Swift Collections: Arrays, Dictionaries, and Sets

# CHAPTER 8

Swift Collections: Arrays, Dictionaries, and Sets #

1. Introduction #

2. Learning Objectives #

• Create, modify, and iterate over Swift Arrays.

• Understand zero-based indexing to extract array items.

• Store and retrieve data using Key-Value pairs in Dictionaries.

• Utilize Sets to perform ultra-fast math and ensure unique data.

• Combine Collections with for-in loops to render data.

3. Arrays (Ordered Lists) #

// 1. Creating an Array of Strings
var highScores: [Int] = [100, 250, 80]
var shoppingList = ["Milk", "Eggs", "Bread"] // Type Inference!

// 2. Adding items
shoppingList.append("Apples")

// 3. Removing items
shoppingList.remove(at: 1) // Removes "Eggs"

Zero-Based Indexing: To grab an item out of an array, you ask for its position (Index). In computer science, we always start counting at Zero!

print(shoppingList[0]) // Prints "Milk" (The 1st item)
// print(shoppingList[3]) // CRASH! Index out of range.

4. Dictionaries (Key-Value Pairs) #

// Syntax: [KeyType: ValueType]

// 1. Creating a Dictionary (Key is String, Value is Integer)
var phoneBook: [String: Int] = [
    "Alice": 5551234,
    "Bob": 5559876
]

// 2. Retrieving a value using the Key!
print(phoneBook["Alice"]) // Prints 5551234

// 3. Adding or Updating a value
phoneBook["Charlie"] = 5551111 // Adds new
phoneBook["Alice"] = 5559999   // Updates existing

5. Sets (Unordered & Unique) #

1. 1. It has NO order (No indexing).

1. 2. Every item MUST be unique. If you try to add "Apple" twice, it simply ignores the second one!

Sets are incredibly fast for searching.

var visitedCountries: Set<String> = ["USA", "Japan", "France"]

// Inserting a duplicate does nothing!
visitedCountries.insert("Japan") 

// Lightning fast check!
if visitedCountries.contains("USA") {
    print("You have been to the US!")
}

6. Iterating through Collections #

Looping an Array:

let students = ["Alice", "Bob", "Charlie"]

for student in students {
    print("Welcome to class, \(student)!")
}

Looping a Dictionary: (It provides both the Key and Value!)

let grades = ["Math": "A", "Science": "B"]

for (subject, grade) in grades {
    print("You scored a \(grade) in \(subject).")
}

7. Higher-Order Functions (Advanced Magic) #

let prices = [10, 20, 30, 40, 50]

// Give me a NEW array containing only items greater than 25!
let expensiveItems = prices.filter { $0 > 25 }
print(expensiveItems) // Prints [30, 40, 50]

8. Common Mistakes #

• Index Out of Range: The absolute most common crash in iOS development is Fatal Error: Index out of range. If an array has 3 items, its indexes are 0, 1, and 2. If your code asks for shoppingList[3], the app will instantly crash because that slot does not exist. Always check .count first!

• Dictionary Nulls: When you ask a Dictionary for a value (phoneBook["Dave"]), what if Dave isn't in the book? Swift doesn't crash; it returns nil (nothing). We will learn how to handle nil in the next chapter!

9. Best Practices #

• Use Arrays for UI Lists: If you are building a feed (like Twitter or Instagram), always use an Array because the *order* of the posts matters.

• Use Dictionaries for JSON: When you download data from the internet (JSON), it almost always comes formatted as Dictionaries (Key-Value pairs).

10. Exercises #

1. 1. Create an Array of Strings named colors containing "Red", "Green", and "Blue". Append "Yellow" to it.

1. 2. Create a Dictionary named inventory where the Keys are Strings (item names) and the Values are Integers (quantity). Add an item.

11. Coding Challenges #

12. MCQ Quiz with Answers #

13. Interview Questions #

• Q: Contrast Arrays, Dictionaries, and Sets. Provide a specific, real-world iOS app scenario for when you would choose each specific collection type over the others.

• Q: Explain the concept of "Zero-Based Indexing". Why does calling array[array.count] result in a fatal "Index out of bounds" crash?

• Q: Briefly describe how higher-order functions like .filter and .map improve architectural readability over standard for loops.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #