30 Days of SwiftUI - Day 4: Mastering Functions and Data Structures in Swift

March 17, 2025

Introduction

Functions are essential building blocks in Swift, allowing you to organize and reuse code efficiently. In today's lesson, we'll explore how to create functions, manage parameters, return values, and handle errors. We'll also compare key data structures like arrays, sets, and tuples to ensure you choose the right tool for the job.

🚀 Reusing Code with Functions

Functions allow you to write code once and reuse it throughout your app, reducing repetition and improving maintainability.

Example:

func greet(name: String) {
    print("Hello, \(name)!")
}

greet(name: "Gati") // Output: Hello, Gati!

🧩 What Belongs Inside a Function?

Use functions when:

You need to repeat code multiple times.
A complex task can be simplified into smaller steps.
The code improves clarity and structure.

Example:

Instead of repeating logic:

print("Loading data...")
print("Fetching user details...")
print("Finalizing setup...")

Use a function to improve clarity:

func loadAppData() {
    print("Loading data...")
    print("Fetching user details...")
    print("Finalizing setup...")
}

loadAppData()

🔢 How Many Parameters Are Too Many?

While Swift allows multiple parameters, aim to keep it minimal (ideally 3 or fewer) for better readability.

Example:

func calculateArea(length: Double, width: Double) -> Double {
    return length * width
}
print(calculateArea(length: 5.0, width: 3.0))

🎯 Returning Values from Functions

Functions can return values using the -> syntax.

Example:

func square(of number: Int) -> Int {
    return number * number
}
print(square(of: 4)) // Output: 16

🔄 When Can You Skip `return`?

If the entire function body is a single expression, Swift allows you to omit return for cleaner code.

Example:

func double(of number: Int) -> Int {
    number * 2 // No `return` needed
}
print(double(of: 5)) // Output: 10

🧳 Packing Multiple Values with Tuples

Swift lets you return multiple values using tuples.

Example:

func getUserDetails() -> (name: String, age: Int) {
    return ("Gati", 30)
}

let user = getUserDetails()
print("Name: \(user.name), Age: \(user.age)")

📊 Choosing Between Arrays, Sets, and Tuples

Array: Use when data is ordered and may include duplicates.
Set: Use for unique, unordered data with fast lookup.
Tuple: Use for grouping fixed, related values with distinct meanings.

🔹 Tuples in Action

Tuples combine multiple values into a single compound value.

Example:

let person = (name: "Gati", age: 30)
print("\(person.name) is \(person.age) years old.")

⚖️ Arrays vs Sets vs Tuples

Feature	Array	Set	Tuple
Order	Yes	No	Yes
Duplicates	Yes	No	N/A
Mutability	Mutable/Immutable	Mutable/Immutable	Immutable

📝 Customizing Parameter Labels

Swift allows you to provide external parameter labels to improve function clarity.

Example:

func sayHello(to person: String) {
    print("Hello, \(person)!")
}

sayHello(to: "Gati")

🚫 Omitting Parameter Labels

Use _ to omit the parameter label for concise function calls.

Example:

func printMessage(_ message: String) {
    print(message)
}

printMessage("Welcome to SwiftUI!")

⚙️ Providing Default Values for Parameters

You can assign default values to function parameters, making them optional when calling the function.

Example:

func greetUser(name: String = "Guest") {
    print("Hello, \(name)!")
}

greetUser() // Output: Hello, Guest!
greetUser(name: "Gati") // Output: Hello, Gati!

💡 Why Use Default Parameters?

Default parameters simplify code when values are frequently the same.

🚨 Handling Errors in Functions

Swift's throwing functions help manage unexpected errors using throw, do, try, and catch.

Example:

enum FileError: Error {
    case fileNotFound
}

func readFile(named fileName: String) throws {
    if fileName != "data.txt" {
        throw FileError.fileNotFound
    }
    print("File read successfully.")
}

do {
    try readFile(named: "data.txt")
} catch {
    print("Error: \(error)")
}

⚠️ When to Write Throwing Functions

Use throwing functions when dealing with tasks that can fail, such as file reading, network requests, or data parsing.

🧐 Why Does Swift Require `try`?

Swift enforces try to remind developers that errors may occur, promoting safer code practices.

Example:

do {
    try readFile(named: "wrongFile.txt")
} catch {
    print("Handled error correctly!")
}

📋 Summary

Today we explored functions — from creating simple reusable logic to managing parameters, returning values, and handling errors. We also covered key data structures like arrays, sets, and tuples.

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