Trailing closure syntax in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the function with a trailing closure.

Swift

func greet(action: () -> Void) {
    action()
}

greet [1] {
    print("Hello!")
}

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

A() -> Void

B()()

C()

D(action:)

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the parentheses when the function requires them.

Trying to put the closure inside the parentheses.

The function greet is called with an empty parameter list followed by a trailing closure in braces.

2fill in blank

medium

Complete the code to use trailing closure syntax with a function that takes one closure parameter.

Swift

func perform(task: () -> Void) {
    task()
}

perform[1] {
    print("Task done")
}

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

A()

B(task:)

C() -> Void

D(task)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to name the closure parameter in the call.

Omitting the parentheses entirely.

The function perform is called with empty parentheses followed by the trailing closure.

3fill in blank

hard

Fix the error in the function call using trailing closure syntax.

Swift

func calculate(operation: (Int, Int) -> Int) -> Int {
    return operation(3, 4)
}

let result = calculate [1] (3, 4) in
    $0 + $1

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

A<

B(

C[

D{

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of braces for the closure.

Trying to put closure parameters inside the parentheses.

Trailing closures use braces { } after the function call parentheses, not parentheses or other brackets.

4fill in blank

hard

Complete the code to correctly use trailing closure syntax with a function that has two parameters, the second is a closure.

Swift

func fetchData(from url: String, completion: (String) -> Void) {
    completion("Data from \(url)")
}

fetchData(from: {BLANK_1}}) { data in
    print(data)

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

A"https://api.example.com"

B{

C(

D"example.com"

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of braces for the closure.

Passing the URL outside the parentheses.

The URL string is passed as the first argument inside parentheses, then the trailing closure starts with a brace.

5fill in blank

hard

Complete the code to create a dictionary using trailing closure syntax with mapValues.

Swift

let numbers = ["one": 1, "two": 2, "three": 3]

let strings = numbers.mapValues { number in
    String(number) + [1] "!" + }

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

A{

B"!"

C"?"

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of braces for the closure.

Using wrong string literals or missing the closing brace.

The closure starts with {, concatenates the string "!", and ends with }.

Practice

1. What does trailing closure syntax allow you to do in Swift?
func perform(action: () -> Void) { action() }

easy

A. Write the last closure argument outside the parentheses

B. Use closures only inside parentheses

C. Avoid using closures altogether

D. Write closures only as named functions

Solution

1. Step 1: Understand closure arguments in Swift functions

   Swift functions can take closures as arguments, usually inside parentheses.

2. Step 2: Identify trailing closure syntax purpose

   Trailing closure syntax lets you write the last closure argument outside the parentheses for cleaner code.

3. Final Answer:

   Write the last closure argument outside the parentheses -> Option A

4. Quick Check:

   Trailing closure syntax = last closure outside parentheses [OK]

Hint: Last closure can go outside parentheses for clarity [OK]

Common Mistakes:

• Thinking closures must always be inside parentheses
• Confusing trailing closures with named functions
• Believing trailing closures remove closures entirely

2. Which of the following is the correct way to use trailing closure syntax in Swift?

func greet(action: () -> Void) { action() }

easy

A. greet() { print("Hello") }

B. greet { print("Hello") }

C. greet { print("Hello") }()

D. greet(print("Hello"))

Solution

1. Step 1: Recall trailing closure syntax format

   Trailing closure syntax allows calling a function with a closure outside parentheses if it's the last argument.

2. Step 2: Check each option

   greet { print("Hello") } correctly calls greet with closure outside parentheses: greet { print("Hello") }.

3. Final Answer:

   greet { print("Hello") } -> Option B

4. Quick Check:

   Trailing closure syntax = function { closure } [OK]

Hint: Use braces after function name without parentheses for trailing closure [OK]

Common Mistakes:

• Adding empty parentheses before trailing closure
• Trying to call closure immediately with () after braces
• Passing closure as a normal argument inside parentheses

3. What is the output of this Swift code?

func repeatTask(times: Int, task: () -> Void) {
    for _ in 1...times { task() }
}

repeatTask(times: 3) {
    print("Hi")
}

medium

A. Hi
Hi
Hi

B. Hi Hi Hi

C. Hi Hi

D. Error: Missing parentheses

Solution

1. Step 1: Understand repeatTask function behavior

   The function calls the closure 'task' the number of times specified by 'times'. Here, times = 3.

2. Step 2: Analyze the closure call

   The closure prints "Hi" each time. So it prints "Hi" three times, each on a new line because print adds a newline.

3. Final Answer:

   Hi Hi Hi -> Option A

4. Quick Check:

   3 prints of "Hi" each on new line = Hi Hi Hi [OK]

Hint: print adds newline, so repeated prints appear on separate lines [OK]

Common Mistakes:

• Assuming prints appear on same line without newline
• Thinking trailing closure causes syntax error
• Confusing number of repetitions

4. Identify the error in this Swift code using trailing closure syntax:

func calculate(operation: (Int, Int) -> Int) {
    let result = operation(5, 3)
    print(result)
}

calculate() { (a, b) in
    a + b
}

medium

A. Trailing closure syntax cannot be used here

B. Missing return keyword inside closure

C. Closure parameters should be outside parentheses

D. Extra parentheses after calculate() when using trailing closure

Solution

1. Step 1: Check function call syntax with trailing closure

   When using trailing closure syntax, do not include empty parentheses before the closure.

2. Step 2: Identify the error in code

   The code calls calculate() with empty parentheses and then a trailing closure, which is incorrect syntax.

3. Final Answer:

   Extra parentheses after calculate() when using trailing closure -> Option D

4. Quick Check:

   Trailing closure means no parentheses if no other arguments [OK]

Hint: Remove empty parentheses when using trailing closure [OK]

Common Mistakes:

• Including empty parentheses before trailing closure
• Adding unnecessary return in single-expression closure
• Thinking trailing closure can't have parameters

5. Given this Swift function:

func fetchData(completion: (String) -> Void) {
    let data = "User info"
    completion(data)
}

How would you call fetchData using trailing closure syntax to print the data received?

hard

A. fetchData() { data in print(data) }

B. fetchData(completion: { data in print(data) })

C. fetchData { data in print(data) }

D. fetchData { print(data) }

Solution

1. Step 1: Understand trailing closure syntax for single closure argument

   When the last argument is a closure, you can write it outside parentheses without empty parentheses.

2. Step 2: Check each option for correct syntax and usage

   fetchData { data in print(data) } correctly calls fetchData with trailing closure and closure parameter 'data' used inside print.

3. Final Answer:

   fetchData { data in print(data) } -> Option C

4. Quick Check:

   Trailing closure with parameter = fetchData { data in print(data) } [OK]

Hint: Write closure after function name without parentheses for trailing closure [OK]

Common Mistakes:

• Including empty parentheses before trailing closure
• Omitting closure parameter when needed
• Trying to use closure inside parentheses and trailing closure simultaneously