Closures are reference types in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a closure that adds two numbers.

Swift

let add: (Int, Int) -> Int = [1] { $0 + $1 }

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

A{

Bfunc

Cclosure

Dlambda

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'func' keyword instead of closure syntax

Trying to use 'lambda' which is not a Swift keyword

The correct syntax to declare a closure in Swift uses curly braces { }. So { $0 + $1 } defines the closure body.

2fill in blank

medium

Complete the code to assign a closure to a variable and call it.

Swift

var multiply = [1] (Int, Int) -> Int = { $0 * $1 }
let result = multiply(3, 4)

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

Afunc

Blet

Cvar

Dclosure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'func' keyword instead of closure type

Using 'var' incorrectly when 'let' is better here

Use let to declare a constant closure. Here, multiply is assigned a closure that multiplies two numbers.

3fill in blank

hard

Fix the error in the closure capturing a variable.

Swift

func makeIncrementer(forIncrement amount: Int) -> () -> Int {
    var total = 0
    let incrementer: () -> Int = {
        total [1]= amount
        return total
    }
    return incrementer
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition

Forgetting to update total inside the closure

The closure should add amount to total each time it is called, so += is the correct operator.

4fill in blank

hard

Fill both blanks to create a closure that captures and modifies a variable.

Swift

func makeCounter() -> () -> Int {
    var count = 0
    return {
        count [1]= 1
        return count [2] 0
    }
}

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

A+

B-

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition

Using wrong comparison operators

The closure increments count by 1 using += and returns whether count is greater than 0 using >.

5fill in blank

hard

Fill all three blanks to create a closure that captures a string and returns its length if longer than 3.

Swift

func makeStringChecker(_ text: String) -> () -> Int? {
    return {
        let length = text.[1]
        if length [2] 3 {
            return length
        } else {
            return [3]
        }
    }
}

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

Acount

B>

Cnil

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property name instead of count

Returning wrong value instead of nil

The closure gets the string length using count, checks if it is greater than 3, and returns nil if not.

Practice

1. What does it mean when we say closures are reference types in Swift?

easy

A. Multiple variables can point to the same closure instance, sharing changes.

B. Closures cannot capture variables from their surrounding context.

C. Each time you assign a closure, a new copy is created.

D. Closures are stored only as values, not references.

Solution

1. Step 1: Understand what reference types mean

   Reference types mean multiple variables can point to the same instance in memory.

2. Step 2: Apply this to closures

   Since closures are reference types, assigning them copies the reference, not the code itself.

3. Final Answer:

   Multiple variables can point to the same closure instance, sharing changes. -> Option A

4. Quick Check:

   Closures share instance references [OK]

Hint: Reference types share instances, not copies [OK]

Common Mistakes:

• Thinking closures always create new copies
• Confusing closures with value types like structs
• Ignoring that closures capture variables by reference

2. Which of the following is the correct way to assign a closure to a variable in Swift?

easy

A. func closure = { print("Hello") }

B. let closure = { print("Hello") }

C. closure := { print("Hello") }

D. var closure = function printHello() {}

Solution

1. Step 1: Recall Swift closure syntax

   Closures are assigned using curly braces and can be stored in variables with let or var.

2. Step 2: Check each option

   let closure = { print("Hello") } uses correct syntax: let closure = { print("Hello") }.

3. Final Answer:

   let closure = { print("Hello") } -> Option B

4. Quick Check:

   Correct closure assignment syntax [OK]

Hint: Closures use curly braces and assign with let or var [OK]

Common Mistakes:

• Using := which is not Swift syntax
• Trying to declare closures like functions
• Using func keyword incorrectly for closures

3. What will be printed by this Swift code?

var closureA = { print("Count: 0") }
var closureB = closureA
closureA = { print("Count: 1") }
closureB()

medium

A. Count: 0

B. Count: 1

C. No output, error occurs

D. Count: 0 and Count: 1

Solution

1. Step 1: Understand closure assignment

   closureB is assigned closureA's original closure printing "Count: 0".

2. Step 2: Reassign closureA

   closureA is changed to a new closure printing "Count: 1", but closureB still points to the old closure.

3. Final Answer:

   Count: 0 -> Option A

4. Quick Check:

   closureB keeps original closure [OK]

Hint: Reassigning closureA doesn't change closureB's reference [OK]

Common Mistakes:

• Assuming closureB changes when closureA is reassigned
• Confusing closure references with copies
• Expecting both closures to print the same after reassignment

4. Identify the error in this Swift code involving closures:

var closure1 = { print("Hello") }
var closure2 = closure1
closure1 = { print("World") }
closure2 = closure1

medium

A. Closure syntax is invalid

B. Cannot assign closure1 to closure2 after reassignment

C. No error, code runs fine

D. Variables closure1 and closure2 must be constants

Solution

1. Step 1: Check closure assignments

   Both closure1 and closure2 are variables and can be reassigned closures.

2. Step 2: Validate syntax and assignments

   All closure assignments use correct syntax and valid variable usage.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   Valid closure reassignment [OK]

Hint: Variables can be reassigned closures freely [OK]

Common Mistakes:

• Thinking reassignment causes errors
• Confusing closure syntax with function syntax
• Assuming closures must be constants

5. Given this Swift code, what will be printed?

func makeCounter() -> () -> Int {
    var count = 0
    let increment: () -> Int = {
        count += 1
        return count
    }
    return increment
}

let counter1 = makeCounter()
let counter2 = counter1
print(counter1())
print(counter2())

hard

A. 1 and 1

B. 0 and 1

C. 2 and 2

D. 1 and 2

Solution

1. Step 1: Understand closure capturing

   The closure increments and returns the same captured variable 'count'.

2. Step 2: Analyze reference sharing

   counter1 and counter2 reference the same closure instance, so increments affect both.

3. Step 3: Trace calls

   First call counter1() returns 1, second call counter2() increments count to 2 and returns 2.

4. Final Answer:

   1 and 2 -> Option D

5. Quick Check:

   Shared closure state increments count [OK]

Hint: Closures share captured variables and state [OK]

Common Mistakes:

• Thinking counter2 is independent copy
• Ignoring closure captures and shared state
• Assuming count resets on each call