Why closures are fundamental in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple closure that adds two numbers.

Swift

let add = { (a: Int, b: Int) -> Int in return a [1] b }

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' will subtract instead of add.

The closure adds two integers using the + operator.

2fill in blank

medium

Complete the code to call the closure with arguments 3 and 5.

Swift

let result = add([1])

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

A(3, 5)

B3, 5

C3 5

D[3, 5]

Attempts:

3 left

💡 Hint

Common Mistakes

Passing arguments without parentheses or commas causes syntax errors.

Closures with multiple parameters are called with parentheses and comma-separated values.

3fill in blank

hard

Fix the error in the closure that captures a variable and increments it.

Swift

var count = 0
let increment = { count [1] 1 }

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

A-=

B++

C+=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '++' causes a syntax error in Swift 3 and later.

The '+=' operator increments the variable by 1 inside the closure.

4fill in blank

hard

Fill both blanks to create a closure that filters even numbers from an array.

Swift

let numbers = [1, 2, 3, 4, 5]
let evens = numbers.filter { $0 [1] [2] 2 == 0 }

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

A%

B==

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' will select odd numbers instead.

The modulo operator '%' checks the remainder, and '==' compares it to zero to find even numbers.

5fill in blank

hard

Fill all three blanks to create a closure that maps strings to their lengths if length is greater than 3.

Swift

let words = ["apple", "cat", "banana"]
let filteredLengths = words.reduce(into: [:]) { dict, word in
    if word.count [1] [2] {
        dict[[3]] = word.count
    }
}

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

A>

B3

Cword

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will filter the wrong words.

The closure checks if the word length is greater than 3 and adds it to the dictionary with the word as the key.

Practice

1. What is a closure in Swift?
Closures are best described as:

easy

A. A type of variable that stores numbers

B. Blocks of code that can be saved and used later

C. A special kind of loop

D. A way to declare constants

Solution

1. Step 1: Understand the definition of closures

   Closures are self-contained blocks of functionality that can be passed around and used in your code.

2. Step 2: Match the description to the options

   Blocks of code that can be saved and used later correctly describes closures as blocks of code saved for later use, unlike variables, loops, or constants.

3. Final Answer:

   Blocks of code that can be saved and used later -> Option B

4. Quick Check:

   Closure = reusable code block [OK]

Hint: Closures are like saved recipes for code [OK]

Common Mistakes:

• Confusing closures with variables
• Thinking closures are loops
• Mixing closures with constants

2. Which of the following is the correct syntax to declare a closure that explicitly takes no parameters and returns an Int?

easy

A. let closure = { () -> Int in return 5 }

B. let closure = (Int) -> { 5 }

C. let closure = { return 5 }

D. let closure = func() -> Int { 5 }

Solution

1. Step 1: Recall closure syntax in Swift

   A closure with no parameters and returning Int is declared as { () -> Int in ... }.

2. Step 2: Check each option

   let closure = { () -> Int in return 5 } matches the correct syntax. let closure = { return 5 } misses return type and parameters. let closure = (Int) -> { 5 } is invalid syntax. let closure = func() -> Int { 5 } uses function syntax, not closure.

3. Final Answer:

   let closure = { () -> Int in return 5 } -> Option A

4. Quick Check:

   Closure syntax = { () -> ReturnType in code } [OK]

Hint: Closure syntax uses braces with 'in' keyword [OK]

Common Mistakes:

• Omitting 'in' keyword
• Using function syntax instead of closure
• Missing return type when needed

3. What will be printed by this Swift code?

let multiplyByTwo = { (number: Int) -> Int in
    return number * 2
}
print(multiplyByTwo(4))

medium

A. Error

B. 4

C. 2

D. 8

Solution

1. Step 1: Understand the closure behavior

   The closure takes an Int and returns that number multiplied by 2.

2. Step 2: Calculate the output for input 4

   4 * 2 = 8, so the print statement outputs 8.

3. Final Answer:

   8 -> Option D

4. Quick Check:

   4 * 2 = 8 [OK]

Hint: Multiply input by 2 inside closure [OK]

Common Mistakes:

• Confusing input and output values
• Expecting the closure to print automatically
• Syntax errors in closure call

4. Identify the error in this closure declaration:

let greet = { name: String in
    print("Hello, \(name)!")
}

medium

A. Missing parentheses around parameters

B. Missing 'in' keyword

C. Closure must return a value

D. Incorrect print syntax

Solution

1. Step 1: Check closure parameter syntax

   Closure parameters must include types inside parentheses, like (name: String).

2. Step 2: Identify the error in given code

   The code misses parentheses around parameters.

3. Final Answer:

   Missing parentheses around parameters -> Option A

4. Quick Check:

   Parameters need types in parentheses [OK]

Hint: Always put parameters and types in parentheses [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Forgetting parameter types
• Assuming closures must return values

5. How can closures capture values from their surrounding context?
Consider this code:

func makeIncrementer(amount: Int) -> () -> Int {
    var total = 0
    let incrementer: () -> Int = {
        total += amount
        return total
    }
    return incrementer
}

let incrementByTen = makeIncrementer(amount: 10)
print(incrementByTen())
print(incrementByTen())

What will be the output?

hard

A. 0 and 10

B. 10 and 10

C. 10 and 20

D. Error because total is not accessible

Solution

1. Step 1: Understand closure capturing

   The closure captures the variable 'total' from its surrounding context and modifies it each time it runs.

2. Step 2: Trace the output of two calls

   First call adds 10 to total (0 + 10 = 10), second call adds 10 again (10 + 10 = 20).

3. Final Answer:

   10 and 20 -> Option C

4. Quick Check:

   Closures capture and update variables [OK]

Hint: Closures remember and update outside variables [OK]

Common Mistakes:

• Assuming closures do not keep state
• Thinking total resets each call
• Expecting errors due to variable scope