Graphics and UI frameworks (like UIKit and SwiftUI) to represent scalar values, such as coordinates, sizes, and angles. On most modern devices, CGFloat is a Double.

The choice between these types depends on the required precision, range, and memory constraints of your application.

Mastering Protocols

Protocols are a powerful feature of Swift that enable code reuse, abstraction, and modular design. Let’s explore some commonly asked questions related to protocols.

What does the Codable protocol do?

We’ve already covered this question in the data structures section, but it’s worth revisiting in the context of protocols. The Codable protocol is a combination of the Encodable and Decodable protocols, allowing you to easily convert custom data types to and from external representations like JSON or property lists. It’s a fundamental protocol for working with data in Swift.

How would you explain protocol-oriented programming to a new Swift developer?

Protocol-oriented programming (POP) is a programming paradigm that emphasizes the use of protocols as the primary means of abstraction and code organization. Instead of relying heavily on inheritance and subclassing, POP encourages the use of protocols to define behavior and capabilities, which can then be adopted by various types (classes, structs, or enums).

In POP, you focus on defining protocols that describe the required functionality, and then create concrete types that conform to those protocols. This approach promotes code reuse, modularity, and extensibility, as protocols can be adopted by multiple types without inheriting from a common base class.

POP also aligns well with Swift’s value types (structs and enums), which cannot inherit from each other but can conform to protocols, enabling composition over inheritance.

What is the difference between an extension and a protocol extension?

Extensions and protocol extensions are both mechanisms for adding functionality to existing types in Swift, but they serve different purposes:

  • Extension: An extension allows you to add new properties, methods, and initializers to an existing type (class, struct, or enum) without modifying the original source code. Extensions are useful for organizing code, providing default implementations, and adding functionality to types you don’t own (like built-in types or third-party libraries).

  • Protocol Extension: A protocol extension allows you to provide default implementations for methods and computed properties defined in a protocol. These default implementations can be overridden by types that conform to the protocol. Protocol extensions are useful for providing shared functionality across types that conform to the same protocol, reducing code duplication and promoting code reuse.

While extensions extend the functionality of a specific type, protocol extensions extend the functionality of all types that conform to a particular protocol.

What are conditional conformances?

Conditional conformances in Swift allow you to specify additional requirements or constraints for a type to conform to a particular protocol. These constraints can be based on the type itself or its associated types (e.g., the element type of a collection).

Conditional conformances are useful when you want to provide protocol implementations that depend on certain type constraints. For example, you might have a protocol that requires a type to be Codable only if its associated type also conforms to Codable.

Here’s an example of a conditional conformance:


extension Array: Codable where Element: Codable {    // Codable implementation for Array<Element> where Element conforms to Codable}

In this case, Array conforms to the Codable protocol only when its Element type (the type of elements it contains) also conforms to Codable.

Conditional conformances allow for more flexible and expressive protocol designs, enabling you to define protocol requirements that depend on the characteristics of the conforming types.


Mastering data structures and protocols in Swift is essential for any iOS developer looking to excel in job interviews and build robust, efficient applications. This guide has covered some of the most commonly asked questions in these domains, providing you with a solid foundation for understanding and effectively communicating your knowledge.

Remember, preparation is key when it comes to interviews. Take the time to practice your answers, explore real-world examples, and continuously expand your understanding of Swift’s powerful features. With dedication and a deep grasp of these concepts, you’ll be well on your way to impressing potential employers and taking your iOS development career to new heights.

iOS Interview Questions | Mock Interview | Tips & Tricks | Swift


How do you stay up to date with changes in Swift?

The Swift Programming Language constantly evolves, and most of its changes result from public proposals inside the Swift Evolution repository. The proposals can tell you what changes are coming up next, which is excellent if you want to stay updated with the latest developments.

What is swift code programming?

Swift is a powerful and intuitive programming language optimized when running on iOS, macOS, and other Apple platforms. Apple offers a wide variety of frameworks and APIs that make applications developed for these platforms unique and fun.

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