Functions and closures in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple function that prints a greeting.

iOS Swift

func greet() {
  print([1])
}

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

A"Hello, world!"

BHello, world!

Cprint("Hello")

Dgreet()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the double quotes around the string.

Trying to call print inside print.

The print function requires a string argument enclosed in double quotes to print text.

2fill in blank

medium

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

iOS Swift

let add: (Int, Int) -> Int = [1]

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

A{ a, b in a - b }

B(a, b) -> a + b

Cfunc(a, b) { a + b }

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

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect closure syntax without 'in'.

Using subtraction instead of addition.

A closure in Swift is defined with curly braces, parameters, 'in', and a return expression.

3fill in blank

hard

Fix the error in the function that returns a closure capturing a variable.

iOS Swift

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

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

Aclosure()

Bfunc()

C{

D-> Int in

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'func()' instead of '{' to start the closure.

Adding extra keywords inside the return statement.

The closure should start with an opening brace '{' without extra keywords.

4fill in blank

hard

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

iOS Swift

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

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

Anumber

Bnum

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names before 'in' and inside the closure.

Using undefined variable names.

The closure parameter name must be consistent; here 'number' is used both times.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension filtering values greater than zero.

iOS Swift

let data = ["a": 1, "b": -1, "c": 3]
let filtered = data.filter { [1], [2] in [3] > 0 }

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

Akey

Bvalue

Cval

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent parameter names.

Checking the key instead of the value.

The closure parameters are key and value; here 'k' and 'value' are used, and 'value' is checked.

Practice

1. What is a closure in Swift?
func greet() { print("Hello") }

easy

A. An unnamed block of code that can be stored and passed around

B. A named function that cannot be reused

C. A variable that holds a number

D. A type of loop

Solution

1. Step 1: Understand closure definition

   A closure is a block of code without a name that can be saved and used later.

2. Step 2: Compare with function

   Functions have names, closures do not but both can be reused.

3. Final Answer:

   An unnamed block of code that can be stored and passed around -> Option A

4. Quick Check:

   Closure = unnamed reusable code block [OK]

Hint: Closures are like functions without names [OK]

Common Mistakes:

• Confusing closures with variables
• Thinking closures must have names
• Mixing closures with loops

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

easy

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

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

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

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

Solution

1. Step 1: Identify closure syntax

   Closures use curly braces with parameters and return type before 'in'.

2. Step 2: Check each option

   'let closure = { () -> Int in return 5 }' matches closure syntax. 'func closure() -> Int { return 5 }' declares a named function. 'let closure = func() -> Int { return 5 }' uses invalid syntax. 'let closure = (Int) -> { return 5 }' lacks 'in' keyword and has wrong parameter placement.

3. Final Answer:

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

4. Quick Check:

   Closure syntax = curly braces + 'in' [OK]

Hint: Closures use braces and 'in' keyword [OK]

Common Mistakes:

• Using func keyword inside closure
• Missing 'in' keyword
• Wrong parameter syntax

3. What will be printed by this Swift code?

let multiply = { (a: Int, b: Int) -> Int in
  return a * b
}
print(multiply(3, 4))

medium

A. 34

B. 7

C. 12

D. Error

Solution

1. Step 1: Understand closure parameters and return

   The closure takes two Ints and returns their product.

2. Step 2: Calculate multiply(3, 4)

   3 * 4 equals 12, so print outputs 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   3 times 4 = 12 [OK]

Hint: Closures return calculated values like functions [OK]

Common Mistakes:

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

4. Identify the error in this closure declaration:

let add = { (a: Int, b: Int) Int in
  return a + b
}

medium

A. Missing '->' before return type

B. Missing 'in' keyword

C. Parameters should not have types

D. Closure cannot return Int

Solution

1. Step 1: Check closure signature syntax

   Return type must be preceded by '->' in closure signature.

2. Step 2: Identify missing arrow

   Code has '(a: Int, b: Int) Int' but should be '(a: Int, b: Int) -> Int'.

3. Final Answer:

   Missing '->' before return type -> Option A

4. Quick Check:

   Closure return type needs '->' [OK]

Hint: Return type needs '->' before it in closures [OK]

Common Mistakes:

• Omitting '->' in closure signature
• Placing 'in' incorrectly
• Removing parameter types

5. You want to sort an array of names alphabetically using a closure. Which code correctly uses a closure with the sorted(by:) method?

hard

A. let sortedNames = names.sorted(by: { s1, s2 -> Bool { return s1 > s2 } })

B. let sortedNames = names.sorted(by: { s1, s2 in s1 + s2 })

C. let sortedNames = names.sorted(by: (s1, s2) -> s1 < s2)

D. let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 })

Solution

1. Step 1: Understand sorted(by:) closure

   It expects a closure that returns true if first element should come before second.

2. Step 2: Check each option

   'let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 })' returns true when s1 < s2 (correct for ascending alphabetical order). The version with s1 > s2 sorts descending. The version with (s1, s2) -> s1 < s2 uses invalid syntax without braces or 'in'. The version with s1 + s2 returns String, not Bool.

3. Final Answer:

   let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 }) -> Option D

4. Quick Check:

   sorted(by:) needs Bool-returning closure [OK]

Hint: sorted(by:) needs closure returning true/false [OK]

Common Mistakes:

• Returning wrong type from closure
• Using wrong comparison operator
• Incorrect closure syntax