Closure expression syntax in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a 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], 5)

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

A4

B3

C5

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 4 or 6 instead of 3 will change the result.

The closure is called with 3 and 5 as arguments, so the first argument is 3.

3fill in blank

hard

Fix the error in the closure syntax to correctly multiply two numbers.

Swift

let multiply = { (a: Int, b: Int) -> Int [1] a * b }

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

Afunc

B->

Cin

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'return' or 'func' instead of 'in' causes syntax errors.

The keyword 'in' separates the closure parameters and return type from the body.

4fill in blank

hard

Fill both blanks to create a closure that returns the square of a number.

Swift

let square = { (num: Int) [1] num [2] num }

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

Ain

B**

C*

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using '**' for exponentiation is not valid in Swift.

The closure uses 'in' to separate parameters from the body and '*' to multiply the number by itself.

5fill in blank

hard

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

Swift

let evens = numbers.filter { [1] in [2] % 2 [3] 0 }

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

Anum

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

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

The closure takes 'num' as input, then checks if 'num % 2 == 0' to filter even numbers.

Practice

1. What is a closure in Swift?

easy

A. A special kind of loop for repeating tasks

B. A block of code that can be passed around and executed later

C. A type of variable that stores numbers only

D. A keyword used to declare functions

Solution

1. Step 1: Understand the definition of closure

   A closure is a block of code that can be saved and run later, similar to a function but more flexible.

2. Step 2: Compare options with closure meaning

   Only 'A block of code that can be passed around and executed later' correctly describes a closure.

3. Final Answer:

   A block of code that can be passed around and executed later -> Option B

4. Quick Check:

   Closure = block of code passed and run later [OK]

Hint: Closures are like saved code blocks you run later [OK]

Common Mistakes:

• Confusing closures with variables
• Thinking closures are loops
• Mixing closures with function keywords

2. Which of the following is the correct syntax for a closure that takes two Ints and returns their sum?

easy

A. { a, b in a + b }

B. { (a, b) -> Int { a + b } }

C. func sum(a: Int, b: Int) -> Int { return a + b }

D. { (a: Int, b: Int) -> Int in return a + b }

Solution

1. Step 1: Identify closure syntax components

   A closure with parameters and return type uses parentheses for parameters, '->' for return type, and 'in' before the code.

2. Step 2: Check each option

   { (a: Int, b: Int) -> Int in return a + b } correctly uses (a: Int, b: Int) -> Int in return a + b. { a, b in a + b } omits types and requires context for type inference to compile. func sum(a: Int, b: Int) -> Int { return a + b } is a function, not a closure. { (a, b) -> Int { a + b } } has wrong braces placement and misses 'in'.

3. Final Answer:

   { (a: Int, b: Int) -> Int in return a + b } -> Option D

4. Quick Check:

   Closure syntax = parameters, return type, in keyword [OK]

Hint: Closure syntax: parameters, arrow, return type, then 'in' [OK]

Common Mistakes:

• Confusing functions with closures
• Missing 'in' keyword
• Incorrect braces placement

3. What is the output of this Swift code?

let multiply = { (x: Int, y: Int) -> Int in x * y }
print(multiply(3, 4))

medium

A. 12

B. 7

C. 34

D. Error

Solution

1. Step 1: Understand closure definition and call

   The closure multiply takes two Ints and returns their product (x * y). It is called with 3 and 4.

2. Step 2: Calculate the result

   3 * 4 = 12, so print outputs 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 times 4 = 12 [OK]

Hint: Multiply inputs inside closure to get output [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Syntax errors in closure call

4. Find the error in this closure syntax:

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

medium

A. Parameters must be in parentheses

B. Missing 'in' keyword after parameters

C. Closure cannot print inside its body

D. Missing parameter types in closure

Solution

1. Step 1: Check closure parameter syntax

   Closure parameters must be enclosed in parentheses, like (name: String).

2. Step 2: Identify the error in given code

   The code uses 'name: String' without parentheses, which is invalid syntax.

3. Final Answer:

   Parameters must be in parentheses -> Option A

4. Quick Check:

   Closure parameters need parentheses [OK]

Hint: Always put closure parameters inside parentheses [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Forgetting 'in' keyword
• Thinking closures can't print

5. How can you simplify this closure expression?

let numbers = [1, 2, 3, 4]
let doubled = numbers.map({ (num: Int) -> Int in return num * 2 })
print(doubled)

hard

A. Use a for loop instead of map

B. Remove 'return' and keep full syntax: numbers.map { (num: Int) -> Int in num * 2 }

C. Use shorthand argument names: numbers.map { $0 * 2 }

D. Cannot simplify closure expressions

Solution

1. Step 1: Understand closure simplification rules

   Swift allows removing parameter types and 'return' when the closure is a single expression, and using shorthand $0 for first parameter.

2. Step 2: Apply shorthand argument names

   Replacing full closure with numbers.map { $0 * 2 } simplifies code and is correct.

3. Final Answer:

   Use shorthand argument names: numbers.map { $0 * 2 } -> Option C

4. Quick Check:

   Shorthand $0 replaces explicit parameters [OK]

Hint: Use $0, $1 for closure parameters to shorten code [OK]

Common Mistakes:

• Keeping full syntax unnecessarily
• Thinking closures can't be simplified
• Replacing map with loops unnecessarily