Collections (Array, Dictionary, Set) in iOS Swift

Introduction

Collections help you store and organize many pieces of data in one place. They make it easy to find, add, or change data.

You want to keep a list of your favorite songs.

You need to store names and phone numbers together.

You want to keep a group of unique items, like a set of visited cities.

You want to quickly find a value using a key, like a username and password.

You want to loop through a group of items to show them on the screen.

Syntax

iOS Swift

var array: [Type] = [value1, value2, ...]
var dictionary: [KeyType: ValueType] = [key1: value1, key2: value2, ...]
var set: Set<Type> = [value1, value2, ...]

Arrays keep items in order and allow duplicates.

Dictionaries store key-value pairs for fast lookup by key.

Sets store unique items without order.

Examples

An array of strings holding fruit names in order.

iOS Swift

var fruits: [String] = ["Apple", "Banana", "Cherry"]

A dictionary with names as keys and phone numbers as values.

iOS Swift

var phoneBook: [String: String] = ["Alice": "123-4567", "Bob": "987-6543"]

A set of integers. The duplicate 3 is stored only once.

iOS Swift

var uniqueNumbers: Set<Int> = [1, 2, 3, 3]

Examples of empty collections ready to be filled later.

iOS Swift

var emptyArray: [Int] = []
var emptyDictionary: [String: Int] = [:]
var emptySet = Set<String>()

Sample App

This SwiftUI app shows three collections: an array of fruits, a dictionary of phone numbers, and a set of unique numbers. It displays each collection's items in a list.

iOS Swift

import SwiftUI

struct ContentView: View {
    @State private var fruits: [String] = ["Apple", "Banana", "Cherry"]
    @State private var phoneBook: [String: String] = ["Alice": "123-4567", "Bob": "987-6543"]
    @State private var uniqueNumbers: Set<Int> = [1, 2, 3, 3]

    var body: some View {
        VStack(alignment: .leading, spacing: 20) {
            Text("Fruits Array:")
                .font(.headline)
            ForEach(fruits, id: \.self) { fruit in
                Text(fruit)
            }

            Text("Phone Book Dictionary:")
                .font(.headline)
            ForEach(phoneBook.sorted(by: { $0.key < $1.key }), id: \.key) { name, number in
                Text("\(name): \(number)")
            }

            Text("Unique Numbers Set:")
                .font(.headline)
            ForEach(Array(uniqueNumbers).sorted(), id: \.self) { number in
                Text(String(number))
            }
        }
        .padding()
    }
}

@main
struct CollectionsApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

Important Notes

Arrays keep the order of items and allow duplicates.

Dictionaries let you find values quickly using keys.

Sets store only unique items and do not keep order.

Use arrays when order matters, dictionaries when you need key-value pairs, and sets when you want unique items.

Summary

Arrays, dictionaries, and sets are basic ways to store groups of data.

Arrays keep order and allow duplicates.

Dictionaries store data as key-value pairs for fast lookup.

Sets store unique items without order.

Practice

1. Which Swift collection type keeps the order of items and allows duplicates?

easy

A. Array

B. Dictionary

C. Set

D. Tuple

Solution

1. Step 1: Understand collection types

   Arrays keep the order of elements and allow duplicates, dictionaries store key-value pairs, and sets store unique unordered items.

2. Step 2: Match collection behavior

   Since the question asks for ordered and duplicate-allowed collection, Array fits this description.

3. Final Answer:

   Array -> Option A

4. Quick Check:

   Ordered + duplicates = Array [OK]

Hint: Remember: Arrays keep order and duplicates [OK]

Common Mistakes:

• Confusing sets with arrays because both store multiple items
• Thinking dictionaries keep order like arrays
• Assuming tuples are collections like arrays

2. Which of the following is the correct way to declare an empty dictionary in Swift?

easy

A. var dict = Set<String>()

B. var dict = [Int]()

C. var dict = [String: Int]()

D. var dict = Array<Int>()

Solution

1. Step 1: Identify dictionary syntax

   In Swift, dictionaries are declared with key and value types inside square brackets separated by a colon, e.g., [KeyType: ValueType]().

2. Step 2: Check options

   var dict = [String: Int]() uses [String: Int]() which is correct for an empty dictionary with String keys and Int values. Others are array or set declarations.

3. Final Answer:

   var dict = [String: Int]() -> Option C

4. Quick Check:

   Dictionary syntax = [Key: Value]() [OK]

Hint: Dictionaries use [Key: Value]() syntax [OK]

Common Mistakes:

• Using [Int]() which is an array, not dictionary
• Confusing sets with dictionaries
• Missing colon between key and value types

3. What will be the output of this Swift code?

let fruits: Set<String> = ["apple", "banana", "apple", "orange"]
print(fruits.count)

medium

A. 4

B. Error

C. 2

D. 3

Solution

1. Step 1: Understand Set behavior

   Sets store unique elements only, so duplicates are removed automatically.

2. Step 2: Count unique fruits

   The array has "apple" twice, so unique items are "apple", "banana", "orange" which is 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Set removes duplicates, count = 3 [OK]

Hint: Sets remove duplicates, count unique items only [OK]

Common Mistakes:

• Counting duplicates as separate items
• Thinking sets keep order and duplicates
• Expecting a runtime error for duplicates

4. Identify the error in this Swift dictionary code:

var capitals = ["France": "Paris", "Japan": "Tokyo"]
capitals["France"] = "Lyon"
capitals["Germany"] = "Berlin"
print(capitals["Germany"])

medium

A. Adding new key-value pair is not allowed

B. No error, code runs and prints Optional("Berlin")

C. Cannot assign new value to existing key

D. Dictionary keys must be Int, not String

Solution

1. Step 1: Check dictionary mutation rules

   Dictionaries in Swift are mutable if declared with var, so you can update existing keys and add new ones.

2. Step 2: Analyze code behavior

   Updating "France" to "Lyon" and adding "Germany" with "Berlin" is valid. Printing capitals["Germany"] returns Optional("Berlin") because dictionary lookup returns an optional.

3. Final Answer:

   No error, code runs and prints Optional("Berlin") -> Option B

4. Quick Check:

   Mutable dictionary updates and adds keys [OK]

Hint: Mutable dictionaries allow adding and updating keys [OK]

Common Mistakes:

• Thinking dictionaries are immutable by default
• Expecting print to show value without Optional
• Believing keys must be Int type

5. Given this code, what is the resulting dictionary?

let names = ["Anna", "Bob", "Anna", "Cara"]
var counts = [String: Int]()
for name in names {
  counts[name, default: 0] += 1
}
print(counts)

hard

A. ["Anna": 2, "Bob": 1, "Cara": 1]

B. ["Anna": 1, "Bob": 1, "Cara": 1]

C. ["Anna": 3, "Bob": 1, "Cara": 1]

D. Syntax error due to default value

Solution

1. Step 1: Understand dictionary default value usage

   The syntax counts[name, default: 0] returns the current value for key or 0 if key is missing, allowing safe increment.

2. Step 2: Count occurrences in array

   "Anna" appears twice, "Bob" and "Cara" once each, so counts dictionary will have Anna: 2, Bob: 1, Cara: 1.

3. Final Answer:

   ["Anna": 2, "Bob": 1, "Cara": 1] -> Option A

4. Quick Check:

   Counting duplicates with default 0 increments [OK]

Hint: Use dictionary[key, default: 0] += 1 to count items [OK]

Common Mistakes:

• Expecting syntax error from default value usage
• Counting duplicates as 1 only
• Confusing dictionary keys and values