Array creation and type inference in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty array of integers.

Swift

var numbers = [[1]]()

Drag options to blanks, or click blank then click option'

ADouble

BString

CInt

DBool

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type that does not match the intended array elements.

Forgetting to specify the type inside the brackets.

The code creates an empty array of integers by specifying Int inside the square brackets.

2fill in blank

medium

Complete the code to create an array with three string elements.

Swift

let fruits = ["Apple", "Banana", [1]]

Drag options to blanks, or click blank then click option'

A"Orange"

B"Carrot"

C"Potato"

D"Milk"

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a non-fruit string.

Forgetting the quotes around the string.

The array contains fruit names as strings. "Orange" fits the list of fruits.

3fill in blank

hard

Fill in the blank so that Swift infers the array type as [Double].

Swift

var values = [1, 2, [1]]

Drag options to blanks, or click blank then click option'

A3.5

B"3.5"

C3

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number.

Using an integer literal (causes inference to [Int]).

3.5 is a double literal, causing Swift to infer [Double]. "3.5" is a string, 3 is an integer (inferring [Int]), and true is a boolean.

4fill in blank

hard

Fill both blanks to create an array of booleans with two elements.

Swift

let flags: [Bool] = [[1], [2]]

Drag options to blanks, or click blank then click option'

Atrue

Bfalse

C"true"

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings instead of booleans.

Using numbers instead of booleans.

The array contains boolean values true and false. Strings or numbers are not valid booleans.

5fill in blank

hard

Fill all three blanks to create a dictionary from strings to integers.

Swift

let scores = [[1]: [2], [3]: 90]

Drag options to blanks, or click blank then click option'

A"Alice"

B85

C"Bob"

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using numbers as keys without quotes.

Using strings as values instead of integers.

The dictionary keys are strings like "Alice" and "Bob". The values are integers like 85 and 90.

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