Array creation and type inference in Swift - Mini Project: Build & Apply

Array creation and type inference

📖 Scenario: You are organizing a small party and want to keep track of the guests' names using an array.

🎯 Goal: Create an array of guest names using Swift's type inference and then print the list.

📋 What You'll Learn

Create an array called guestNames with exactly these names: "Anna", "Brian", "Catherine"

Create a variable called newGuest and assign it the value "David"

Add newGuest to the guestNames array

Print the guestNames array

💡 Why This Matters

🌍 Real World

Arrays are used to store lists of items like guest names, product lists, or scores in many apps.

💼 Career

Understanding arrays and type inference is essential for Swift developers building iOS apps that manage collections of data.

1

Create the initial array

Create an array called guestNames with these exact names: "Anna", "Brian", "Catherine". Use Swift's type inference by not specifying the type explicitly.

Swift

// Create the array guestNames with the names "Anna", "Brian", "Catherine"
// Your code here

Hint

Use square brackets to create an array and assign it to guestNames.

2

Add a new guest variable

Create a variable called newGuest and assign it the value "David".

Swift

var guestNames = ["Anna", "Brian", "Catherine"]
// Create a variable newGuest and assign it "David"
// Your code here

Hint

Use var newGuest = "David" to create the variable.

3

Add the new guest to the array

Add the newGuest variable to the guestNames array using the append method.

Swift

var guestNames = ["Anna", "Brian", "Catherine"]
var newGuest = "David"
// Add newGuest to guestNames using append
// Your code here

Hint

Use guestNames.append(newGuest) to add the new guest.

4

Print the guest list

Print the guestNames array to show all guest names.

Swift

var guestNames = ["Anna", "Brian", "Catherine"]
var newGuest = "David"
guestNames.append(newGuest)
// Print the guestNames array
// Your code here

Hint

Use print(guestNames) to display the array.

Practice

1. What does Swift do when you create an array with values like [1, 2, 3] without specifying a type?

easy

A. Swift infers the array type as [Int] automatically.

B. Swift treats the array as [Any] type.

C. Swift throws a syntax error because type is missing.

D. Swift creates an empty array instead.

Solution

1. Step 1: Understand type inference with array literals

   When you write an array with values like [1, 2, 3], Swift looks at the values and guesses the type. Here, all values are integers.

2. Step 2: Determine the inferred type

   Since all values are integers, Swift infers the array type as [Int]. No explicit type declaration is needed.

3. Final Answer:

   Swift infers the array type as [Int] automatically. -> Option A

4. Quick Check:

   Array with integers = [Int] [OK]

Hint: Look at items; Swift guesses array type from them [OK]

Common Mistakes:

• Thinking Swift defaults to [Any]
• Assuming a syntax error without explicit type
• Believing empty array is created instead

2. Which of the following is the correct way to create an empty array of Strings in Swift?

easy

A. let names = []

B. let names = String[]

C. let names: [String] = []

D. let names = Array()

Solution

1. Step 1: Recognize empty array syntax with type annotation

   Empty arrays need a type so Swift knows what kind of items will be stored. let names: [String] = [] declares names as [String] and assigns an empty array.

2. Step 2: Check other options for correctness

   let names = [] lacks type info, so it's invalid. let names = String[] uses invalid syntax. let names = Array() creates an empty array but type is not specified, so it defaults to [Any].

3. Final Answer:

   let names: [String] = [] -> Option C

4. Quick Check:

   Empty array needs type annotation [OK]

Hint: Empty arrays must declare type explicitly [OK]

Common Mistakes:

• Omitting type for empty arrays
• Using Array() without type annotation
• Assuming [] alone is enough

3. What is the type of the array created by let numbers = [1, 2.5, 3] in Swift?

medium

A. [Int]

B. [Double]

C. [Float]

D. Error: mixed types not allowed

Solution

1. Step 1: Analyze the array elements' types

   The array contains 1 (Int), 2.5 (Double), and 3 (Int). Swift will promote Int values to Double to keep the array type consistent.

2. Step 2: Determine the inferred array type

   Because of the Double value 2.5, Swift infers the array type as [Double], converting Ints to Double.

3. Final Answer:

   [Double] -> Option B

4. Quick Check:

   Mixed Int and Double = [Double] [OK]

Hint: Mixed Int and Double become [Double] array [OK]

Common Mistakes:

• Assuming array is [Int]
• Expecting a syntax error for mixed types
• Thinking array becomes [Float]

4. Identify the error in this Swift code:

let items = []
items.append("apple")

medium

A. Cannot append to a constant array declared with let.

B. Array type cannot be inferred from empty array literal.

C. Appending a String to an array of Ints causes error.

D. No error; code runs fine.

Solution

1. Step 1: Check array declaration and type inference

   The array is declared as let items = [] with no type or initial values. Swift cannot guess the type from an empty array literal.

2. Step 2: Understand why append fails

   Because items is declared with let, it is immutable and cannot be appended to. To append, it must be declared with var.

3. Final Answer:

   Cannot append to a constant array declared with let. -> Option A

4. Quick Check:

   let arrays are immutable [OK]

Hint: let arrays cannot be modified after declaration [OK]

Common Mistakes:

• Thinking append fails because array type is unknown
• Assuming default array type is [Int]
• Believing code runs without error

5. You want to create an array that holds either integers or strings in Swift. Which approach correctly creates such an array?

hard

A. let mixed = Array()

B. let mixed = [1, "two", 3]

C. let mixed: [Int | String] = [1, "two", 3]

D. let mixed: [Any] = [1, "two", 3]

Solution

1. Step 1: Understand Swift's type system for mixed arrays

   Swift does not support union types like Int | String. To hold different types, use the type Any.

2. Step 2: Check each option for correctness

   let mixed: [Any] = [1, "two", 3] declares mixed as [Any] and initializes with Int and String values, which is valid. let mixed = [1, "two", 3] tries to infer type but fails due to mixed types. Options A and C use invalid syntax for union types.

3. Final Answer:

   let mixed: [Any] = [1, "two", 3] -> Option D

4. Quick Check:

   Mixed types array uses [Any] [OK]

Hint: Use [Any] for arrays with mixed types [OK]

Common Mistakes:

• Trying to infer mixed types without [Any]
• Using invalid union type syntax
• Assuming Swift allows Int | String type