Type inference by the compiler in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type inferred as Int.

Swift

var number = [1]

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

A"42"

B3.14

C42

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the number makes it a String, not Int.

Using a decimal number makes it a Double, not Int.

The compiler infers the type of number as Int because the value 42 is an integer literal.

2fill in blank

medium

Complete the code to declare a constant with type inferred as Double.

Swift

let pi = [1]

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

A3.14159

B3

C"3.14159"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using an integer literal will infer Int, not Double.

Using quotes makes it a String, not Double.

The compiler infers pi as Double because 3.14159 is a floating-point literal.

3fill in blank

hard

Fix the error in the code by completing the assignment with correct type inference.

Swift

var isActive = [1]

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

A1

Btrue

C"true"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes makes it a String, not Bool.

Using numbers like 1 or 0 does not infer Bool.

The variable isActive is inferred as Bool when assigned the Boolean literal true.

4fill in blank

hard

Fill both blanks to create a dictionary with type inferred by the compiler.

Swift

let ages = [[1]: [2]]

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

A"Alice"

B30

C"Bob"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Boolean values as dictionary values causes type mismatch.

Using numbers as keys without quotes causes errors.

The dictionary ages has keys of type String (like "Alice") and values of type Int (like 30), inferred by the compiler.

5fill in blank

hard

Fill all three blanks to filter the dictionary using a trailing closure with type inference and a condition.

Swift

let filteredAges = ages.filter { [1], [2] in [2] [3] 25 }

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

Aname

Bage

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong parameter names for keys or values.

Using the less than symbol reverses the filter condition.

This filter creates a dictionary with only entries where the age is greater than 25. The compiler infers the types from ages.

Practice

1. What does type inference mean in Swift?

easy

A. The program runs slower because types are guessed.

B. You must always write the type explicitly.

C. The compiler automatically figures out the variable's type.

D. You cannot use variables without specifying types.

Solution

1. Step 1: Understand type inference concept

   Type inference means the compiler guesses the type of a variable based on the value assigned.

2. Step 2: Compare options with definition

   Only The compiler automatically figures out the variable's type. correctly describes automatic type detection by the compiler.

3. Final Answer:

   The compiler automatically figures out the variable's type. -> Option C

4. Quick Check:

   Type inference = automatic type detection [OK]

Hint: Type inference means compiler guesses type automatically [OK]

Common Mistakes:

• Thinking you must always write types explicitly
• Believing type inference slows down the program
• Confusing type inference with dynamic typing

2. Which of the following Swift variable declarations uses type inference correctly?

easy

A. let number = 10

B. let number: = 10

C. let number Int = 10

D. let number : Int 10

Solution

1. Step 1: Check syntax for variable declaration with type inference

   Type inference allows omitting the type, so let number = 10 is correct.

2. Step 2: Identify syntax errors in other options

   Options B, C, and D have syntax errors: missing type name, wrong order, or missing assignment operator.

3. Final Answer:

   let number = 10 -> Option A

4. Quick Check:

   Correct syntax with inference = let number = 10 [OK]

Hint: Look for correct Swift syntax without explicit type [OK]

Common Mistakes:

• Adding colon without type name
• Writing type before variable name incorrectly
• Omitting assignment operator

3. What is the type of variable value in this Swift code?

let value = 3.14

medium

A. Double

B. Int

C. String

D. Float

Solution

1. Step 1: Identify the literal type assigned

   The number 3.14 is a decimal number, which Swift treats as Double by default.

2. Step 2: Confirm Swift default for floating-point literals

   Swift infers floating-point literals as Double unless specified otherwise.

3. Final Answer:

   Double -> Option A

4. Quick Check:

   3.14 literal = Double type [OK]

Hint: Decimal numbers default to Double in Swift [OK]

Common Mistakes:

• Assuming 3.14 is Float by default
• Confusing Int with floating-point types
• Thinking literals default to String

4. Identify the error in this Swift code snippet:

var name = "Alice"
name = 42

medium

A. No error, type inference allows changing type.

B. Error: Cannot assign Int to a variable inferred as String.

C. Error: Missing semicolon after first line.

D. Error: Variable must be declared with explicit type.

Solution

1. Step 1: Check initial type inferred for variable

   Variable name is assigned a String "Alice", so compiler infers type String.

2. Step 2: Analyze reassignment with different type

   Assigning an Int (42) to a String variable causes a type mismatch error.

3. Final Answer:

   Error: Cannot assign Int to a variable inferred as String. -> Option B

4. Quick Check:

   Type mismatch on reassignment = Error [OK]

Hint: Variable type fixed by first assignment, cannot change later [OK]

Common Mistakes:

• Thinking type inference allows changing variable type
• Assuming semicolons are required in Swift
• Believing explicit type declaration is always needed

5. Given this Swift code, what is the type of result?

let numbers = [1, 2, 3]
let result = numbers.map { $0 * 2 }

hard

A. Array<Any>

B. Int

C. [Double]

D. [Int]

Solution

1. Step 1: Understand the type of numbers

   Array numbers contains Int values, so its type is [Int].

2. Step 2: Analyze the map operation and type inference

   The map multiplies each Int by 2, producing another Int for each element, so the result is an array of Ints.

3. Step 3: Confirm inferred type of result

   Since map returns an array of transformed elements, result is inferred as [Int].

4. Final Answer:

   [Int] -> Option D

5. Quick Check:

   map on [Int] with Int output = [Int] [OK]

Hint: map returns array of same type as transformed elements [OK]

Common Mistakes:

• Thinking map returns a single Int
• Assuming map changes type to Double automatically
• Believing result is Array<Any> without type inference