Array creation and type inference in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code?

Consider the following Swift code that creates an array using type inference. What will be printed?

Swift

let numbers = [1, 2, 3, 4]
print(type(of: numbers))

A Array<Int>

B Array<String>

C Dictionary<Int, Int>

D Set<Int>

💡 Hint

Look at the values inside the array and what type Swift infers from them.

Since the array contains integers, Swift infers the type as Array<Int>.

❓ Predict Output

intermediate

What is the output of this Swift array creation?

What will this Swift code print?

Swift

let mixedArray = ["apple", "banana", "cherry"]
print(type(of: mixedArray))

A Array<String>

B Array<Int>

C Array<Any>

D Array<Character>

💡 Hint

All elements are strings, so what type does Swift infer?

All elements are strings, so Swift infers the array type as Array<String>.

❓ Predict Output

advanced

What is the output of this Swift code with mixed types?

What will this Swift code print?

Swift

let mixed = [1, 2.5, 3]
print(type(of: mixed))

A Array<Int>

B Array<Double>

C Array<Any>

D Array<Float>

💡 Hint

Swift will promote integers to a common type with the floating-point numbers.

Swift promotes the integers to Double to match the floating-point literal 2.5, so the array type is Array<Double>.

❓ Predict Output

advanced

What error does this Swift code raise?

What error will this Swift code produce?

Swift

let invalidArray = [1, "two", 3]
print(type(of: invalidArray))

A Array<Int>

B Array<String>

C Error: Heterogeneous types in array without explicit type annotation

D Array<Any>

💡 Hint

Swift arrays must have elements of the same type or an explicit common type.

Swift does not allow arrays with mixed types without explicit type annotation. This code causes a compile-time error.

🧠 Conceptual

expert

How does Swift infer the type of an empty array literal?

Given the code let empty = [], what is the inferred type of empty in Swift?

A Array<Int>

B Array<Any>

C It causes a compile-time error due to ambiguous type

D Array<String>

💡 Hint

Think about how Swift needs type information to infer the type of an empty array.

Swift cannot infer the type of an empty array literal without context, so it causes a compile-time error due to ambiguous type.

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